Download PRAXIS II Earth Science Remediation Part One: Introduction, Rocks

PRAXIS II
Earth Science
Remediation
Part One: Introduction, Rocks & Minerals
1.1: Introduction
What is Earth science? Earth science is a collective, or “umbrella” term for all the sciences that seek to better
understand the planet Earth. We usually don’t call someone an “Earth scientist,” for example, as scientists almost
always specialize within more narrow disciplines, such as within geology or meteorology. And these disciplines of
Earth science are specialized even further. A geologist might specialize in geomorphology (the study of
landforms), for example, or a meteorologist might work in the field of atmospheric chemistry. Additionally, many
people who have entered into one of the Earth sciences have gone on to study other planets, such as Mars.
We will divide Earth science into four general areas of study: geology, oceanography, meteorology, and
astronomy.
Geology is a word that literally means "study of Earth." The science of geology is further divided into physical
geology which examines the composition of the planet and the processes taking place within it and upon its
surface. Historical geology looks at the development of Earth from its formation to the present, a vast length of
time during which Earth has undergone much change. Historical geologists have worked toward establishing a
chronology of events which gives us a better understanding of the processes at work on the planet today, and
what the fate of the Earth and its inhabitants might hold in the future.
Oceanography is not distinct science, it is the application of the sciences (chemistry, physics, biology, and
geology) seeking to understand the oceans, a vast portion of Earth that remains largely unexplored. In fact,
portions of Mars have been investigated in more detail than many depths of Earth’s waters.
The study of the atmosphere of our world is called meteorology, and includes investigations of the composition of
the atmosphere, and the processes creating weather phenomena and global climate patterns.
When we examine our planet from the perspective of outer space, we can gain better insights as to its
composition and the dynamic processes taking place on, and within, the Earth. This focus is one of the aspects
of the science of astronomy.
It is important to view Earth as a system, with interacting parts. We will broadly group these parts into four
“spheres”:
Geosphere: the solid portion of Earth
Hydrosphere: the water portion
Atmosphere: the gases
Biosphere: the life on Earth
Some courses and texts in Earth science also distinguish the cryosphere (the ice of the planet) and the
pedosphere, the soils. We shall take up those portions as part of the discussion of the hydrosphere and the
geosphere. The ice on Earth, for example, plays a part in shaping the solid Earth: ice carves away at the solid
Earth, breaking the rock down into smaller particles, which can become mixed into… soil! This is just one
example of the myriad relationships of these spheres. By looking at the Earth as a system, these relationships
and interactions become a foundation for a clearer understanding of our planet. Many of the interactions are
cyclical, such as the hydrologic (water) cycle and the rock cycle.
The energy driving the processes within Earth system can ultimately be traced to either of two sources: energy
from the Sun and the internal heat with the Earth itself. Earth receives less that one-half a trillionth of the total
energy produced by the Sun, yet this is sufficient to power the external processes affecting Earth, such as
weather and ocean circulation. The internal energy is responsible for processes such as mountain building and
volcanic eruptions.
It is important to note that a change to a portion of one sphere in the system often produces a response in one, or
even all, of the other spheres. Here is an illustrative scenario:
Lightning from a thunderstorm (atmosphere) causes a brush fire which greatly
reduces the vegetation (biosphere) in an area. The reduced vegetation allows for greatly
increased soil moisture content (soil is an interface of all the spheres as will see later)
and runoff (hydrosphere) when heavy rains occur in future storms. This results in
mudflows that remove much of the topsoil in the area (geosphere). The loss of soil is
accompanied by loss of much of the habitat dependent upon it (biosphere). Smoke from
the fire places particles and carbon dioxide into the air (atmosphere). The loss of
vegetation will further affect the atmosphere as plants produce oxygen and absorb
carbon dioxide (the significance of carbon dioxide will be seen later as well).
Two things we should ask ourselves about the scenario. First, the event portrayed above has played out
countless times over the hundreds of millions of years land flora have existed. So why have the grasslands and
forests of Earth not been lost to fire, and the land they grew on washed away to the sea? The answer, as you
may know, is that seeds and animal life can begin to encroach from the area outside the damage zone. A
process of slow rejuvenation begins, including the replenishment of soil. The interactive balance of the spheres is
reestablished.
Secondly, and to emphasize the recent understanding of human activities and their effect on the Earth system,
what if we change the scenario by removing the vegetation through land development? In that case, humans
may very well introduce enough change to the area (by bulldozing and paving, for instance) that the pre-existing
balance of the spheres in this locale may never return.
1.2: Minerals: The Building Blocks of Rocks
Rocks and minerals. The terms seem inexorably combined, yet rocks and minerals need to be
considered distinctly. A mineral has a definite chemical makeup and structure. [We will define a rock in
the next section]. A mineral must be a natural solid, and is generally inorganic (usually not made of
molecules that were a part of a living thing). It must be crystalline, that is to say the atoms within it are
arranged in a geometric structure that repeats itself as the crystal grows during its formation. This
structure, or crystal lattice as it is known, is akin to the scaffolding constructed around buildings under
repair.
Various geometric shapes are possible in crystals due to the nature of the bonding between the atoms
comprising them. Salt crystals, which are known to mineralogists as halite, have a cubic crystal structure,
due to the way the atoms of sodium and chlorine bond together. The silicate minerals, such as quartz,
have a crystal lattice based on a pyramid-shaped structure of silicon bonded with oxygen atoms. As
crystals grow, the basic lattice shape is repeated, and the shape of the crystal can become visible to the
naked eye if the process continues for a sufficiently long time.
The composition and structure of each mineral gives it a set of observable properties that can help to
identify it. Among these properties are the hardness and luster of the mineral, how it breaks apart when
struck, and its density.
All the known elements on (and in) planet Earth are grouped in the periodic table of elements that we see
in chemistry and physics classrooms.
Most minerals are composed from the more common elements found in the geosphere. There are
thousands of minerals, in fact a few are discovered (in trace amounts) every year, but only a few dozen
are abundant, and these are made up of combinations of eight elements: silicon, oxygen, aluminum,
calcium, sodium, iron, magnesium, and potassium.
Minerals are grouped based on the primary constituent atom(s) or molecule(s) making up their structure.
The largest group by far is the silicates.
Minerals are important to Earth science as they tell us of the chemical makeup and history of the
geosphere. The presence of a particular mineral can provide evidence of the environmental conditions
that existed on Earth when the crystals formed. They are important in a more familiar way: much of our
world depends of the supply of mineral resources needed for building materials, chemicals, generating
and carrying electricity, and much more.
1.3: Rocks: Materials of the Solid Earth
A rock is defined less strictly than a mineral. A rock is any solid, naturally occurring mass of mineral or
mineral-like material. Some rocks are made up of almost entirely of one mineral, however most are
aggregates of two or more minerals. For examples, limestone is composed of the mineral calcite with a
few impurities but granite is often composed of at least quartz and a few feldspar minerals, with some
mica minerals entering the mix on occasion.
Rocks are important not only as a resource of mineral material. They hold clues to how they were
formed. This formation process in turn gives us a portrait of the forces and environmental conditions at
work on Earth, going back billions of years.
Rocks are divided into three types, based on the general way in which they formed. The groups are:
• Igneous rocks: formed by the cooling (crystallization) of molten rock material (magma)
• Sedimentary rocks: formed by the compaction and cementation of sediments of weathered rock
(a process called lithifaction) or through chemical processes such as precipitation of dissolved
minerals
• Metamorphic rocks: rocks which have been altered by significant changes in the environment the
original rock resided in, such as greatly increased pressures or temperatures, or exposure to
chemical action
A key component of the Earth system is the rock cycle. The vast and eternal forces at work within the
solid Earth have altered nearly all of the original crustal materials from Earth’s formation. Rock is being
formed in many places on the planet, rock is being altered into different forms in others. Not all of the
physical and chemical mechanisms that are at work in the rock cycle are fully understood.
In the rock cycle, igneous rock is forming form the cooling of molten rock material (magma) into crystalline
solids. Some igneous rock is the result of volcanic activity, but much of the magma rising toward the
upper crust does not reach the surface, but cools at depth in what are called plutonic formations. Igneous
rocks are classified by their mineral makeup, and their texture, i.e. the size and arrangement of crystals.
The texture of igneous rock is governed largely by the environment within which the magma has
undergone crystallization as represented in Bowen’s reaction series. Typical igneous rocks are granite
and basalt.
Sedimentary rock requires first the deposition of weathered rock material. As the sediment accumulates
into deeper thicknesses, the pressure acting on the sediment increases with depth. Sufficient pressures
result in the particles to become compacted and cemented together into a detrial sedimentary rock, a
process called lithifaction. Limestone and sandstone are examples. Chemical sedimentary rocks are
formed from dissolved minerals becoming solid once more through precipitation or evaporation.
Examples of chemical sedimentary rock are limestone and rock gypsum.
The layering of sedimentary rock is of great help to the historical geologist. The layers, or strata as they
are known, read like a time line over the periods of Earth’s history. The nature of the sediment gives
clues as to the extent of the parent (weathered) rock and the environmental conditions in place during
ancient times. Fossils embedded within the strata tell us of the living organisms in existence in these
eons past.
When rock is subjected to a change in its surroundings which results in the rock being heated (but not to
its melting point, or it will become magma), placed under the stress of pressure, or is exposed to
chemically active fluids, its crystal may be altered physically and sometimes chemically. The rock is said
to undergo metamorphism. Metamorphic rock is often found surrounding a molten igneous body (contact,
or thermal metamorphism) or in regions of mountain building where rock is under tremendous stresses
and pressures (regional metamorphism). Marble is the metamorphic rock hwose parent rock was
limestone, granite is often the parent rock of the metamorphic rock gneiss.
Many mineral resources, particularly metals, are found concentrated in areas where igneous and
metamorphic process have taken place.
Part Two: External Processes
2.1: Weathering, Soil, and Mass Wasting
Planet Earth has undergone immense change since it formed over four billion years ago. Landmasses have
been raised and lowered, they have collided with other landmasses, ocean crust has been created and
consumed. Mountains have been thrust upward, exposing them to the elements of wind and water that then
begin to reduce the rock and wash it away. The forces that create mountains are internal, produced
ultimately by the heat of Earth’s interior. The opposing forces breaking down and lowering mountains are
external, driven by the Sun. And these forces continue to act on the features of Earth right now, and will play
out for millions of years to come. It is impossible for human perception to witness most of these changes due
to their slowness. For example, the Appalachian Mountains were once roughly the height of today’s Rocky
Mountains, but external forces have been at work for tens of millions of years so that they have been reduced
in elevation and have been shaped into less-rugged features with gentler slopes.
Immense quantities of rock material were taken from the peaks of the old Appalachians to sculpt the smaller
mountains we see now. Where did this material go? The rock and minerals were broken down by the action
of wind and water, and the wind and water removed the material, carrying it to more distant locales downwind
and downstream. Then it is distributed as sediment. In order for rock to undergo this process of removal and
transport, it must be broken from its position and made small enough to be carried away by the medium of
transportation. The medium we will examine first will be liquid water, later we will examine the actions of wind
and ice.
Rock masses, such as we see built up into huge mountains, are made of minerals and minerals, when
exposed to air and water, can be altered in size and/or chemistry. Weathering is the term used to describe
the reduction of mineral material physically or chemically so that it is loosened or dissolved. Continued
weathering will reduce the mineral material to a size small enough to be carried away. We will define the
physical (size) reduction of rock as mechanical weathering, and the decomposition of material by the altering
of mineral chemistry as chemical weathering.
Mechanical weathering is often accomplished through frost wedging, the action of water expanding as it
freezes in small cracks in the rock. You may have had the misfortune of discovering that water expands
when it freezes by trying to chill a sealed beverage in the freezer. If you wait until the liquid freezes,
expansion can result in the bursting of the container. This is the force involved in frost wedging, strong
enough to break apart rock into looser pieces. Another type of mechanical weathering is the sheeting of rock
layers due to removal of overlying rock from erosion, called unloading. The reduction of pressure upon the
body of rock causes it to expand, and crack. Rock may also be broken up through the action of biological
activity, such as burrowing animals, and by the roots of plants.
The removal of a piece of rock exposes more surface area of the rock to the elements, due to the geometry of
the situation. Weathering of the chemical type is especially accelerated by the increase in surface area.
Chemical weathering takes place on the molecular level. The internal structure of the mineral material is
altered by the addition or removal of compounds or elements. Water often plays a key role in the chemical
reactions, as do gases in the atmosphere (another illustration of the interactions of the “spheres” of the Earth
system, and an interface of the hydrologic and rock cycles). One of the most significant types of chemical
weathering involves the natural acidity of rainwater, caused by the dissolution of atmospheric carbon dioxide
(creating carbonic acid, the acid in carbonated beverages). The acid attacks many mineral structures. Other
acids caused by human-generated pollutants have resulted in what has been dubbed “acid rain.”
The rate of weathering in a given rock formation is governed by the amount of surface area exposed, the
composition of the rock, and the climate conditions. Colder climates might have more frost wedging and less
biological activity, for example, whereas the opposite would generally be true in tropical locales. Rock
formations do not weather at a uniform rate. Slight variations in the mineral composition of a rock formation
can result in what is known as differential weathering.
Weathered rock material covers nearly all the land surface of Earth, a covering called regolith. The regolith
provides the minerals we see in the soils of the planet. But soil is more than mineral particles, it is a mixture
of mineral, water, air, and decayed organic matter (humus). Therefore, from our perspective of the Earth
system, we have in soil an interface between all four spheres. Soil is an indispensable resource, which is
often taken for granted by humans, especially if allowed to erode due to a change to the landscape in an
area. For example, the removal of vegetation for land development or overgrazing by livestock can result in
the loss of topsoil.
Soils form from the surface downward, the processes at work thereby creating a particular type of soil, and a
particular soil profile with depth. The processes of soil formation and the rate soils form are governed by
several factors, including climate, the topography of the region, amount of biological activity, and the length of
time the processes have been at work.
The force of gravity plays a role in the shaping of earth’s landforms. Mass wasting is the downward transport
of rock, regolith, and soil due to the action of gravity. Gravity can produce a spectacular and rapid lowering of
material such as in a rockfall, or landslide. But many mass wasting events are imperceptibly slow, such as in
the case of creep. In the evolution of most of Earth’s land features, mass wasting is the step occurring
between weathering and erosion (the removal of the weathered material via water, wind, or ice).
2.2: Running Water and Groundwater
Earth is sometimes called “The Water Planet.” Seventy-one percent of the planet is covered with water. But
only a tiny fraction of Earth’s hydrosphere is water flowing upon its land surfaces. Nevertheless, the role of
stream flow is a major one when examining the external forces acting to shape Earth’s land features. Rivers
and streams have an obvious significance in providing a supply of fresh water to much of the biosphere,
including humans. They are critical for navigation upon the globe. Civilizations have relied on the
replenishment of topsoil along Earth’s fertile river valleys from silt deposition during floods, floods that can
also bring death and destruction.
Streams from the standpoint of geologic processes do three kinds of work:
•
•
•
Erosion, the incorporation of weathered rock material, regolith and soil into the water
Transportation of this solid Earth material to a different location, perhaps thousands of kilometers
away
Deposition, the laying-down of the material as sediment
These are the three actions that carry out the work of removing vast amounts of rock weathered from rugged
mountains as they are sculpted into smoother, less lofty features, and placing this subtracted material
elsewhere, often at the bottom of a large body of water. This sediment may, after hundreds of millions of
years, become the rock of a future range of mountains. [How sedimentary rock can be forced from the
bottom of a sea upward thousands of meters into a mountain system will be discussed later.] Again, note the
interplay of the hydrologic cycle and the rock cycle.
