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Earth Systems
Chapter 2
© 2011 Pearson Education, Inc.
What you will learn
• Origin, structure, and character of the four
Earth systems
• Nature of the geologic time scale and the
major periods of Earth history
• Methods for determining the age of Earth
materials
• Rates of select Earth systems processes
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The Spheres
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•
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•
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Atmosphere
Hydrosphere
Geosphere, and
Biosphere
All spheres interactive
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Earth’s Interactive Systems
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Much like the body with its many systems
Transfers of energy between systems
Open or closed system?
Considering little change in amount of
matter in or out of Earth’s realm—it
functions as a closed system
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Origin of the Geosphere
Birth of our planet—immense cloud of gas and
interstellar debris called a nebula
Figure 2.2a
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The Geosphere
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•
•
•
What is it?
Solid Earth with its deep molten portions
Is it a static or nonmoving mass?
No, it’s dynamic and it has constant energy
transfers that build mountain chains, create
ocean basins, cause hazards such as
volcanism, earthquakes, etc.
• It also provides many benefits such as
mineral wealth and energy resources
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Origin of the Geosphere
4.5 billion years before
present (BYBP)
immense cloud of gas
and interstellar debris
(nebula) began to
collapse under its own
gravity.
Figure 2.2 The Formation of Earth and
Our Solar System (a) Most stars, like the
Sun, are born in clouds of gas and dust called
nebulae. (b) The solar system was formed
along with the Sun when part of a nebular
cloud became dense enough to collapse.
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Geosphere Origin (cont.)
• Denser and hotter center became the Sun
• Dust, rocks, and gases swirling around
Sun coalesced to form solar system
planets
• Rocky planets—Mercury, Venus, Earth,
and Mars— formed closer to the Sun
• Large gaseous planets—Jupiter, Saturn,
Uranus, and Neptune—formed farther
away
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The Geosphere Is Born
Third planet from the Sun is Earth
Figure 2.3 Location of the Planets Relative to the Sun
The four small, rocky planets—Mercury, Venus, Earth, and Mars—are closer to
the Sun than are the large gaseous ones—Jupiter, Saturn, Uranus, and Neptune.
(Note that the distances are not drawn to scale. If they were, Earth would be
almost 11 meters, or 36 ft, from the Sun, and Neptune about 330 meters, or 1080
ft.)
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Early Earth
• Aggregation of nebular debris via many
collisions of various sized particles
• Heat generated by debris collisions,
gravitational compression as the
aggregated debris compacted, and by
decay of radioactive elements such as Ur,
Th, and K
• Early Earth fairly uniform compositionally
and partially molten
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Earth’s Moon
• Probably formed after early Earth, half the
size of present day Earth, was impacted
by body roughly size of Mars—ejecta
generated
• Some ejected mass flung out from Earth
formed the Moon
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Earth’s Early Surface a
“Moonscape”
• Views of the Moon
not unlike early Earth
Figure 2.4a Hints of Earth’s Early
Appearance
(a) Impact craters on the far side of
the Moon show what Earth’s very
early surface may have looked like.
The Moon’s far side, the oldest
surface on the Moon, preserves very
old impact structures possibly formed
between 4 and 4.5 billion years ago.
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Rare to Find Impact Evidence
on Earth
• Why?
• Geospheric Change through Time
Figure 2.4b Hints of Earth’s Early
Appearance
(b) Such scars are rare on today’s Earth.
Most extraterrestrial objects burn up
from friction as they pass through the
atmosphere, and the craters formed by
those that do reach the surface
are soon erased by natural processes
such as erosion. This crater, 1.2
kilometers (0.75 mi) across, formed in
the Arizona desert when an asteroid
20 meters (65 ft) in diameter struck
Earth. The impact occurred a mere
50,000 years ago—recently enough for
the evidence to still be visible.
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Geospheric Compositional
Structure
• Earth began to differentiate into
layers with varying compositions.
• Denser elements such as Fe and
Ni migrated to center of the
planet
• Less dense elements like Si, Al,
O, K, and Na floated to surface
• Once all elements moved and
Earth differentiated, 3 distinct
layers resulted—core, mantle,
and crust
Figure 2.5 Compositional Structure of the Geosphere
(a) The geosphere’s internal structure formed early in Earth’s history. Its structure,
based on composition, includes the inner and outer core, thick mantle, and thin crust.
(b) The crust under continents is thicker and less dense than crust under the oceans.
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Earth’s Core—Two Parts
• Solid inner core, 1200 kilometers (760 mi) thick, 1.7%
of Earth’s mass; composition: Fe and Ni ( an alloy)
• Liquid outer core, 2250 kilometers (1460 mi) thick,
30.8% of Earth’s mass (30.8%); composition Fe,
some Ni, up to 10% lighter elements, perhaps O, S
• Composition of core inferred from studies focusing on
density, composition of iron-rich meteorites that are
thought to be pieces of other planetary cores, and the
manner seismic (earthquake) waves pass or don’t
pass through it
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Earth’s Mantle—Below the
Crust
• Majority of Earth’s Interior is the mantle
• 2850 km (1800 mi) thick, 67.1% of Earth’s total
mass; composition somewhat variable but
relatively homogeneous with mostly Mg and in
decreasing abundance: Si, O, and Fe
• Compositional knowledge draws on studies of
Earth’s density, data from meteorites, and
seismic waves and more information from upper
mantle that has been brought to Earth’s surface
(e.g., in volcanic eruptions)
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Earth’ Crust—
the Thick and Thin of It
• The outer thin skin of Earth is its crust
• 80 km (50 mi) thick at its deepest point,
contains rocks that can be examined at
Earth’s surface
Two different types of crust :
A) Continental ~ 30 to 40 km (12 to 25 mi)
thick (granite)
B) Oceanic ~5 to 8 km (3 to 6 mi) thick (basalt)
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Comparison: Oceanic and
Continental Crust
Crust
Type
Oceanic
Typical
Thickness
Oldest
Age Dated
5-8 km
(3-6 mi)
180 MY
Basalt
4 BY
Granite
Continental 30-40 km
(12-25 mi)
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Dominant
Rock
Defining Earth’s Interior
• Abrupt changes in
seismic wave velocities
mark boundaries
between various regions
of the core, mantle, and
crust
• These include inner
core, outer core, lower
mantle, mantle
transition zone, upper
mantle, asthenosphere,
and lithosphere
Figure 2.6 The Physical Structure of the
Geosphere
Abrupt