Streams have a set of characteristics which change as the water makes its way from the headwaters, or
source, of the stream, down to their end, the mouth of the stream. One of these characteristics is the
velocity of the stream, that is, the speed the water is traveling relative to a stationary point on the bank, or
above the water. The velocity of a stream is greater near the channel, or deepest part of the stream, and
slower near the banks. A bend in the stream will accelerate the water toward the outside of the turn of the
water, creating increased erosion on that bank. The water is slowed on the inside of the river bend, causing
sand deposition there. Streams over the course of their length undergo a gradual slowing of their velocity
from head-to-mouth because the gradient (slope) of the land the water is flowing over decreases.
Streams are not only carrying eroded material from the headwaters but material from the banks along their
length, and from the bed (bottom) the water is flowing through. The overall effect of this erosion is for the
channel to erode downward to a base level of bedrock in a region, or to an ultimate base level, sea level. As
long as there is a gradient, the work of a stream continues.
As a stream’s profile decreases, its velocity slows, and the stream looses what is called competence.
Competence is the largest-size sediment the stream can carry. Fast-moving streams (steep gradient, near
the source) can carry large rocks, even boulders. As the stream slows with decreasing gradient, the size of
the sediment becomes smaller and smaller. So streams are sorting mechanisms, depositing large material
near the source, and small particles at the mouth. This is why we find rocky bottoms in mountains streams
and sandy and muddy (silt) beds in the slow-moving streams of flatlands.
Slower moving streams tend to form wide bends called meanders, which can themselves migrate along the
floodplain. Some meanders narrow and bend enough to jump through to an adjacent meander, forming a
cutoff.
Deposition at the mouth of a stream may be great enough to build a triangular-shaped, or fan-shaped area of
land called a delta. The Mississippi River delta has been built over several millennia as the river deposited
sediment in one portion of the region after another, with the river mouth location switching back-and-forth
when sediment forced the stream flow to find an easier path to the Gulf of Mexico.
While streams lose competence as they approach the mouth, they gain capacity, that is the amount of
material being transported by the water. This is due to the increase in the discharge of the stream, the
amount of water it carries. As tributaries and runoff empty into the stream along its profile, discharge
increases for most streams. The discharge, however, can vary greatly due to changes in runoff based on the
weather conditions, and can even be controlled by a dam. If a stream’s discharge exceeds the channel’s
capacity, a flood results.
Dams are one of several measures taken to mitigate flooding, other actions include artificial levees, artificial
cutoffs, and re-zoning of the floodplain.
Groundwater is an indispensable source of freshwater (more than half the world’s population depends on it).
Contrary to what is commonly thought, groundwater is not usually flowing in “underground rivers.” Nearly all
groundwater is stored in the pores in the soil and sediment, and cracks in the bedrock. Precipitation and
runoff can infiltrate through the soil and rock in an area, in what is called the zone of aeration. Once enough
water accumulates in the pores and cracks via this percolation, the pores and cracks become filled, in the
zone of saturation. The top of the zone of saturation is called the water table. Water tables vary with wet vs.
dry weather conditions, and with the lay of the land. Large zones of saturation that are permeable (able to
transmit water easily) are called aquifers, and are used as sources for wells. Aquifers are often composed of
sand and gravel (loose/porous material). Drawdown of the water table occurs near wells when drawing
water from the saturated zone.
Localized water tables can occur above impermeable layers (called aquitards, made of clay or rock, typically)
as water is blocked on its way downward toward the main water table. Springs can occur when aquitards
intersect the ground surface. Hot springs are caused by groundwater moving through the increasedtemperature environment deep below the surface. Cooling magma formations may also act as a source of
heat, and may result in the water reaching the boiling point. The resulting steam rushing up through the
underground caverns eroded by the water exits through an outlet hole at the surface, making a fountain of
steam and boiling water called a geyser.
In addition to caverns, erosion of rock features by groundwater may result in cavern formations, but also
sinkholes, sinking creeks, and other related formations. These features make up what is known as Karst
topography.
2.3: Glaciers, Deserts, and Wind
Ice covers a significant portion of the planet, and is of course important as an agent controlling the climate
of Earth. This ice, however, also plays an important role in geologic processes. Ice is a powerful agent
as it can move large pieces of solid Earth, much larger than can be transported by liquid water.
The vast, continent-sized areas of ice are called ice sheets. At present, there are two ice sheets, over
Antarctica, and on Greenland. The ice filling valleys in mountains terrain are called valley glaciers, or alpine
glaciers.
These glaciated areas are formed as snow accumulates of hundreds, or even thousands of years. The
snow packs under the weight of previous snows, forming ice. The ice in Antarctica is in places nearly two
miles deep, where the deepest ice is believed to be some half-million years old!
The ice in glaciers is not stationary, it moves slowly down the valleys in alpine glaciers, and from the interior
of ice sheets to the perimeter. The motion of the ice is due to plastic flow, that is, the ice is within the
glacier, when placed at the pressures found under about 50 meters or more of ice, becomes flexible enough
to move without breaking. Crevasses (cracks) in the ice occur above this depth. The flow of the glacier is
also caused by ice slipping along the rock surface underneath it, a process that causes great erosion.
The upper reaches of a glacier, where snowfall exceeds melting during most years, is called the zone of
accumulation. As the ice works its way slowly toward the base of the valley (the pace of this flow varies
greatly among glaciers) it reaches an elevation warm enough for melting of ice to exceed replenishment.
This is the zone of wastage. At the front end, or terminus of the glacier, large portions of ice may break
away. This is called calving. The overall length of a glacier may vary also, depending of changes in the
climate, which affect the budget of the glacier, that is the rate of accumulation of new ice vs. the rate of ice
loss (called ablation).
Glaciers perform the same three agents of work to alter the landscape as does running water (erosion,
transportation, and deposition), but do so in more spectacular fashion. Ice is capable of lifting much
larger pieces of rock, and much greater capacities of load, than rivers. This results in the rugged alpine
features revealed if and when the climate of a region warms sufficiently for the ice to retreat. The removal
of rock occurs through plucking (wedging & pulling away) and abrasion (grinding and scraping). At the
terminus, the eroded and transported rock is deposited as unsorted load (till). If the terminus of the
glacier does not advance or retreat for many years, the till will accumulate in a vast pile called terminal
moraine. Retreating glaciers cause changes to the landscape by deposits (called drift) in various forms
as seen in. The large ice sheets that covered much of North America and Europe during the Ice Age left
immense deposits when the ice retreated.
Lastly, the external forces that shape Earth’s land features are also at work even where liquid water and ice
are seldom present. Earth’s deserts are shaped by occasional rains, and by wind erosion. Since desert
streams are short-lived, the length of transportation of eroded material is shortened. Deserts are said to
exhibit interior drainage, so that the eroded rock material is deposited closer to its parent rock location,
often at the very base of he mountains the rock was removed from. This results in a landscape where relief
(elevation differences) are gradually diminished, as mountains are lowered and the valleys are filled with the
remains.
Wind erosion is more prevalent in desert areas due to the loose regolith (mostly sand and silt) and the
reduced coverage of vegetation. A process called deflation lowers the floor of the desert: small particles
removed from the floor leaving a pavement of coarser material. Wind can remove and transport vast
quantities of silt for thousands of miles under the right conditions. Deflation of the topsoil of the Great
Plains of the United States (the “Dust Bowl”) occurred in the 1930’s due to a severe drought and poor
agricultural practices.
Sand is also blown considerably, but its heavier size limits the distance of transportation. Sand deposits are
called dunes, which migrate across deserts and onto their surrounding perimeter. Dunes have a variety of
shapes due to different textures of sand, and variations of wind speed and direction.
Part Three: Internal Processes (Earthquakes, Igneous Activity, Plate Tectonics, and Mountain Building)
3.1: Earthquakes
Earthquakes are the result of a buildup of stress in rock layers resulting in fracture and slippage of the rock
layers into a stress-relieving position. The stress upon the rock is due to the forces within the earth acting to
push crust vertically or horizontally, due to tectonic processes. The rock layers are able to accommodate the
stress up to a point, but if that point is exceeded, a fracture occurs called a fault. The rock then undergoes a
slippage along the fault to relieve the stress. Elastic energy is suddenly released in the form of seismic
waves. The vibrations continue until the rock layers find a position that is stable enough to lock together and
the earthquake then ends, although several aftershocks, as they are known, may occur while this newly
stress-relieved position is being achieved. Most earthquakes are slippages along preexisting faults, as the
stress within the crust along the fault begins to build again as soon as the previous displacement and shocks
have ended.
Seismic waves propagate outward from the focus of the earthquake. The point directly above the focus of the
quake is called the epicenter. Seismic waves shake the surface rock and soil, and in the case of strong
earthquakes, can cause vast amounts of destruction and many casualties. The seismic waves also travel
within the body of the Earth, and despite having their energy dissipated over distance, and by volume and
density differences inside the planet, they can be detected by sensitive instruments called seismographs. The
body waves are of two types, primary (P) waves, that are longitudinal (waves of compression, like sound
waves), and the secondary (S) waves, that are oscillating waves akin to waves on water. P waves travel
faster than S waves, and the difference in their velocities allows seismologists to calculate the distance to an
epicenter. With at least three seismographs, the location of the epicenter can be found. The size of the
earthquake can be determined by examining damage caused by the shaking, using the Mercalli scale, but this
is rather subjective, and dependent on the quake affecting inhabited areas, the stability of the structures in the
area, and the nature of the surface soils and bedrock. A more consistent and widely used analysis is to
calculate the magnitude of the quake, which is the energy released in the event. The most commonly
reported scale of magnitude is the Richter scale, which uses the amplitude of the seismograph wave traces.
Another is called the moment magnitude. It is calculated using field observations of displacement along the
fault, and data about long-period seismic waves.
The destruction form earthquakes can vary greatly even between quakes of the same magnitude. Such
factors as the integrity of the structures in the area, the duration of the quake, and the nature of the surface
materials upon which structures are built are important. Other destructive agents are soil liquefaction (the
mixing of water with soil particles during the shaking, turning the soil into a fluid, like quicksand), landslides,
tsunamis (sea waves produced by underwater crustal displacement), and fire.
Despite their destructive nature, seismic waves have been beneficial to seismologists and geologists, as they
have provided clues as to the internal structure and composition of Earth below the crust.
3.2: Plate Tectonics
The theory of plate tectonics is one of the most revolutionary ideas in science; and many branches of science
have been applied in the important roles of testing and refining it. In short, the theory states that the crust of
Earth, the continents and ocean basins, are divided into sections (plates) that are not fixed, but are slowly
moving. The theory has helped to explain many of Earth’s features and processes, such as the pattern of
earthquakes and volcanic activity on the globe. Some plates are separating, creating ocean basins; some are
colliding, causing mountain ranges to form. The boundaries between plates are regions where great dynamic
activity occurs, including the creation of new ocean floor, the plunging of old seafloor into the mantle. Other
boundaries represent great faults in the crust which are grinding against one another.
The theory began with a hypothesis by the German meteorologist Alfred Wegener. In 1915, he proposed the
Earth’s continents had once been joined together, much like pieces of a jigsaw puzzle can be rejoined, into a
“supercontinent,” around 200 million years ago (MYA). He called this landmass Pangea (“all land’). To
support the hypothesis, he cited fossil evidence, located mountain and rock formations that appeared to have
be formed as one but are now separated, and noted clues to ancient climate conditions. But because
Wegener was unable to provide a mechanism with enough force to move continents over what most geologist
felt was a mostly rigid planetary surface, he came under intense criticism and his hypothesis was largely
discredited. He died seeking more climate data in Greenland in 1930, long before his hypothesis would be
confirmed.
Today we have a wide body of evidence that the continents were joined into Pangea as Wegener had
surmised, but we have an even more comprehensive model of Earth’s dynamism than Wegener had
proposed. It is the theory of plate tectonics, which gained wide acceptance beginning in the 1960’s. It states
that Earth’s crust is divided into several sections, called plates, which move about upon the partially molten
mantle below. Where the plates are in contact, at plate boundaries, great geologic activity is taking place.
There are three types of plate boundary:
• Divergent boundaries, where plates are separating, and magma from the mantle below wells upward
into the gap. This creates new seafloor, which begins to move away from the boundary on each side in what
is called seafloor spreading.
• Convergent boundaries, where plates are in collision. Usually this results in one plate being pushed
under the other, called subduction. The result is for the subducted plate to descend into the mantle, thereby
causing crustal consumption. A few plates are marked by the collision of continental crust creating a
mountain system.
• Transform fault boundaries, where two plates are moving parallel to each other in opposite directions,
grinding against one another. No crust is consumed or created with this type, but the displacement results in
earthquake activity.
Geologists had long noticed that earthquake and volcano activity were found in zones across the planet, but
the reason would not come until the development of plate tectonic theory. The theory explains many other
observations about Earth’s geosphere, such as why deep ocean trenches occur, and why the age of the
oceanic crust is much younger than the crust of the continents. It provides an explanation for the increased
age of the volcanic rock of Hawaiian Island chain (and the Emperor Seamounts) with distance from a “hot
spot” presently under the island of Hawaii. There is much uncertainty, however, concerning what mechanism
is driving the motion of the plates. It is thought that the partially molten rock of the mantle is moving in some
type of convective flow, that is, the kind of heat flow where density differences within a fluid cause a
circulation to develop.
3.3: Igneous Activity
The upper part of Earth’s mantle, the asthenosphere, is the source of the molten rock material, or magma,
that can rise from below into the Earth’s crust, where it cools and crystallizes into igneous rock. If the magma
reaches the surface, volcanic activity results. This is known as extrusive igneous activity, best recognizable
to us in the spectacle of a volcanic eruption. However most igneous activity occurs below the surface to
create intrusive igneous formations, bodies of rock which may become exposed if the crust in which they are
implanted is eventually weathered away.
Volcanic eruptions may be violent and explosive, such as the eruption that exploded from Mt. St. Helens in
Washington State in 1980. Other eruptions are marked by more gentle flows of lava, such as those
commonly seen from Kilauea in Hawaii. And the shape and size of volcanoes varies considerably. These
differences are in large part due to the composition of the magma extruding (coming from) the upper mantle.
The key ingredient in the mixture of minerals and gases that is magma is silica (SiO2). Silica affects the
viscosity (thickness) of the magma; the greater the silica content, the lower the viscosity (greater thickness).
Viscous magma has a more difficult time moving from the source to the surface. Pressures from dissolved
gases (volatiles) in the magma build to a point where the molten rock is ejected explosively. The material
produced from these explosions is called pyroclastic material. The blast can pulverize the rock into volcanic
ash, with larger pyroclasts (such as cinders and volcanic bombs) falling near the conduit, piling up to build the
slopes of the volcano. The most viscous magmas result in a high degree of pyroclastic material, which in turn
results in a volcano with steep slopes, called a cinder cone. The magma associated with cinder cones has
silica content generally around 70%. Cinder cones are the smallest type of volcano.
A lower-silica magma (around 50%) is often fluid enough to flow gently from the volcano, with lava pouring
over the surface and spreading out in sheets, or streams. Volatile gases are able to escape more easily with
this type of magma, but still may propel some of the molten rock upward in occasional fountains. Lava may
also flow from the flanks of the volcano from cracks (fissure eruptions). The spreading-out of lava of this type
produces a broad, large mountain called a shield volcano (as they resemble the broad-domed shape of a
warrior’s shield), the type seen on the island of Hawaii. Shield volcanoes are the largest volcanic mountain.
Many of the world’s volcanoes are of a third type, the stratovolcano, or composite cone as it is also known.
The composite cone is an intermediate size between the cinder cone and shield types), and is shaped with an
intermediate slope. They are the picturesque conical mountains exemplified by the Cascade Range
volcanoes such as Mt. Shasta, or by Mt. Fuji in Japan. The lava of these volcanoes is, not coincidentally, of
intermediate silica content (~60%) and the eruptions associated with this type are marked with both
pyroclastic events and flows of lava, sometimes alternating between the two, or occurring simultaneously.
Along with the formation of volcanoes, extrusive igneous activity is responsible for other significant geologic
events. One is the fissure eruption, when lava flows form a large cracks in the crust. An especially large
series of flows of this type spewed lava over much of the Pacific Northwest millions of years ago. Another
event is the caldera collapse, when the magma chamber beneath a volcano collapses inward due to loss of
magma within it, after a series of eruptions. This type of event caused the formation of Crater Lake in
Oregon.
While volcanoes offer us a view of igneous activity while it takes place, most igneous activity is intrusive, that
is, below the surface. Magma intrudes into existing rock layers and begins a slow cooling process resulting in
a variety of igneous rock formations called plutons. The main magma chamber forms a massive batholith,
and smaller projections cut through the existing rock to form dikes (diagonal features) and sills (horizontal).
Small domes called laccoliths are another type of pluton. These formations may become visible (surface)
formations if the surrounding rock is removed through weathering, usually after the region experiences uplift.
The location of volcanoes around the planet is explained through the theory of plate tectonics. The
distribution of volcanoes around the Pacific Rim (the “Ring of Fire”), for example, is due to the convergent
plate boundaries that surround much of the Pacific Plate. The Hawaiian Islands, on the other hand, are an
example of intraplate volcanism: magma rising into a “hot spot” under the plate.
3.4: Mountain Building
Mountains provide sightseeing spectacle for most of us, but to the geologist, they tell of the immense forces at
work inside Earth. Earth’s crust is in a constant state of change. External processes such as weathering are
constantly working to lower rock material from higher elevations to lower ones, that is to “wash mountains
down to the sea.” The Appalachian Mountains, for example, were once about the height of today’s Rocky
Mountain range, but over 300 million years of weathering and erosion have lowered them considerably. Why
not then the Rockies, or the even higher and more rugged Himalayas? Because they are younger mountains,
indicating that forces continue to build new mountains while older ones are lowered.
We have already seen how one type of mountain range, the volcanic arc, is created. The collision of an
oceanic crustal plate with another, or with a continental plate, results in the subduction and melting of the
crust as it enters the upper mantle. But not all mountain systems are caused this way.
The forces that move continental plates are in turn placing enormous stress on the rock of the plate. The
rock, when placed under sufficient stress, will begin to deform, that is to change from its original shape. The
rock will fold, bend, flow, or fracture, depending on the nature and strength of the stress, the composition of
the rock material. Researchers in the laboratory have been able to better understand the processes of rock
deformation. Experiments have shown that most rock can undergo elastic deformation. The rock can deform
and then return to its original shape if the stress is below what is called the elastic limit. If the limit is
exceeded the rock will fracture (brittle deformation), or will undergo a solid-state flow called ductile
deformation. High temperature and great pressures aid ductile deformation. The length of time the stress is
applied affects the deformation, although since very long (geologic) time spans are usually involved, this
factor cannot be added into the experimental studies.
If you take a piece of paper and push in on the ends, the paper folds into undulations (waves). Rock layers
can be deformed into folds in a similar fashion, by compressional forces. The arched (upward) parts of the
folds are called anticlines, and the troughs (downfolds) are termed synclines. Monoclines occur when strata
are deformed by fault displacement underneath the strata.
Broad upwarps in crustal rocks can produce domes, such as the Black Hills (Paha Sapa) of South Dakota.
Downwarping in a region can produce the converse formation, the basin. Several basins exist in the U.S.,
including the Permian Basin in Texas and New Mexico, and the Michigan Basin. Due to the erosion of the
slanted rocks in these formations, the basins are marked by a roughly concentric pattern with the oldest rock
in the centers of domes (and the toungest rocks on the outside flanks), whereas the opposite pattern is found
in basin formations.
Another type of deformation is fracturing of the rock. The fracture produces displacement of the rock on
either side of cracks called faults, which can be only a few meters in size, or hundreds of kilometers long.
Types of faults are classified depending on the nature of the displacement: vertical displacement due to
compressional or tensional forces results in a dip-slip fault, whereas horizontal displacements along the fault
from shear (rock moving parallel to, but in opposite directions relative to, each other) are strike-slip faults.
Fracturing of rock layers may occur without appreciable displacement, causing cracks to exist in the rock
layers called joints. Joints allow for water seepage and therefore increase the rate of weathering in the rock.
Orogenesis is the term applied to the processes resulting in the building of mountain belts. Geologists have
begun to understand the immense forces involved, and have been aided in their understanding by
determining when various mountain systems were built. Younger (more recent) belts include the mountains
of the western North America, and the Himalayas. Older mountain systems include the Appalachians of
eastern North America, and the Urals in Asia.
We have already seen some of the processes involved in mountain building with the formation of volcanic and
island arcs at subduction zones (convergent plate boundaries). Another mountain building processes found
at convergent plate boundaries is the development and uplift of an accretionary wedge. This is caused as
rock is “scraped” from the colliding plates and begins to accumulate, and may with time increase in size
sufficiently to rise above sea level. These two processes combined create the Andean-Type continental
margin, so-named as the Andes of South America are an excellent example of this type of mountain belt.
They consist of the volcanic arc on the continental block, and the roughly parallel belt of mountains of the
accretionary wedge at the contact point of the plates at the continental margin. Another example of this type
are the Sierra Nevada and Coast Ranges of the western United States.
Another type of mountain belt are the collisional mountains. These are due to accretion of crustal materials
onto continental landmasses in what are called terranes. A terrane can consist of an accreted volcanic island
arc, a microcontinent (islands of continental crust, similar to the present-day island of Madagascar), or thick
sections (plateaus) of oceanic crust that have been “scraped” from the subducting ocean crust and forced into
a continental margin. The collision and accretion of terranes has happened along ancient subduction zones
along the Eastern Seaboard of North America to form the Blue Ridge Mountains, the Piedmont region, and
the Valley and Ridge Province. The North American Cordillera was formed much more recently, as the
continental crust of the North American Plate collected terranes through its westward motion against the
Pacific Plate. Orogeny also results when whole continents can collide, as exemplified by the Himalayan
mountains. They are in their infancy, having begun their building only about 45 million years ago, as India
collided with the Asia.
Mountains not only form due to the compression of crust, they can form by way of tensional forces
(stretching). Fault-block mountain systems form this way. Crust can be elongated though continental rifting,
or broad uplifting of crust. The crust is faulted, and the blocks so formed tilt to give rise to roughly parallel
ridges. Examples of this type are the Tetons of Wyoming and the mountains of the Basin and Range
Province of the western United States.
Plate tectonic movement causes much of the crustal deformation resulting in mountain building. However,
vertical movement of continental crust (gradual “bobbing up-and-down”) takes place in many regions of the
planet. This is due to the fact that continental crust is less dense than the material of the mantle. The
continents are essentially “floating” on the mantle. The buoyancy of crustal rock is called isostacy. Increasing
the mass upon the crust of a region, say by the accumulation of sediment or by the formation of ice sheets
can cause the crust to sink. Removal of mass (from ice retreating for example) cause rebound. These
response of the crust to move vertically to changes in the weight upon it is called isostatic adjustment.
Part Four: Earth’s History
4.1: Geologic Time
The age of the earth was a great mystery to early scientists, but beginning in the late 1700’s, with the
landmark work by James Hutton “Theory of the Earth,” geologists began to unravel many of the mysteries of
Earth’s past using scientific principles and methods. Before these first true geologists, it had been widely
believed that Earth’s features, such as mountains and canyons, were the result of huge and sudden
catastrophies. The unknown causes these cataclysms were thought to have ceased to operate, that the
features of the planet were static apart from minor effects like weathering, or an occasional rockfall. This
notion is called the doctrine of catastrophism.
Hutton ushered in the principle of uniformitarianism. It states that the processes that influence geologic
features in the present are the same as those that have been at work throughout geologic history. The pace
of most geologic processes is so slow we simply cannot see the actions taking place. Many geologic features
have taken tens, even hundreds of millions of years.
One of the most vexing problems for early geologists was determining the age of the Earth. Various ages
were proposed, but, even through the nineteenth century into the early 1900s, none would prove close to
correct. An accurate age would eventually require an understanding of radioactive decay, which we will
examine shorly. First, we look at the principles used by geologists to place rock formations in the order which
they formed. This is called relative dating.
One such principle is known as the law of superposition, which states, simply, that strata of sedimentary rock
are older with depth. This is why the Grand Canyon is of such significance to geologists, as it is an easily
viewable example of how layers of sediment are laid down one atop the other. The oldest layers of rock, in
the bottom of the canyon, are nearly 2 billion years old, the uppermost (at the rim) are about 270 million years
old. The layers in between hold clues to processes and environments during the vast time line in between
(although parts of the timeline are missing, as we shall see).
Other principles used in relative dating are the principle of original horizontality, which sates that layers of
sedimentary rock form in horizontal, flat strata; any tilting in such rock must be due to disturbances that came
after the sediment was laid down. The principle of cross-cutting relationships states that intrusive igneous
formations and faults must be younger than the layers of surrounding rock through which they cut.
Other features that play a role in relative dating methods:
• Inclusions: pieces of one rock layer embedded in another. The mass of rock providing the inclusions
must be older
• Unconformities, formed when deposition ceased, followed by a period of erosion, then another period
of deposition after that. This results in some of the rock record being lost. They occur during significant
geologic events that caused the rock layers to be lifted above sea level, followed by subsidence and new
sedimentation. Unconformities are further divided into:
o angular unconformities (erosion of disturbed strata)
o disconformities (erosion of undisturbed strata), and
o nonconformities (breaks with sedimentary rock overlying metamorphic or igneous rock)
By the process of correlation, the information in the rock record in a local formation can be matched up with a
larger region of rock layers, such as has been done with the rock layers in the Colorado Plateau. This
correlation can build a much more comprehensive view of geologic events on a large scale.
Fossils in the rock layers play a dual role. They provide paleontologists (scientists who combine biology with
geology) with clues about ancient life forms and the environments they lived in. Fossils also help to correlate
the rock record between widely separate regions, or separate continents. The principle of fossil succession is
critical: fossils organisms succeed each other in a recognizable order throughout time. Fossils that are
widespread and lived in a short span of geologic time are especially helpful time markers, and are called
index fossils.
While relative dating has provided information regarding the order of geologic events, numerical dates can be
obtained by analyzing radioactive decay of atoms with a rock sample. Certain kinds of elements (radioactive
isotopes) undergo a transition whereby the nuclei of the atoms in the isotope break apart. This is known as
radioactivity, and we say the original nucleus is the parent isotope, and the isotopes resulting from the decay
are called daughter products. The key to radiometric dating is the fact the rate of decay for radioactive
(parent) isotopes can be determined with great accuracy. The proportion of daughter products to the
remaining parent isotope determines how long the sample has been undergoing decay (i.e. how long since
the sample crystallized from magma). The time required for half of the parent nuclei in a sample to decay into
daughter products is called the half-life. The radioactive isotopes commonly used in radiometric dating
include uranium-238, rubidium-87, and potassium-40. Carbon-14 is useful for more recent samples (younger
than 75,000 years or so).
4.2: Earth’s History: A Brief Summary
Because of the vast amount of time that has transpired since the formation of Earth approximately 4.5 billion
years ago, a geologic time scale has been developed. It is divided and subdivided for convenience into eons,
eras, periods, and in the case of the most recent spans of the time line, epochs.
The Precambrian time, the portion of the time line from 4.5 billion years ago (BYA) to about 540 million years
ago (MYA) represents 88% of Earth’s history. Not much information can be gathered about this period, as the
dynamic processes that take place on Earth have obscured the rock record. There are very few Precambrian
fossils as well. This is a summary of what is known about the Precambrian:
• A few core areas of the present continents contain Precambrian rocks in what are called
“shields.”
• The primordial atmosphere was largely devoid of oxygen at first, and was mainly the result of
volcanic outgassing. Water vapor, nitrogen, and carbon dioxide were major constituents.
• Condensing water vapor lead to the formation of Earth’s first oceans.
• Early plants began to produce a gradual increase in oxygen
• The few fossils available from this time include microfossils (bacteria, algae) and stromatolites
deposited by algae.
• The extent and positions of landmasses is speculative.
The Paleozoic era (540 MYA to 248 MYA) begins where the rock record of Earth begins to show many more
fossils, as there was an “explosion of life” on the planet. The presense of the first animals with solid body parts,
such as calcium shells, make the fossil record’s first chapters possible, along with the more abundant rock
formations of this era as opposed to the Precambrian. Here is a summary of the Paleozoic era:
• There is a dramatic diversification of life forms, with only aquatic life at first (invertebrates such as the
trilobites, then fish), followed by land plants, insects, amphibians, then reptiles.
•
Vast swamps cover much of the land, which will become our present-day coal beds.
•
Plate movements bring together the landmasses of the era into two groups called Laurasia and
Gondwanaland. At the close of the era, these two groups will join to form the supercontinent of Pangea.
• At the end of the Paleozoic, a mass extinction of unknown cause takes place. Over 80% of the marine
species perished. The climate and sea level changes brought about by the formation of Pangea may have
played a role.
The extinction at the close of the Paleozoic (248 MYA) made for a great enough change in the fossil record that
is was decided that a new era be marked on the geologic time line: the Mesozoic era. Here is a summary of
that era, which will last until 65 MYA (when another mass extinction occurs):
• The surviving species diversify, and are affected by drier climate, with more land available due to the
existence of Pangea.
• Reptiles, including the dinosaurs thrive, becoming the dominant animals on land. The were suited to
the changing to a drier climate at this time, their ability to lay (shelled) eggs on land being a critical factor. The
Mesozoic is also called the “Age of Reptiles,” or the “Age of the Dinosaurs.”
•
The first birds appear, also large seed-bearing trees, and first flowering plants
• Pangea splits into what will become the present continents, and the breakup forms the Atlantic Ocean.
The North American plate colliding with the Pacific plate will produce many of the igneous formations found in
the western part of North America
•
This era ends with another mass extinction (probably due to the impact of an asteroid).
The present era, the Cenozoic era, started with the demise of the dinosaurs 65 MYA. [Perhaps it will end with
another mass extinction!] Here are some of the key events:
• Mammals become the dominant land animals (including humans). This era is also called the “Age of
Mammals.”
• Flowering plants develop, which in turn affects the evolution of birds and mammals. Grasses develop,
bringing about the emergence of grazing, plant-eating mammals.
• The continents assume their preset orientation. The western margin of North America continues its
tectonic activity, while the passive margin of eastern North America undergoes extensive sedimentation and
isostatic adjustment which shape the Appalachian Mountains to the slopes we see today. The building of the
Rocky Mountains ends, sediment from their erosion forms the Great Plains.
Part Five: The Oceans
5.1: The Ocean Floor
Earth oceans are geographically divided into the Pacific Ocean, the largest; the Atlantic, which is about half
as large; the Indian Ocean (nearly the same size as the Atlantic); and the Arctic, the smallest and shallowest,
and is covered in large part with ice. Not only are the oceans the dominant feature from a geographical
standpoint, they are also significant in their overall depth. The average height of the continents is about 840
meters above sea level, but the average depth of the seafloor is about 3730 meters.
The seafloor was for centuries steeped in mystery, and as was pointed out above, there is still much to be
discovered. Mapping of the ocean bottom was first undertaken by the H.M.S. Challenger expedition in the
1870’s. The laying of trans-oceanic cables gave some clues as well. Not until sonar (the use of sound waves
reflecting off the seafloor) was any detail available. But despite decades of sonar mapping, only about 5
percent of the seafloor has been mapped in detail.
Oceanographers have noted three basic types of topographic feature on the seafloor: the continental
margins, the ocean basin floor, and the mid-oceanic ridge.
Continental margins are considered either passive or active: the difference is whether the margin is along a
convergent plate boundary where subduction of oceanic lithosphere is occurring. If so, it is considered active,
and is characterized by a steep plunging from the continental shelf (the portion of the continent submerged by
shallow ocean) down to the ocean basin floor, where nearby, a deep-ocean trench is found. The sediments
and crustal rock of this type of margin, being forced into the subduction zone, become highly deformed and
broken. This material is being compressed into an accretionary wedge. The passive margin is marked by
more tranquil geologic processes, particularly the gradual accumulation of mostly undisturbed sediment,
much of which is transported and deposited from the continent by stream runoff. The passive margin has a
less-steep plunge to the ocean basin known as the continental slope. Here, submarine canyons are found,
believed to be eroded by what are called turbidity currents, downward flows of sediment-heavy water.
The ocean basin floor is mostly an extremely flat surface, called the abyssal plain. It is dotted with seamounts
(underwater volcanic “peaks”), guyots (submerged peaks that have had their tops eroded, also called
tablemounts), and oceanic plateaus (analogous to flood basalts on the continents). The flatness of the
abyssal plain is in large part due to the thick layers of accumulated sediment which “smoothes over” the
irregularities of the crustal rock.
Oceanic ridges are the result of divergent plate boundaries, where magma rises into the rifting crust to create
fresh igneous rock. The ridges are generally 1000 to 4000 kilometers wide, and typically have crests of 2 to 3
kilometers above the adjacent ocean basin floor. The oceanic ridges of the planet are often near the middle
of ocean basins, and most of the ridges are interconnected (like the seam of a baseball) making them the
longest topographic feature on Earth, some 70,000 kilometers (43,000 miles) long. Along their center-lines,
the ridges often feature a deep rift valley, a feature also found with the only continental divergent boundary,
the East African Rift. Rift valleys are flanked with rugged volcanic structures produced by the upwelling
magma. The Mid-Atlantic Ridge has been the most examined by oceanographers. It has, for most of its
length, volcanic structures nearly 3000 meters above the floor, and has a rift valley as much as 30 km wide.
At a few points, most notably Iceland, the ridge has grown above sea level. Extensive transform faulting, and
associated earthquakes, are another characteristic of the oceanic ridges. The high temperatures imparted by
the shallow magma to the pressurized water in the deep rift zones create hydrothermal vents.
The sediment covering the seafloor (typically to depths of 500 to 1000 meters) has three sources:
•
Terrigenous sediment comes form weathered grains of continental rocks that have been transported
via stream runoff, wind erosion, and glacial drift, if it reaches the ocean shores.
•
Biogenous sediment is made up of the shells and skeletons of small marine animals and algae that
have died and drifted to the bottom. Sampling of this type of sediment provides clues to ancient
climate as the types and numbers of these small organisms is dependent on climatic conditions.
•
Hydrogenous sediment consists of minerals that have crystallized from the seawater due to various
chemical reactions. This type of sediment can provide a resource of metals and salts
Other resources found on and below the seafloor include a sizeable fraction (about 30%) of the world’s oil and
natural gas supply, and sand and gravel deposits that are dredged up from the bottom (with a few areas having
deposits containing precious gems and metals). Of future potential are manganese nodules and gas hydrates.
Increasingly, seawater itself is becoming a valuable resource, that of drinking water, once the salt and impurities
are removed and filtered. Removal of salt from seawater is called desalination. Due to the increasing difficulty of
finding new supplies of freshwater as the world’s population increases, more and more countries and
communities are looking toward desalination to supply drinking water.
5.2: Ocean Water and Ocean Life
Seawater is also called saltwater, as it contains on average about 35 grams of various salts per kilogram of water.
This concentration (about 3.5%, or 35 parts per thousand) is called the salinity of the seawater. If we evaporate
seawater, we would find that almost all of the is salt sodium chloride (table salt), followed by magnesium chloride,
sodium sulfate, calcium chloride, and traces of several other salts.
These dissolved materials come from two sources: weathered minerals from the continental rocks, and from
volcanic outgassing. The salinity of seawater varies from place to place in the oceans. It is affected by several
processes that effectively add or remove the water portion of the seawater mix. Processes that increase the
amount of water (thereby reducing the salt concentration, i.e. the salinity) include precipitation, runoff from land,
and sea ice melting. Conversely, the salinity is increased by evaporation, or by the formation of sea ice (the salt
in seawater does not become part of the ice upon freezing). Salinity in most of the oceans varies from about 33
to 38 parts per thousand (3.3% to 3.8%). Salinity in the Red Sea and Persian Gulf, for example, where
evaporation exceeds runoff and precipitation, can exceed 40 parts per thousand
The surface temperature of Earth’s oceans varies considerably, particularly with latitude. The surface
temperature is governed by the amount of solar radiation striking the surface, which in turn is a function of
latitude. This is the same principle that affects temperatures on land.
Temperature lowers with depth in most of the ocean, with the exception being at the higher latitudes. The surface
temperature in most of the ocean falls steadily (due to a decrease in penetrating sunlight) until a depth of around
1000 meters, then remains relatively constant (just a few degrees above freezing) below that level, down to the
ocean floor. This temperature change with depth is called the thermocline, and is noted primarily at lower
latitudes, and in mid-latitude waters during the warm seasons. Because the surface temperatures near the pole
are already just above freezing, there is no significant thermocline in the high latitudes, although during summer
months a slight thermocline may exist.
Numerous studies of the temperatures of Earth’s oceans are being conducted, as these temperatures are a
critical factor in the stability of marine life, and they play a key role in the weather and climate on Earth. The
nature of these relationships is complex and not understood to a degree that oceanographers, marine biologists,
and climatologists can predict how changes in ocean temperatures might affect the other spheres of the Earth
system. One set of predictions, from the 2007 report of the Intergovernment Panel on Climate Change, forecasts
temperatures to rise between 1.8° to 4° C (about 3.5° to 8° F), and sea levels to rise between 18 to 59 cm (7 to 23
inches) by the end of this century.
The ocean can be given a layered structure (except in the high latitudes) based on the variance of density (mass
per unit volume) of the seawater with depth [the ocean is also structured in another way by oceanographers,
based on the marine life found at various depths]. The largest factor affecting the density of seawater is its
temperature, with salinity having a smaller effect. Much as in the thermocline of the low latitudes, the pycnocline
(change of density with depth) increases as it is measured from the surface down to about 1000 meters. This is
due to the temperature falling through that depth, and a decrease in temperature increases the density of the
water. Below this depth, as we have seen, the temperature remains fairly constant, so the density stays nearly
constant as well. So, as was the case with the thermocline, there is no appreciable pycnocline in the high
latitudes as the water temperature there (through most of the seasons) is roughly the same at all depths. The
uppermost layer, down to a few hundred meters generally, is called the surface-mixed zone, where the
temperature is fairly uniform due to the mixing of waves, currents, and turbulence. Below this is the transition
zone, or pycnocline zone, where the temperatures begin to fall with the thermocline, until the deep zone is
reached.
5.3: The Dynamic Ocean
The waters of Earth’s oceans are in constant motion. In fact, the power of the ocean waves and currents is
now being tapped as a source of energy in many countries. The currents moving at the surface of the ocean
are driven by the frictional force of the wind blowing over the water. Here, then, we have an interface
between two spheres, the hydrosphere and atmosphere. Large scale currents, thousands of miles long, can
be identified in the oceans. The currents in fact move in vast circles called gyres. The gyres tend to turn
clockwise in the oceans of the Northern Hemisphere, and counterclockwise in the southern oceans. The
reason for this rotational tendency is the Coriolis effect, which is a deflection imparted to moving objects due
to Earth’s rotation. The Coriolis effect plays a role in the motion of the atmosphere as well.
One of the best-known currents, the Gulf Stream along the east coast of North America, is part of the North
Atlantic gyre. It is responsible for moving warm waters from the Gulf of Mexico and the Caribbean northward
toward New England and the Canadian Maritimes. The gyre continues to move the warm water across the
Atlantic toward northern Europe, making the wintertime temperatures there warmer than would be expected
for that latitude. Other currents have a critical role in the climates of much of the world’s maritime regions.
From the Earth-system perspective, ocean currents are important in maintaining Earth’s heat balance by
moving warm waters from the equatorial latitudes northward, where they cool at higher latitudes.
Ocean currents can produce vertical movement of the water, most notably by what is called upwelling.
Coastal upwelling occurs when winds and currents move toward the equator and parallel to coasts. This
forces the water away from the continental margin, which is replaced in part by colder water welling up from
the depths.
Circulation of water in the deep ocean also occurs, by way of slight density differences, often caused by sea
ice formation (which increases salinity) in the high latitudes. This is called thermohaline circulation. Vertical
movement of water between the deep ocean and the surface currents has created a global “conveyer belt” of
water exchanging heat in the oceans and with the atmosphere, and providing a mechanism for moving
nutrient-rich deep water to shallower depths.
The movement of water at the surface is also marked by waves, which transfer energy from winds and from
large storms along the surface, sometimes for thousands of kilometers. This energy is transferred to the
shoreline, and is responsible for shaping the landforms on the coasts of the planet. Waves create a circular
motion to the molecules of water. The distance between two crests (or two troughs) of the waveform is called
the wavelength. The vertical distance from the bottom of the trough to the crest is the wave height. The time
between the arrival of each crest is the wave period. This is also the time between each oscillation. Waves
increase in height as they approach the shoreline, since the energy carried in the wave is imparted to a
smaller and smaller (shallower) volume of water.
Waves reaching the shore result in erosion through impact and abrasion. Coastal features created by wave
action include wave-cut cliffs and platforms, and marine terraces. Waves bending (refracting) around
headlands (protrusions of the coast) create features called sea stacks and arches. Waves reaching the shore
at an angle (obliquely) can transport sand on the beach down the coast in what is called beach drift, which
can deplete a beach of its sand over time. Oblique waves can cause a flow of water parallel to the shore
called a longshore current which can also transport a large amount of sand down the coast. The removal of
beach sand is a major problem for residents of coastal area, requiring expensive mitigation and correction
measures (akin to flood prevention measures) to stabilize the shore.
In coastal areas where wave energy is diminished, deposition of sand, gravel, pebbles, and debris occurs. So
as the ocean can carve and reduce features on the shore, the ocean is also responsible for depositional
features such as barrier islands, spits, bars, and tombolos.
The features found on a particular coast are in large part due to whether the coast is emergent, or
submergent. Emergent coasts are undergoing lift due to tectonic processes, or due to a lowering of sea level.
This exposes new rock to wave erosion resulting in wave-cut platforms, such as those exhibited on the
California coast, which is being uplifted as a result of the tectonic activity there. The Atlantic coast, by
contrast, is submergent. It is part of the passive continental margin of North America, and is lowering due to
the accumulation of sediment eroding from the continent, and due to rising sea level.
Oceans are affected by the gravitational pull of the Moon, and to a lesser extent, the Sun. This gravitational
pull produces a bulge of the water on each side of the Earth. This bulging results in a higher sea level along
the gravitational axis, and the effect is made more significant by local and regional coastal characteristics.
Part Six: The Atmosphere
6.1: Atmospheric Composition, Structure, and Temperature
Most of the air we breathe is not oxygen, it is mostly nitrogen (78%), then oxygen is second in abundance at
21%, followed in a distant third by argon (0.93%). Trace gases, including carbon dioxide, make up the
remainder. This mixture is remarkably consistent throughout the latitudes of the planet, and up to a height of
80 to 100 km (50-65 miles). There are variable components in the mixture, however, most notably water
vapor, ozone, and aerosols (fine particles such as smoke, dust, and sea salt).
Earth’s atmosphere begins at the surface, of course, but how deep is the atmosphere? Or in other words,
how high up is the top of the atmosphere? The answer: the atmosphere has no defined upper limit, the
molecules of the atmosphere get thinner with altitude until there are too few molecules to detect, where we
are then said to be in “outer space.” This thinning of the density of air molecules with height is due to the
reduction of atmospheric pressure with altitude. Just as pressure increases with depth in the ocean (due to
the increased weight of water above increases as one dives deeper), or decreases as one rises back upward,
the pressure of air decreases as one rises higher through the “ocean” of air. Atmospheric pressure is simply
the weight of the air above the location where it is measured, per unit area. It is also called barometric
pressure, as the device used to measure this pressure is called a barometer. Typical atmospheric pressure
at sea level is about 14.7 pounds per square inch, but meteorologists use units of pressure called millibars,
and this value translates to just over 1000 millibars. This is also about 29.9 inches of mercury (still used on
weather broadcasts, based on old-style mercury barometers). Regardless of how pressure is measured,
however, it is important to note that the pressure decreases with height (altitude) and that slight differences in
the pressure from place-to-place on Earth’s surface are responsible for winds.
Since atmospheric pressure falls off steadily with height, it is an inconvenient basis to give any structure to
the atmosphere. We can, however, divide the atmosphere into four layers based on temperature changes
with altitude. The layers are the troposphere, stratosphere, mesosphere, and thermosphere:
•
The troposphere (surface up to about 10 to 12 km, i.e. about 7 miles) is the lowest layer and is the
realm of Earth’s weather phenomena. It is marked with a decrease in temperature with altitude,
something you may have noticed if you have climbed a mountain. This temperature decrease is
known as the environmental lapse rate, and averages about 6.5 degrees per kilometer (3.5° per
1000 ft), but varies considerably with weather conditions, and in turn it has a great effect on weather
phenomena. So meteorologists send instruments upward through the troposphere frequently to
determine the environmental lapse rate at a given time and place to help their predictions. The top of
the troposphere is called the tropopause.
•
The stratosphere (10-12 km up to about 50 km, or 30 miles) is marked by a reversal, or inversion, of
the lapse rate with height. In other words, the temperature in the stratosphere begins to warm with
altitude, despite the tinning of the air. The reason for this reversal is the presence of ozone in the
stratosphere, which absorbs harsh ultraviolet (UV) radiation from the Sun, converting it to heat
energy. Ozone is a form of oxygen with three atoms of oxygen in its molecule (O3), as opposed to the
common form we find in the troposphere with two atoms (O2). [Ozone does occur in small quantities
as a pollutant that forms when emissions of certain hydrocarbons and nitrogen oxides react to
sunlight. It is harmful to the respiratory system. But the ozone in the stratosphere is beneficial, as its
absorption of UV radiation reduces significantly the dosages of UV incurred by the biosphere. The
stratopause marks the top of this layer.
•
The mesosphere (about 50 km up to about 80 km, or 50 miles) is demarked by the return of a drop in
temperature with altitude, due to the lack of ozone. Here the atmosphere becomes very thin,
although enough molecules exist to impart a frictional force on incoming meteors from space causing
most of them to burn up as they move through this layer, creating the phenomenon we call “shooting
stars.”
•
After passing through the mesopause at about 80 km (50 mi), we enter the thermosphere. Here the
temperature again begins to rise with altitude, but the temperature we speak of in this instance is not
sensible warmth. The temperature in this case is measured by the average speed of the molecules
of air. The intense solar radiation striking the few molecules of nitrogen and oxygen in this layer,
cause them to speed up to velocities which represent much warmer temperatures. We would not be
able to “feel” the temperature of the air here, however: there would be too few molecules striking
your skin for a sensation of the temperature. But, from a physical standpoint, the temperature is
technically warming with altitude in the thermosphere. In fact, the molecular motion can increase to
speeds that represent temperatures over 1000° C! The intense solar bombardment in this layer can
strip electrons from the molecules, leaving positive ions and free electrons. This creates auroras, and
results in layers within the thermosphere collectively known as the ionosphere.
Many atmospheric scientists, and some texts, identify yet another layer of the atmosphere called the
exosphere, where atmospheric molecules, atoms, and ions begin to escape into outer space. This region is
estimated to begin at 500 to 1000 km high (and again has no clearly defined upper boundary).
The energy for atmospheric processes comes from the sun. Yet the amount of solar energy striking the
planet varies greatly from place to place, and this has a tremendous effect on the weather and climate around
the globe. The distribution of solar energy is affected greatly by the latitude and time of year. The tilt of
Earth’s axis results in the changing angle of the Sun’s rays (which affects the amount of energy per unit area)
and the length of daylight and darkness periods each day. As the Earth orbits the Sun through the course of
one year, these changes cycle back-and-forth to create the seasons. The temperature swings are greatest in
the middle and higher latitudes, and less pronounced in the tropics.
Once solar energy reaches the Earth, it begins to be affected by the atmosphere. Some of the sunlight (and
UV light) is scattered and reflected back into space, some is absorbed and reflected by clouds. What is left
(about 50%) strikes the earth’s land and ocean surfaces, and most of this is absorbed, but a small portion is
reflected. The amount reflected is called the albedo of the planet; the average albedo of Earth’s surface is
about 5%.
Once the solar radiation, which is in the form of visible light and ultraviolet radiation (UV), is absorbed by the
land or water, it is converted into heat, or infrared radiation (IR). Earth’s infrared (heat) radiation heads back
out toward space, but a sizeable fraction is absorbed by the atmosphere before it escapes. This absorbed
heat warms the troposphere above the temperatures that would exist if the atmosphere allowed all of the IR
heat to escape. The situation is a bit analogous to what happens inside an automobile with the windows
closed on a sunny day. The sunlight enters through the glass, is absorbed by the materials in the interior of
the car (seats, dashboard, etc.) and converted to hear (IR). The heat (IR) radiation is not allowed to penetrate
the glass and escape back to the outside, however. This is due to the physics of radiation and the nature of
glass: shorter-wavelength radiation (sunlight and UV rays) can pass through the glass, but not longerwavelength radiation like heat (IR). So the heat is trapped inside the car. This is the same situation which
allows a greenhouse to stay warm in the winter: sunlight comes in through the glass, but the heat produced
when it is absorbed by the plants and materials inside is unable to exit back outward through the glass. The
glass is said to be “opaque” to IR rays. For this reason, the absorption of outgoing heat by Earth’s
atmosphere is called the “Greenhouse Effect.” This effect has had a profound effect on warming Earth’s
atmosphere: it is believed that the global temperature would be at least 30° C colder, making the development
of life as we know it impossible.
Two gases in the atmosphere are particularly strong absorbers of the outgoing IR radiation from Earth’s
surface: water vapor and carbon dioxide (CO2). [Methane levels in the atmosphere may also play a role, as
methane (CH4) is a absorbing gas, but it exists in much lower concentrations. More studies are being
conducted concerning methane’s possible role.] These gases have been in the atmosphere for at least a
billion years, so the greenhouse effect should not be confused with the concepts behind human-induced
global warming. Global warming is the term used to describe an increase in absorptive gases in recent
decades, primarily an increase in CO2, as a result of fossil fuel burning and deforestation.
Although direct sampling of carbon dioxide (CO2) levels through the eons of prehistory is not possible, ice
cores from Antarctica do allow samples as far back as about 400,000 years ago. These show swings from
about 200 to 300 parts per million (ppm).
These deviations are thought to have had profound effects on climate throughout Earth’s history, including the
episodes of glaciation (“Ice Ages”) during these ancient times, although the causes and effects of these
variations is not fully understood. Note however the close correlation between CO2 levels and temperature
(the blue line, based on isotopes in the samples). Other factors are thought to play a role in ancient climate
change: note the dust content in the samples for example (dust can scatter more sunlight back into space).
Changes in solar radiation intensity have been observed and may play a role, along with slight variations in
Earth’s orbit around the Sun. Adding to the complications are feedback mechanisms, as were discussed
earlier.
An observatory on Mauna Loa (one of the large shield volcanoes of Hawaii, far from large sources of
pollution), has been making regular CO2 samples of the atmosphere since the late 1950’s. They show levels
of CO2 in the atmosphere greater than any of those in the 400,000 years previous, and they have been rising
steadily from 315 ppm to a present level of about 385 ppm (the oscillations from the mean are due to
seasonal effects each year: CO2 levels fall slightly when vegetation absorbs the gas during the warm
seasons in the Northern Hemisphere.)
This steady increase is believed to produce a corresponding increase in Earth’s atmospheric and oceanic
temperature in recent decades.
This is the basis of concern over the effect on Earth’s temperatures due to human activity. Much research
continues, but predictions regarding the effects of global warming made over a decade ago have been largely
borne correct, and often were too conservative. For more on the latest predictions and results of further study on
global warming, visit the website of the Intergovernmental Panel on Climate Change, at http://www.ipcc.ch/
As was discussed earlier, solar radiation received by Earth’s surface varies greatly with latitude, and with the
seasons. This in turn affects the seasonal temperature pattern found at any given location on Earth. There are
other controls on temperature from place to place. These include whether a location is near a body of water, or is
situated in a continental setting. Water tempers the swings of temperature through the seasons since water can
absorb more heat per degree of temperature rise (a property known as specific heat). Note that despite the two
cities lying at nearly the same latitude, Vancouver has less extremes in temperature than Winnipeg due to its
marine location. Other controls on temperature include altitude, and geographic position (e.g. the presence of
mountains and the direction of prevailing winds.
6.2: Moisture, Clouds, and Precipitation
Water on our planet can exist in three phases: solid (ice), liquid water, and water vapor (an invisible gas).
Water vapor is a critical element in understanding many processes in the atmosphere, including those
responsible for weather phenomena.
Water vapor, is produced by adding sufficient heat to liquid water. The heat used to change liquid water to
vapor is called the latent heat of vaporization, and is stored in the vapor until it is condensed back to liquid,
and the heat is released back into the surroundings. This heat release is important, for example, when water
vapor condenses into cloud droplets, the latent heat released warms the surrounding air, making it more
buoyant.
The air we breathe has a varying amount of water vapor. The amount of vapor in the air is called the
humidity. There are different methods of calculating the humidity, one way is to determine how many grams
of water vapor are mixed in a kilogram of air (this is about 0.8 cubic meters of air, a little more than the
volume in a typical refrigerator). The fraction of vapor using this method is called the mixing ratio of vapor.
Air at a given temperature has an upper limit to the vapor amount it can hold, a saturation point. This amount,
when expressed as a mixing ratio, is called the saturation mixing ratio. The warmer the air, the greater the
capacity of vapor it can hold. A kilogram of air at 10°C, for example, can hold a maximum of 7 grams of
vapor, and at 25°C it has a capacity of 20 grams. This fact that air has a saturation point is important in
several ways, as we shall see.
Another way to measure the vapor amount is the relative humidity, which is the fraction (in percent) of the
vapor content of the air divided by the capacity at the temperature the air is being sampled. If a kilogram air
at 25°C has a vapor content of 10 grams, then its relative humidity is 50%, since air at 25°C can hold 20g. In
other words, this air is “half full” of vapor. We can add more vapor to this air, in fact we can add another 10
grams, but then the air can hold no more, and the air is said to be saturated, and its relative humidity is 100%.
There is another way to saturate air (achieve 100% relative humidity), and that is too cool the air, rather than
add more vapor. We begin in this example with a kilogram of air containing 7g of water vapor at 20°C. A
kilogram of air at that temperature has a saturation mixing ratio of 14 g, so the relative humidity is 7/14, or
50%. But if the air is cooled to 10°C, it becomes saturated, since 7 grams is the maximum the air can hold at
10°C. Further cooling will force vapor to condense to liquid: cooling the air in this case to 0°C, which has a
saturation mixing ratio of 3.5 g, means that 3.5 g (7 minus 3.5 equals 3.5) of water will condense as liquid,
falling to the bottom of the flask. Also we can define using his example, the dew point: the temperature to
which the air was cooled to achieve saturation, in this example, 10°C. This is often how fog, clouds, and dew
form: the air is cooled to achieve saturation.
One way air cools is through radiational heat loss, that is, the loss of heat (infrared radiation) by the air at
night. Another way air can be cooled is to expand it, by lifting the air to a higher altitude, where the pressure
is lower. You may have noticed air spraying out of a canister in which it was compressed feels cool as it exits
the nozzle. This kind of cooling of a gas by expansion is called adiabatic cooling. If air is lifted, the reduced
pressure at the higher altitude allows it to expand and cool adiabatically. Air cools about 10°C per 1000
meters of lifting (about 5.5°F per 1000 feet). This rate is known as the dry adiabatic lapse rate. Saturated air
has a different rate, the wet adiabatic rate, which is less than the dry rate because the condensing vapor is
releasing latent heat, slowing the pace of cooling. The wet adiabatic lapse rate varies depending on how
much vapor is present in the saturated air; values can range from 5°C to 9°C per 1000 meters.
How does air get lifted in order to undergo this cooling-by-expansion? Here are four common ways:
1. Orographic lifting, when air is forced to rise over a mountain barrier.
2. Frontal wedging, when warmer, less dense air flows up and over cooler, denser air below.
3. Convergence, when air flows into a region from relatively opposite directions, causing it to collide and
move upward.
4. Localized convection, caused by unequal heating of the surface in an area, creating small bubbles of
warmer, buoyant air. These bubbles are called parcels by meteorologists.
Since cooling the air increases its density, adiabatic lifting results in a parcel of air becoming heavier, and if
the lifting force ceases, the parcel will tend to fall back downward. The air in this case is said to be stable.
The important factor as to whether a parcel of air is stable, and will try to return to its previous altitude, is the
temperature of the air in the environment the parcel is moving through, that is, the temperature outside the
parcel.
As we saw earlier, the troposphere cools with height, but the environmental lapse rate varies considerably
with day-to-day weather conditions, and with seasonal changes. The environmental lapse rate of 6.5°C per
1000 m cited earlier is an average figure. What happens when a parcel of dry air is lifted through this
“average” troposphere? The air in the parcel cools adiabatically, so after 1000 meters of lifting it will cool
about 10°C. But outside the parcel (in the tropospheric environment), the temperature is 6.5°C cooler at this
new height. So the air in the parcel is 3.5°C cooler than the air outside, making it more dense that the air in
its environment, and it will tend to sink (it is stable).
But under different weather or seasonal conditions, the environmental lapse rate can be faster. Let us see
what happens if the environmental lapse rate is 12°C per 1000 m. A dry air parcel under these conditions
must still cool 10°C per 1000 m (the dry adiabatic rate remains the same regardless of external conditions),
but upon rising to this level, the air outside the parcel has cooled 12°C, so the parcel is 2 degrees warmer
than the environment it is surrounded by: the parcel is less dense than its surroundings, and is buoyant. This
is why hot air balloons rise: the air inside the balloon is heated to a point where it is significantly warmer than
the air outside. Conditions such as this, when the environmental lapse rate is greater than the adiabatic rate,
are said to be unstable. And if the parcel cools to its situation point, the parcel will cool at the wet adiabatic
rate, and will be even warmer than its surroundings, due to the release of latent heat.
It is possible to force air upward (by the four means outlined earlier) through altitudes where it is stable, into
an unstable situation. One common situation is when the environmental lapse rate is between the dry
adiabatic rate and the wet rate (between around 6°C and 10°C per 1000 m). This is a situation called
conditional instability.
If a parcel becomes unstable, it will rise without the need for forced lifting, and parcels can rise and create
billowing clouds upon reaching the level where they cool to their dew point (called the cloud condensation
level). Very unstable conditions often result in massive cloud buildup resulting in thunderstorms and
tornados, that are fueled by the energy from huge columns of rising, unstable air. Conversely, stable
conditions mean air is settled, and the resulting inability for the air to rise can lead to stagnation of the air in a
region.
Clouds have a wide variety of shapes and sizes, depending on the atmospheric conditions and any lifting
mechanisms that may be occurring. Clouds are classified by their shape, and their height above the surface.
[Cloud names are based on Latin root words, as in many other scientific taxonomies.] Three shapes, or
forms, are used in the classification scheme:
•
•
•
Cirrus [Latin from “a curl of hair] shape: wispy, or feathery (“mare’s tails)
Cumulus shape [from cummulus, a pile]: puffy, or globular shapes
Stratus [stratum, a layer]: flat, or mostly featureless sheets, often covering all or much of the sky
Three heights are used in the classification: low (below about 2000 meters), middle (2000-6000m), and high
clouds (above about 6000m). Cloud names often are a combination of a prefix based on their height,
combined to their shape designation. The high cloud prefix is “cirro-” and the middle clouds have the prefix
“alto-”. Low clouds usually have no prefix, nor do cirrus clouds. Puffy-shaped clouds in the high levels are
called cirrocumulus, and in the mid levels, altocumulus. At low levels, puffy clouds are known simply as
cumulus clouds. Flat clouds are cirrostratus if high, altostratus if middle, or just stratus if low. If stratus have
rain falling from them, they are called nimbostratus (from nimbus, Latin for rain).
Some clouds reach from low levels into the higher levels. These are the clouds of vertical development, as
they are called. The most significant example is the cumulonimbus cloud, or thunderhead. As was
mentioned earlier, this cloud is the result of unstable atmospheric conditions.
Clouds can form near the ground: this is known as fog. Fog can be caused by increased evaporation in an
area, or by a drop in temperature.
If cloud droplets grow to sufficient size, they become heavy enough to fall, generating precipitation. This can
happen through continued condensation of vapor onto the droplets, making them grow. Droplets can also
grow by collision. A more complex growth occurs in cold clouds, where snow crystals develop at the expense
of liquid droplets due to the physics of supercooling and supersaturation. This is the Bergeron process. Once
a snow crystal or drop reaches a large enough mass, it will fall, and often collide with other crystals or drops
to effect growth by collision.
The names given to types of precipitation are based on whether the particles are liquid or frozen, and on their
size. The largest type is hail, which his formed by strong updrafts in a cumulonimbus cloud. Rain is blown
upward into the freezing level of the storm, freezes and falls back into the liquid region. Repeated up-anddown trips through the cumulonimbus result in a large, layered stone of ice.
6.3: Atmospheric Pressure and Wind
As we have seen earlier, the pressure of the air (the weight of the air above a given location, per unit area)
decreases with altitude. But even at a constant altitude, at sea level for instance, slight variations in the
atmospheric pressure occur. Sea level pressure averages about 1 kilogram per square centimeter (about
14.7 pounds per square inch) about the same pressure as is produced by a column of water 10 meters
(roughly 33 feet) high. This pressure is measured with a device called a barometer, and a few different types
of barometers exist. Meteorologists generally record barometric pressure in millibars (metric units), which
they often convert to inches of mercury for weather reports in the U.S. The average sea level pressure is
about 1013 millibars (mb), or about 29.92 inches of mercury. But again, this is the average pressure. Over
time, it is common to see barometric pressure fluctuate between 990 and 1030 mb (about a 4% variance),
and much greater extremes are possible. The highest barometric pressure on record is 1078.4 mb (31.85 in.)
and the lowest, 870 millibars (or 25.69 in.), a difference of some 20%.
These differences are not a result of compositional change in the atmosphere (the composition of the
atmosphere is nearly constant), but due to density variation of the air, and this is in turn due to changes in
temperature. The imbalances energy distribution on our planet result in areas where the atmosphere is
warmer, and therefore less dense, than colder areas. Warmer, less dense air has a lower pressure than
colder, denser air. [Recall that temperature and density differences resulted in deep ocean water circulation
and it is responsible in part for the circulation patterns of air in Earth’s atmosphere.]
In order to establish a balance between areas of higher air pressure with areas of lower pressure, air begins
to flow from the higher pressure region to the lower, and the horizontal movement of air is what we call wind.
The force creating this movement is called the pressure gradient force.
As air flows from a high pressure area toward a low pressure region, it begins to be deflected (to the right in
the Northern Hemisphere) due to the Coriolis effect, the effect of the Earth’s rotation on free-moving objects
and fluids (such as ocean currents, as we saw earlier). So air moving from a center of high pressure to the
west of a center of low pressure will begin a southeastward trend as the Coriolis force and pressure gradient
forces act at right angles to create a net deflection.
Continued air motion controlled by these forces, and when frictional forces are added, results is air flow that is
counterclockwise into the low, and is said to be cyclonic. Air, conversely, flows clockwise out of the high, and
is said to be anticyclonic. The lines of equal barometric pressure on these diagrams are called isobars, and
are a way for meteorologists to plot the areas of high and low pressure on maps based on the readings of
numerous barometers over the country, in fact over whole continents and hemispheres. The winds resulting
from the forces discussed above blow at an angle over the isobars, rather than parallel to them, due to friction
with the surface. Higher up in altitude, the lesser friction results in the flow being more parallel to the isobars,
in what is called geostrophic wind.
Because air is flowing into centers of lower pressure, the flow is converging. Convergence is one of the ways
air is forced upward, which in turn can create clouds and precipitation. This is why areas of low pressure are
associated with active, inclement weather. Areas of low pressure on the surface are often created, moved,
and strengthened, by upper level winds. High pressure areas have settling (descending air) air, and this
usually results in greater stability and more tranquil weather conditions. It is important to remember these key
facts about high and low pressure areas:
Airflow at surface
Rotation
Vertical air motion
Associated weather
Low Pressure Areas (“Lows”)
Inward (converging)
Cyclonic (counterclockwise in the
Northern Hemisphere)
Upward (ascending)
Clouds, precipitation (unsettled)
High Pressure Area (“Highs”)
Outward (diverging)
Anticyclonic (clockwise in the
Northern Hemisphere)
Downward (descending)
Clearing, settled, usually dry
The temperature difference between the poles and the equator drives a globalized circulation pattern to the
atmosphere, which is complicated by the Earth’s rotation, and the different heat absorbing properties of land
and water. On a non-rotating planet with a uniform surface, we would expect to see the heat at the
equatorial region create rising air that would be replaced by cooler air sinking from the poles. This represents
two convective cells (convection is the circular motion of a fluid caused by its unequal heating) in each
hemisphere.
Earth’s rotation causes a more complicated, three-cell (in each hemisphere) model.
The three cells are called the Hadley cell (over the tropics), the Ferrel cell (over the mid-latitudes), and the
polar cell. Near the equator, rising air (heated by abundant solar radiation) creates a belt of low pressure
called the equatorial low. Upon rising to upper tropospheric altitudes, it begins to move northward, but
instead of traveling all the way to the pole before descending (as would be the case in the single-convective
cell model), the air begins to descend at about 30° latitude, resulting in a belt of high pressure, the subtropical
high. The anticyclonic flow in this zone sends some air poleward to produce winds from a westerly direction
above 30°, which are called the westerlies. Below 30°, the subtropical high sends air toward the equator
(feeding the equatorial low) from an easterly direction, causing what are called the trade winds. In the high
latitudes (above 60° latitude), sinking air from the pole heads southward at the surface, and is deflected to the
right resulting in the polar easterlies. The trade winds, the westerlies, and the polar easterlies are said to be
prevailing winds.
The westerlies and polar easterlies converge at around. 60° latitude; this is the zone called the subpolar low,
and the colliding warm and cold air in this belt produce what is called the polar front, an area of active weather
due to the low pressure and frontal wedging frequently occurring here. This is one reason why the weather
over much of the United States is so changeable.
Adding the continents and oceans, and seasonal cycles, and the three-cell circulation model in becomes even
more complex.
Local wind patterns are produced through heating differences on a smaller scale. Common examples of local
winds are the land and sea breezes, and mountain and valley breezes. Land and sea breezes are the result
of diurnal (day-to-night) temperature differences between land and water, and the mountain and valley
breezes are caused by airflow being driven by daytime heating of mountain slopes, followed by descending
(cooler) nighttime flow back down the valley. Chinook and Santa Ana winds are gusty, warm, dry winds that
are forced down from higher elevations to lower, causing the reverse of adiabatic cooling called katabatic
warming. The air moving down slope is compressed by the higher surrounding air pressure, and compressed
air heats (at the dry lapse rate of 10°C per km). Winds of this type occurring in the Rocky Mountain region
are called Chinook winds, in southern California they are the Santa Ana winds. In Europe, katabatic winds
include the foehn, and the mistral.
Prevailing winds are responsible for driving the large-scale ocean currents, as we have seen (an interface in
the Earth system). Meteorologists and oceanographers have found that the reverse can also occur: ocean
currents can have an affect on the global wind patterns. This is exemplified by the changes in the equatorial
currents in the Pacific Ocean, which are normally driven to the west by the trade winds allowing for cool
upwelling of water in the eastern Pacific offshore of Peru and Ecuador. Every three to seven years, however,
a equatorial countercurrent (to the east) develops which in turn weakens the trade winds and causes the
Peruvian/Ecuadorian waters to be abnormally warm. This situation is called an El Nino event, as it had been
noticed by South American fishermen to occur frequently during the Christmas season (El Nino means “the
child”). In an El Nino situation, the pressure difference between the equatorial low and the subtropical high
weakens, triggering a number of abnormal trends in the weather around the globe. El Ninos often result in
warmer than average winters in the western Unites States, wetter weather in the Gulf Coast states and the
eastern seaboard, and fewer hurricanes in the Atlantic.
Eventually equatorial current begins to reverse again and the El Nino ends, usually after 12 to 18 months.
Some years see abnormally cold water off Peru and Ecuador, creating what is essentially the opposite of El
Nino, called La Nina. This situation often results in colder than normal winters in the western U.S., warmer
winters in the East, and in increase in hurricanes.
6.4: Weather Patterns and Severe Storms
Weather in the mid-latitudes, the day-to-day changes in temperature, humidity, sky conditions, winds, and so
on, is greatly influenced by air masses: huge bodies of air (generally over 1500 km across, and several
kilometers thick) that have a fairly uniform temperature and humidity content. They form when air stagnates
over a particular region (called the source region) long enough to acquire this uniformity. Air remaining over a
large body of tropical water, for example, will become warm and humid. If this air mass begins to move
northward, and onto land, the population will see the weather change to a warmer and more humid regime for
a time. Source regions are considered polar or tropical based on the latitude of the source region, and are
either continental or maritime depending on the surface of the source region (mostly land or water,
respectively). So an air mass with a source region of the north Pacific and Gulf of Alaska, for example, is
maritime polar, abbreviated mP. Extremely cold air masses in the winter months are sometimes designated
as arctic.
Air masses often collide, and often as the result of the subtropical high pushing tropical air masses poleward,
intercepting polar air masses driven into the mid-latitudes by the polar high. Due to the density and moisture
differences of the clashing air masses, active weather usually results. When air masses of different
characteristics collide, the boundary where they come into contact is called a front, a term first coined by
Norwegian meteorologists in the World War I era, who likened the situation to armies aligning to do battle on
a military front.
Fronts are of four types:
•
Warm front: a warm air mass is pushing back, or replacing a retreating cooler air mass (the
warm air is “winning the battle”, i.e., advancing). The boundary takes on a ramp-like structure as
the less-dense warmer air overrides the denser cool air as it advances. The ramp is a frontal
wedge as was discussed earlier: a mechanism for lifting air, thereby creating adiabatic cooling
which can lead to precipitation. The gentle slope of the warm frontal typically brings a gradual
lowering of the cloud levels (the “ceiling”) and as the surface boundary passes, a rain event of
several hours or more.
•
Cold front: cold air is advancing into a region of warmer air. This boundary is steeper than the
warm front, so the frontal wedging effect is greater. This more forceful lifting often produces more
intense precipitation from thunderstorms. If the warm air ahead of the front is unstable, the
storms may be severe. Cold fronts tend to move more quickly that warm front, so the frontal
passage and its associated weather tend to be of shorter duration than with a warm front
passage.
•
Stationary front: air masses may collide and then lose momentum so that the resulting front
shows no appreciable movement toward either the warm side or the cold. The flow of air
becomes parallel to the boundary rather than into it, providing no “push” to the front. Stationary
fronts are associated with prolonged periods of unsettled weather and can result in flooding.
•
Occluded front: forms when a cold front catches up to, and collides with a warm front.. A
variety of weather can be found along occluded fronts, but a typical passage of an occluded front
is marked by a slow lowering of the ceiling as with a warm front, followed by light to moderate
precipitation for 6-18 hours, ending with a brief period of heavy precipitation and some
thunderstorms, as the cold front portion of the occlusion passes.
Fronts are often connected to a low pressure center. Much of the active weather we experience is caused by
the development and movement of these lows, known as mid-latitude cyclones. These are areas of low
pressure that are usually initiated by a disturbance (often due to a change in the airflow at higher altitudes)
along a stationary front. Disturbances sometimes dampen out, and the front remains stationary, but if not, the
wave can begin to increase in amplitude and generate an advancement of warm air on the forward side of the
wave, and of cold air on the rear side. [This is somewhat like a water wave: water is lifted ahead of the crest,
and after the passage of the crest, a downward movement of water occurs.] This process creates a cyclonic
circulation (low pressure) as well as a warm and cold front, as the air masses begin to be advanced by the air
movement around the low pressure center. Since cold fronts move more quickly than warm fronts (primarily
due to their having denser air behind them), the mid-latitude cyclone will begin to occlude (produce an
occluded front). Eventually this occluding brings all of the warm air to upper levels of the system, and cold air
overtakes the entire lower levels of the circulation. Once this happens the low has lost its main source of
energy (horizontal temperature contrast) and it begins to dissipate. The life span of a typical mid-latitude
cyclone is about a week, during which time it can move over a large part of the United States.
Upper-level winds play a role in generating, strengthening (or weakening), and steering mid-latitude cyclones.
These systems are often associated with jet streams: narrow belts of strong winds at the top of the
troposphere related to the boundaries between the convective cells discussed earlier [there are two jet
streams in each hemisphere, the polar jet between the polar cell and the Ferrel cell, and the mid-latitude jet
between the Ferrel and Hadley cells].
The mid-latitude cyclone generates active weather through convergence of air into the low, and through
frontal wedging (if it is not in the dissipating phase). If the air being lifted by the system is unstable, the
weather produced is often destructive, as severe thunderstorms develop.
A thunderstorm is a large cumulonimbus cloud, or a complex of joined cumulonimbus, resulting from updrafts
of rising air. The air may be lifted through the mechanisms discussed earlier, and if the parcels are unstable
the lifting will be accelerated. Especially unstable conditions with an ample supply of water vapor in the air
can produce thunderstorms that can reach heights over 15 km high. The string updrafts that produce these
tall clouds condense large quantities of water, which it turn releases an immense amount of heat which adds
energy to the storm. A mature thunderstorm contains both updrafts and downdrafts that can carry hail stones
up and down through he cloud allowing them to gain more ice. Strong updrafts also create the potential for
tornado development, especially if the thunderstorm complex is set into a rotation called the mesocyclone.
Tornadoes are one of nature’s most destructive forces, although lightning kills more people in the U.S. in most
years. The improvement of warning systems has lowered the death toll considerably, although the tornado is
still one of the more unpredictable and sudden weather phenomena, so there are still many fatalities,
especially in large tornados that hit in the late night hours, or strike communities with weak structures like
trailer home neighborhoods. The table below shows an overall decrease:
The strength, duration and size of tornadoes varies greatly, and the system of categorizing the strength of an
event is a subjective one, the enhanced Fujita scale, which is based on examining the damage after the
storm.
Hurricanes are the strongest form of the tropical cyclone, a low pressure system that derives its energy form
the latent heat release by the rising air from the tropical ocean waters (rather than the thermal differences of
air masses as in the mid-latitude cyclone). These storms are essentially vast walls of cumulonimbus clouds
rotating around a low pressure center called the eye which is surrounded by a ring of cumulonimbus called
the eye wall. The winds generated through the pressure gradient of the cyclone, combined with the inflow due
to the cumulonimbus bands, create the destructive winds of the tropical cyclone. The winds are of greatest
sustained speeds in the eye wall region. If a tropical low has sustained winds below 39 mph, it is called a
tropical depression, but if sustained winds exceed this speed, the system is called a tropical storm (and is
given a name). If sustained winds reach 74 mph or more, the storm is called a hurricane in the Atlantic, Gulf
of Mexico, and the Eastern Pacific. In the western Pacific hurricanes are called typhoons, and in the Indian
Ocean they are called, simply, cyclones. The strength of a hurricane is rated on the Saffir-Simpson scale,
based on the sustained winds.
Although the winds are a major destructive agent in a hurricane, the greatest danger to coastal residents is
from the storm surge, a dome of water pushed ahead of the storm. Once it moves onto land, the storm
weakens (as it is no longer able to be sustained by the tropical ocean evaporation) but still creates extensive
wind and flooding damage well inland, and can spawn tornadoes as well.
6.5: Climate
The terms “weather” and “climate” are used frequently enough that they might seem interchangeable, but they
are not the same. Weather is the day to day changing of atmospheric conditions, particularly those conditions
that affect humans the most, such as temperature, wind, and precipitation. But climate is the representation
of conditions over long periods of time, made possible with accumulated data taken over many years, at least
30 years being a generally accepted minimum. Preferably at least 100 years of data would be available to the
climatologist.
With the vastly different kinds of atmospheric conditions found over the Earth, it serves to classify the kinds of
climate into a system that can simplify how to apply the vast amount of data used to observe climate. The
most widely used classification system is the Köppen classification, based on two readily available sources of
data: the (mean monthly and annual) values of temperature and precipitation. From this information,
Wladimir Köppen, around 1900, devised five basic climate groups:
A: Humid tropical. No winter, i.e. all months have a mean temperature of at least 18°C (64°F).
B: Dry. Evaporation exceeds precipitation. (A formula using precipitation and temperature data is
applied to determine this criterion.)
C: Humid mid-latitude, with mild winters. Average temperature of coldest month is below 18° C but
above -3°C (27°F).
D: Humid mid-latitude, with severe winters. Average temperature of coldest month is below -3° C but
the warmest monthly average is above 10°C (50°F).
E: Polar. No summer, i.e. average temperature of the warmest month is below 10°C.
These principal groups are further subdivided to add more detail to the climate analysis. A map of the
climates of the planet using the Köppen classification system.
These are the present climates found on Earth, climate as we have seen, has changed considerably over
recent geologic time. The causes of climate change in ancient times are not fully understood, but we can
hypothesize that the movement of the continents in relation to the oceans have played a key role. Also,
variations in Earth’s orbit, and solar energy, and volcanic activity may have been involved. And now we are
becoming aware of human influences on climate. Even before the Industrial Revolution, humans have likely
made some alterations in climate through agricultural practices, such as through clearing land with fire, and
overgrazing. And it has become increasingly likely that mankind has had a great influence on global climate
since the Industrial Revolution, with the burning of fossil fuels and deforestation. The most notable effect is
the increase of carbon dioxide in the atmosphere, which we have seen is a heat-absorbing gas. The
predictions of what this trend will bring have been summarized in the reports of the Intergovernmental Panel
on Climate Change (IPCC), the most recent of which was released in 2007 (see http://www.ipcc.ch/ ).
Changes in climate, as in many other changes to the complex Earth system, can be subject to feedback
mechanisms. Changes that enhance a trend are called positive-feedback mechanisms, those that are
opposing a trend are negative-feedback mechanisms. In one scenario, for example, the melting of sea ice
(which is currently exceeding earlier predictions) reduces the amount of reflected sunlight (the albedo) which
would in turn increase the absorbed energy, speeding up the warming even further (positive feedback). In
another, however, the increase in ocean temperatures would increase evaporation, making for more cloud
cover in Earth’s atmosphere, resulting in a higher planetary albedo. More reflected sunlight would thereby
reduce Earth’s temperatures (negative feedback). Researchers with complex computer models are working
to resolve these mechanisms, but the consensus is that Earth’s atmospheric and ocean temperatures will
continue to rise in the near term.
Part Seven: Astronomy
7.1: Origin of Modern Astronomy
Astronomy is widely considered the oldest science, as humans have eternally looked upward in wonder at the
night sky. Why do the objects there behave with such regularity? Why do a few of the bright objects in the
sky move with respect to the others? Why do the seasons change? Many early civilizations made records of
the motions of celestial objects and events. These include the Mesopotamians, Egyptians, Persians, the
Mayans of Central America, the ancient Greeks, peoples of India, China, and in much of the Islamic world.
These societies were able to make celestial charts, and remarkably accurate calendars in many cases, but
there was no scientific method applied in these early times to explain the movement of the stars, the Sun and
the Moon, and the visible planets. This lead to much mysticism and misconception for millennia, particularly
the notion that Earth was located at the center of the universe (what is known as the geocentric model) and
the objects in the sky all revolved around Earth.
Around 500 to 600 B.C., the Greeks began to apply philosophic arguments to explain astronomical
observations. In the third century B.C., using basic principles of geometry, Aristarchus measured the sizes of
the Sun and Moon and their distances from Earth, and Eratosthenes closely calculated the size of Earth.
Then in the second century A.D., the Greek mathematician Ptolemy devised a system using various circular
orbits and epicycles that so closely agreed with observation, it went unchallenged for nearly 13 centuries,
despite being a geocentric model.
The birth of modern astronomy, and many other modern sciences, would begin as the Dark Ages ended, with
the publishing of the work of Nicolaus Copernicus, De Revolutionibus Orbium Coelestium, in 1543.
Copernicus placed the Sun in the center of a planetary system, making Earth a planet as well. This is known
as the heliocentric model. Copernicus still had a series of circular orbits and epicycles to get his system to
better agree with observation. It would be left to astronomers in the remainder of the 16th century, and into
the 17th century, to refine the heliocentric model.
The major figure in developing an Earth-centered system that had a precise mathematical basis, and
removed circular orbits and epicycles as a way of correcting anomalies, is Johannes Kepler. He derived the
three laws of planetary motion in the early 17th century. Kepler had used the meticulous observations of his
patron, Tycho Brahe, to aid in the calculations, which led to the three laws. The first law states that the orbits
of the planets are not circular, but elliptical. The second law states that an imaginary line between a planet
and the Sun sweeps out equal areas over equal time intervals. Thirdly, he showed the mathematical
relationship between a planet’s distance to the Sun and the period of its orbit.
During this same time, Galileo Galilei, in Italy, became a figure of powerful intellect and is regarded my many
to be the “Father of Modern Science.” Galileo worked extensively in many scientific studies, and his
accomplishments on astronomy were crucial in confirming many aspects of the Copernican system by using
an early telescope which he built himself, having heard of its invention in the Netherlands. He discovered four
satellites orbiting the planet Jupiter (which are known today as the Galilean satellites), and saw that the planet
Venus showed phases of illumination by the Sun which can only be produced if the Sun is centrally located in
the planetary system. He also first noted the features on the surface of the Moon, and sunspots.
A satisfactory, scientific explanation for Kepler’s laws was still wanting, until Sir Isaac Newton developed the
law of universal gravitation. Newton showed that Kepler’s mathematical solutions were due to the nature of
the force of gravity, which acts proportionally with the masses of objects, and inversely proportional to the
square of the distance between them.
Even though Earth is not at the center of the system, it is still convenient for many astronomical observations
to place Earth in the center of what is called the celestial sphere, upon which are fixed the stars, which have
been grouped into constellations (often mythological figures) by stargazers for thousands of years. The
celestial sphere can be imagined to be rotating around Earth once every 24 hours, even though it is really
Earth that is rotating. The axis of rotation is an extension of Earth’s axis, so the North celestial pole is directly
above Earth’s North Pole, the South celestial pole above the South Pole, and the celestial equator is a ring
directly above Earth’s equator. A latitude/longitude-like system of coordinates is used to locate objects on the
celestial sphere. The coordinates are: declination, which is the angle of an object above or below the
celestial equator, and right ascension, which is the angular distance measured eastward from the point on the
celestial equator where the sun crosses on the vernal equinox. The right ascension is measured in hours and
minutes rather than degrees, with an hour being equivalent to 15°.
Earth is tilted 23½° to its orbital path around the sun, known as the plane of the ecliptic. The Sun appears to
move on the celestial sphere one revolution per year along the plane of the ecliptic, which intersects the
celestial equator twice a year during the equinoxes. The planets and our Moon move at various rates around
the celestial sphere, near the same plane of the ecliptic, although they do not coincide with it exactly. This is
why there are not lunar and solar eclipses every month: the Moon is often a bit above or below the plane of
the ecliptic so its shadow (or the shadow of Earth on the Moon) is a little above or below the plane of the
ecliptic, and the shadow “misses.”
On a few occasions, usually a few times a year, the lunar-orbital and ecliptic planes do coincide for lunar or
solar eclipses. Mercury, and more rarely Venus, can be seen to pass in front of the Sun as seen form Earth,
if the planet’s orbital plane intersects with the plane of the ecliptic, and the planet is coincidentally along a line
between Earth and the Sun. This is called a transit.
Although it may be convenient to view Earth as being stationary and the rest of the visible universe rotating
around it, it is important to remember it is Earth that is rotating on its axis (once every 24 hours), that the axis
is tilted 23 ½ ° to its orbit around the Sun, and that it takes Earth about 365 ¼ days to complete a revolution of
the Sun. This pair of motions, rotation and revolution, creates two ways to calculate one day on Earth: a
mean solar day, the time interval between one noon and the next, and the sidereal day, the time it takes Earth
to make a rotation with respect to a star other than the Sun.
The motion of the Moon around Earth is also instructional. A complete revolution of the Moon around Earth
can also be calculated two ways. A synodic month is the time needed to complete a cycle of its phases (e.g.,
new moon to next new moon), which is 29 ½ days, and it the basis for the use of the moon in the Roman
calendar (the word “month” is derived from “moon”). The siderial month is the time the moon takes to
complete a revolution with respect to a fixed frame, such as a distant star.
Like the planets about the Sun, the Moon’s orbit is elliptical, with a closest point to Earth (called perigee) of
about 357,000 km (221,000 mi) and the farthest point (apogee) of 406,000 km (251,000 mi). This
eccentricity, as it is known, is just noticeable when viewing the full Moon at the close point versus one at the
apogee point as seen below (although sometimes atmospheric effects can also make the Moon appear
larger):
7.2: Tour of the Solar System
We can gain insight about out home planet when we compare it to the other bodies in the solar system. Earth
is unique (as far as we know) in many ways, most notably for its biosphere, but there are characteristics it
shares with other planets, particularly the other inner, or terrestrial, planets: Mercury, Venus, and Mars.
These bodies are primarily made up of rock and metal, while the outer, or Jovian planets (Jupiter, Saturn,
Uranus, and Neptune) have cores of rock and metal, but are also comprised of vast amounts of gases. [What
about Pluto? More about Pluto later.]
The nebular hypothesis states the sun and planets formed from a large cloud of gas and dust which began to
collapse (about 5BYA) in on itself due to gravitational forces. The cloud began to rotate and flatten into a disk
(which accounts for the plane of the ecliptic, within which the planets orbit closely), with nearly all of the
nebular material collapsing to form the Sun. The remaining gas and dust collided, fused, and gradually
accreted, eventually forming the planets, their larger satellites, and countless smaller bodies such as meteors
and asteroids.
Before the planets cooled (they are sometimes referred to as “proto-planets” at this early time) into more solid
bodies, the material that was amassing into them went through a density differentiation, whereby the heaviest
materials sank toward their centers, forming the metallic cores, surrounded with lighter rocky material. Gases
“floated” into an envelope surrounding the proto-planets. Near the young Sun, however, it was too hot for the
accumulation of lighter gases and ices to make a significant contribution to the inner planets’ composition, so
much of the primordial atmospheres of the terrestrial planets were likely dissipated by the heat and solar
radiation. But farther out, temperatures allowed for gases and ices as well, giving the outer planets a more
massive composition that included vast atmospheres of even the lightest gases such as hydrogen and
helium, and cold enough for ices of methane, ammonia, and many others that cannot exist in a hotter
environment, or on a small body without the gravitational pull to hold them.
There were countless bodies beside the proto-planets inhabiting the early solar system, smaller objects called
planetesimals, many of which were drawn into a collision with the Sun, or the planets. Evidence of these
collisions is seen on the impact-strewn surface of the planet Mercury, and on Earth’s Moon. The impact
craters on Earth from this early bombardment have been erased through external processes (weathering, ice,
etc.) and through tectonic processes. Also friction with Earth’s atmosphere now incinerates all but the largest
incoming objects. Mercury and the Moon have no atmosphere (therefore no weathering) and lacked tectonic
processes, so the craters remain there, testimony to the violent “rain of bombardment” in the early solar
system. Here on Earth, only much more recent impact craters are found in a few locations. Collisions will
continue with the remaining planetesimals (or “minor bodies”) in the solar system, objects we call asteroids
and meteors, and comets.
Mercury is the planet closest to the Sun. It appears similar to our Moon (i.e. heavily cratered), and is noted for the
extreme temperatures at its surface. On the daylight side, these are estimated to be about 430°C (800°F), and
the night side about -170°C (-280°F). Mercury, like Earth’s Moon, is too small and hot for an atmosphere, which
has left it unprotected from meteor impacts. With no erosion and weathering (also no apparent tectonic activity),
the surface is pocked by countless impact craters.
Venus is sometimes called Earth’s “sister planet” as it is nearly the same size, density, mass as Earth. It is
covered in thick, dense clouds of mostly carbon dioxide, so its surface is obscured from our viewing. However the
Magellan spacecraft that orbited Venus in the early 1990’s used radar imagery to map its surface. It appears to
have a young surface, as impact craters are in small numbers. Numerous volcanoes are found on the surface,
and it appears that volcanic and probably tectonic processes have been at work in the recent geologic past.
Because carbon dioxide is an efficient heat-absorbing gas, the temperatures on Venus are estimated to reach
475°C (900°F) and the atmospheric pressure on the surface is about 90 times Earth’s. Venus spins “backward”,
i.e. clockwise, on its axis compared to the Sun and most of the other large bodies in the solar system, for reasons
unknown, but this may be due to tidal forces on the thick atmosphere.
Since we have extensively examined Earth already, we will move on to the next planet, Mars. But first, it is worth
noting some facts about Earth’s Moon. One is that it is unusually large, most natural satellites of the planets are
much smaller than their parent planet, but our Moon is about one-quarter the size of Earth. It has been
hypothesized that when Earth was in its proto-planetary stages, another proto-planet about the size of Mars
struck Earth. This impact is thought to have dislodged and vaporized a large amount of rock material from Earth,
which later collected in nearby Earth orbit forming the Moon. This idea is based on the Moon’s lack of a
significant iron core, which it should have had it formed though density differentiation as the other planets and
large satellites.
The dark regions or maria (“seas”) the we see on the “face” of the Moon are the result of basaltic lava flows
similar to the Columbia Basalts. Much of the moon is the brighter highland terrain, though to be the result of
crustal deformation by the heavy bombardment of the Moon. The fewer craters found on the maria suggest they
are younger than the highlands. The Moon shows only one side to Earth as it rotates in the same time it takes to
revolved around Earth. This is due to Earth’s tidal force on the Moon, which slowed its rotation. Much we have
learned about the Moon’s composition and history (and therefore Earth’s) is due to the Apollo missions which
placed astronauts on its surface beginning in 1969 and ending in 1972. Hundreds of kilograms of lunar samples
were brought back for study by Earth’s geologists.
We now move on to Mars. It is called the “Red Planet” as it has a reddish appearance to the unaided eye, due to
the large amount of iron-bearing minerals in its dusty regolith. Mars id the most-studied planet after Earth:
several probes have visited it, including craft that have landed on the surface with roving robots that can perform
chemical analyses. Mars has a thin atmosphere (about 1% of Earth’s, mainly of carbon dioxide and a small
amount of water vapor) but it is substantial enough for weather phenomenon, such as winds, and dust storms.
Mars has numerous large extinct volcanoes and large, deep canyons. Perhaps these may have been due to
tectonic processes that have long ago ceased. There is considerable evidence that Mars was once covered with
large bodies of liquid water. There still may be areas where water exists trapped in the Martian soil. But more
research will be required to answer the questions of whether Martian “seas” existed, what became of this water,
and if life forms of some kind developed.
Mars has two small moons, Deimos and Phobos, that appear to be captured asteroids rather than satellites that
formed with Mars during the proto-planetary stage.
Jupiter is the largest planet (about 2 ½ times more massive than the rest of the planets combined), primarily a
gaseous body (mostly hydrogen and helium) with core of metallic hydrogen. The atmosphere is marked by cloud
bands generated by strong belts of winds and convection currents, the result of internal heat. A giant storm called
the Great Red Spot has been raging for at least 300 hundred years. Jupiter has a powerful magnetic field,
probably produced by currents in the hydrogen core, and auroras occur near the Jovian magnetic poles similar to
those on Earth.
Jupiter has dozens of moons, but the four first seen by Galileo are of the greatest interest. For example, Io has
active volcanoes of sulfur, and Europa is covered with an icy crust under which may exist an ocean of liquid
water. The other two Galilean moons are Ganymede, the largest satellite in the solar system, and Callisto.
Saturn is, of course, known as the Ringed planet (although the other Jovian planets have much less spectacular
rings that are barely visible). The origin of the rings is still much debated: they may have formed with the planet,
or by the collision of a large object with one of its moons, or perhaps some other mechanism. Saturn is currently
being intensively studied with the Cassini orbiting probe, which may provide the answer to this and other
mysteries of Saturn.
Saturn is similar to Jupiter in composition, and has bands of clouds driven by internal heat, although its magnetic
field is much weaker. Its largest moon, Titan, has an atmosphere of methane.
Uranus and Neptune are nearly identical in size and composition (again, mainly hydrogen and helium, much
lesser amounts of methane and ammonia) but they are icier than Jupiter and Saturn. Uranus is remarkable for its
greatly tilted axis of rotation, nearly 90 degrees. Why this is so is another mystery, perhaps a large body collided
with Uranus during its formation. Neptune has strong winds shaping its clouds, some measured at speeds over
1000 km/hr (over 600 mph). It also has a storm similar to Jupiter’s called the Great Dark Spot. Neptune’s moon
Triton appears to have geysers of liquid nitrogen bubbling on a portion of it surface.
Beyond Neptune lies Pluto, which has fallen from planetary status recently, after a contentions vote by the
International Astronomical Union in 2006. Pluto was then demoted to a “dwarf planet” status; then in 2008 it was
re-designated to a category of objects known as “plutoids.” [For more on the controversy you can see the link in
the supplemental reading/web sites section.] These changes of status are due primarily to Pluto’s small size, but
it has other idiosyncrasies, such as its orbit, which is well outside the plane of the ecliptic and is much more
elliptical that the other planets. Pluto has not been visited by a space probe, so very little is known about it. The
New Horizons spacecraft is scheduled to arrive there in 2015. It has three moons, Charon, Nix, and Hydra.
Pluto may one of an untold number of bodies that occupy what is called the Kuiper Belt. This is a collection of
dwarf bodies (several have been seen by the most powerful telescopes, such as 2003 UB, as it has been
cataloged) believed to reside in orbits outside Neptune, which is where many comets are thought to drift until a
gravitational “bump” sends them into the inner solar system. Comets are a group of the minor members of the
solar system, along with the asteroids (rocky bodies which are mostly located in a belt of their own, between Mars
and Jupiter that are as small as sand grains up to the largest, Ceres (technically a dwarf planet, about 1000 km in
diameter.) Comets are generally about 5 to 10 km in size, and are made up of not only rock, but have a
significant amount of ice in their composition. When thrust into a trajectory close to the Sun, the ice heats and
vaporizes, causing the lengthy tail associated with their apparitions. Also among the minor occupants of the solar
system are meteors, which are from three sources, debris from comets, planetesimals from the formation of the
solar system, or caused through the collisions of other bodies.
Beyond the Kuiper Belt, another “deep-freeze” source of comets has been hypothesized, called the Oort cloud
(some 10,000 times farther from the Sun than Earth) where comets of very long orbits are though to begin their
journey toward the inner solar system a push form another comet, or even a nearby star, can get them to drift into
the gravitational well of the Sun.
7.3 Extra-Solar Planets
Although astronomers have long assumed that many other stars have planets, they have been unable to detect
these other solar systems until recently. Planets orbiting around stars other than the Sun are called extrasolar
planets. Planets are small and dim compared to stars, so they are lost in the glare of their parent stars and are
invisible to direct observation with telescopes.
Astronomers have tried to detect other solar systems by searching for the way a planet affects the movement of
its parent star. The gravitational attraction between a planet and its star pulls the star slightly toward the planet, so
the star wobbles slightly as the planet orbits it. The first discovery of a planet orbiting a star similar to the sun
came in 1995. The Swiss team of Michel Mayor and Didier Queloz of Geneva announced that they had found a
rapidly orbiting world located blisteringly close to the star 51 Pegasi. Their planet was at least half the mass of
Jupiter and no more than twice its mass. These announcements marked the beginning of a flood of discoveries.
By the end of the 20th century, several dozen worlds had been discovered, many the result of months or years of
observation of nearby stars.
Astronomers now know of more planets (more than 300) outside our solar system than inside our solar system.
Most of these planets, surprisingly, are more massive than Jupiter and are orbiting so close to their parent stars
that some of them have “years” (the time it takes to orbit the parent star once) as long as only a few days on
Earth. These solar systems are so different from our solar system that astronomers are still trying to reconcile
them with the current theory of solar system formation.
7.4 Stars
Stars are balls of gas that shine or used to shine because of nuclear fusion in their cores. The most familiar star is
the Sun. The nuclear fusion in stars produces a force that pushes the material in a star outward. However, the
gravitational attraction of the star’s material for itself pulls the material inward. A star can remain stable as long as
the outward pressure and gravitational force balance. The properties of a star depend on its mass, its
temperature, and its stage in evolution.
Astronomers study stars by measuring their brightness or, with more difficulty, their distances from Earth. They
measure the “color” of a star—the differences in the star’s brightness from one part of the spectrum to another—
to determine its temperature. They also study the spectrum of a star’s light to determine not only the temperature,
but also the chemical makeup of the star’s outer layers.
Many different types of stars exist. Some types of stars are really just different stages of a star’s evolution. Some
types are different because the stars formed with much more or much less mass than other stars, or because
they formed close to other stars. The Sun is a type of star known as a main-sequence star. Eventually, mainsequence stars such as the Sun swell into giant stars and then evolve into tiny, dense, white dwarf stars. Mainsequence stars and giants have a role in the behavior of most variable stars and novas. A star much more
massive than the Sun will become a supergiant star, then explode as a supernova. A supernova may leave
behind a neutron star or a black hole.
In about 1910 Danish astronomer Ejnar Hertzsprung and American astronomer Henry Norris Russell
independently worked out a way to graph basic properties of stars. On the horizontal axis of their graphs, they
plotted the temperatures of stars. On the vertical axis, they plotted the brightness of stars in a way that allowed
the stars to be compared. (One plotted the absolute brightness, or absolute magnitude, of a star, a measurement
of brightness that takes into account the distance of the star from Earth. The other plotted stars in a nearby
galaxy, all about the same distance from Earth.) The resulting Hertzsprung-Russell diagram, also called an H-R
diagram or a color-magnitude diagram (where color relates to temperature), is a basic tool of astronomers.
•
Main-Sequence Stars Hertzsprung and Russell found that most stars fell on a diagonal line across the
H-R diagram from upper left to lower right. This line is called the main sequence. The diagonal line of
main-sequence stars indicates that temperature and brightness of these stars are directly related. The
•
•
•
•
•
•
•
hotter a main-sequence star is, the brighter it is. The Sun is a main-sequence star, located in about the
middle of the graph. More faint, cool stars exist than hot, bright ones, so the Sun is brighter and hotter
than most of the stars in the universe.
Giant and Supergiant Stars Stars that fall in the upper right of the H-R diagram are known as giant
stars or, for even brighter stars, supergiant stars. Supergiant stars have both larger diameters and larger
masses than giant stars.
Giant and supergiant stars represent stages in the lives of stars after they have burned most of their
internal hydrogen fuel. Stars swell as they move off the main sequence, becoming giants and—for more
massive stars—supergiants.
White Dwarf Stars A few stars fall in the lower left portion of the H-R diagram, below the main
sequence. These smaller, dimmer stars are hot enough to be white or blue-white in color and are known
as white dwarfs. White dwarf stars are only about the size of Earth. They represent stars with about the
mass of the Sun that have burned as much hydrogen as they can. The final stage of life for all stars like
the Sun is the white dwarf stage.
Variable Stars Many stars vary in brightness over time. These variable stars come in a variety of types.
One important type is called a Cepheid variable, named after the star delta Cephei, which is a prime
example of a Cepheid variable. These stars vary in brightness as they swell and contract over a period of
weeks or months. Their average brightness depends on how long the period of variation takes. Thus
astronomers can determine how bright the star is merely by measuring the length of the period. By
comparing how intrinsically bright these variable stars are with how bright they look from Earth,
astronomers can calculate how far away these stars are from Earth. Cepheid variables are only one type
of variable star. Stars called long-period variables vary in brightness as they contract and expand, but
these stars are not as regular as Cepheid variables. Eclipsing binary stars are really pairs of stars. Their
brightness varies because one member of the pair appears to pass in front of the other, as seen from
Earth.
Novas Sometimes stars brighten drastically, becoming as much as 100 times brighter than they were.
These stars are called novas (Latin for "new stars"). They are not really new, just much brighter than they
were earlier. A nova is a binary, or double, star in which one member is a white dwarf and the other is a
giant or supergiant. Matter from the large star falls onto the small star. After a thick layer of the large star’s
atmosphere has collected on the white dwarf, the layer burns off in a nuclear fusion reaction. The fusion
produces a huge amount of energy, which, from Earth, appears as the brightening of the nova. The nova
gradually returns to its original state, and material from the large star again begins to collect on the white
dwarf.
Supernovas Sometimes stars brighten many times more drastically than novas do. A star that had been
too dim to see can become one of the brightest stars in the sky. These stars are called supernovas.
Sometimes supernovas that occur in other galaxies are so bright that, from Earth, they appear as bright
as their host galaxy. There are two types of supernova. One type is an extreme case of a nova, in which
matter falls from a giant or supergiant companion onto a white dwarf. The other type of supernova occurs
when a supergiant star uses up all its nuclear fuel in nuclear fusion reactions. The star uses up its
hydrogen fuel, but the core is hot enough that it provides the initial energy necessary for the star to begin
“burning” helium, then carbon, and then heavier elements through nuclear fusion. The process stops
when the core is mostly iron, which is too heavy for the star to “burn” in a way that gives off energy. With
no such fuel left, the inward gravitational attraction of the star’s material for itself has no outward
balancing force, and the core collapses, and a shock wave that tears apart the star’s atmosphere. The
core continues collapsing until it forms either a neutron star or a black hole, depending on its mass.
Neutron Stars and Pulsars Neutron stars are the collapsed cores sometimes left behind by supernova
explosions. Pulsars are a special type of neutron star. Pulsars and neutron stars form when the remnant
of a star left after a supernova explosion collapses until it is about 10 km (about 6 mi) in radius. At that
point, the neutrons—electrically neutral atomic particles—of the star resist being pressed together further.
When the force produced by the neutrons balances the gravitational force, the core stops collapsing. At
that point, the star is so dense that a teaspoonful has the mass of a billion metric tons.
Neutron stars become pulsars when the magnetic field of a neutron star directs a beam of radio waves
out into space. The star is so small that it rotates from one to a few hundred times per second. As the star
rotates, the beam of radio waves sweeps out a path in space. If Earth is in the path of the beam, radio
astronomers see the rotating beam as periodic pulses of radio waves.
Black Holes Black holes are objects that are so massive and dense that their immense gravitational pull
does not even let light escape. If the core left over after a supernova explosion has a mass of more than
about fives times that of the Sun, the force holding up the neutrons in the core is not large enough to
balance the inward gravitational force. No outward force is large enough to resist the gravitational force.
The core of the star continues to collapse. When the core's mass is sufficiently concentrated, the
gravitational force of the core is so strong that nothing, not even light, can escape it. Astronomers have
various ways of detecting black holes. When a black hole is in a binary system, matter from the
companion star spirals into the black hole, forming a disk of gas around it. The disk becomes so hot that it
gives off X rays that astronomers can detect from Earth. Astronomers use X-ray telescopes in space to
find X-ray sources, and then they look for signs that an unseen object of more than about five times the
mass of the Sun is causing gravitational tugs on a visible object.
7.5 Galaxies
Our Sun is part of the Milky Way Galaxy. Galaxies also contain dark strips of dust and may contain huge black
holes at their centers. Galaxies exist in different shapes and sizes, and are classified by shape. The three types
are spiral, elliptical, and irregular. Spiral galaxies consist of a central mass with one, two, or three arms that spiral
around the center. An elliptical galaxy is oval, with a bright center that gradually, evenly dims to the edges.
Irregular galaxies are not symmetrical and do not look like spiral or elliptical galaxies. Irregular galaxies vary
widely in appearance. A galaxy that has a regular spiral or elliptical shape but has some special oddity is known
as a peculiar galaxy. For example, some peculiar galaxies are stretched and distorted from the gravitational pull
of a nearby galaxy.
Some galaxies are spirals, some are oval, or elliptical, and some are irregular. Galaxies tend to group together in
clusters.
The Milky Way The Milky Way is a spiral galaxy. Our Sun is only one of a trillion stars in the Milky Way
galaxy. On a dark night, far from outdoor lighting, a faint, hazy, whitish band spans the sky. This band is the Milky
Way Galaxy as it appears from Earth. The Milky Way looks splotchy, with darker regions interspersed with lighter
ones.
The Milky Way Galaxy is a pinwheel-shaped flattened disk about 75,000 light-years in diameter. The Sun is
located on a spiral arm about two-thirds of the way out from the center. The galaxy spins, but the center spins
faster than the arms. At Earth’s position, the galaxy makes a complete rotation about every 200 million years.
When observers on Earth look toward the brightest part of the Milky Way, which is in the constellation Sagittarius,
they look through the galaxy’s disk toward its center. This disk is composed of the stars, gas, and dust between
Earth and the galactic center. When observers look in the sky in other directions, they do not see as much of the
galaxy’s gas and dust, and so can see objects beyond the galaxy more clearly.
7.6 The Universe
The ultimate goal of astronomers is to understand the structure, behavior, and evolution of all of the matter and
energy that exists. Astronomers call the set of all matter and energy the universe. The universe is infinite in
space, but astronomers believe it does have a finite age. Astronomers accept the theory that some 13 to 14 billion
years ago, the universe began as an explosive event, resulting in a hot, dense, expanding sea of matter and
energy. This event is known as the “ Big Bang.” Astronomers cannot observe that far back in time. Many
astronomers believe, however, the theory that within the first fraction of a second after the big bang, the universe
went through a tremendous inflation, expanding many times in size, before it resumed a slower expansion.
As the universe expanded and cooled, various forms of elementary particles of matter formed. By the time the
universe was one second old, protons had formed. For approximately the next 1,000 seconds, in the era of
nucleosynthesis, all the nuclei of deuterium (hydrogen with both a proton and neutron in the nucleus) that are
present in the universe today formed. During this brief period, some nuclei of lithium, beryllium, and helium
formed as well.
When the universe was about 1 million years old, it had cooled sufficiently for the protons and heavier nuclei
formed during nucleosynthesis to combine with electrons to form atoms. Once protons and electrons combined to
form hydrogen, and various forms of radiation were released. Much of the radiation had the characteristic
spectrum of a hot gas. Since the time this radiation was first released, it has cooled and is now at about 3 K (270° C or –450° F). It is called the primeval background radiation and has been definitively detected and studied,
first by radio telescopes and then by the Cosmic Background Explorer (COBE) spacecraft.
In the billions of years since, two populations of stars formed out of the interstellar gas and dust that contracted to
form galaxies. This first population, was made up almost entirely of hydrogen and helium. The stars that formed
evolved and gave out heavier elements that were made through fusion in the stars’ cores or that were formed as
the stars exploded as supernovas. The later generation of stars, to which the Sun belongs, contains heavy
elements formed by the earlier population. The Sun formed about 5 billion years ago and is almost halfway
through its 11-billion-year lifetime.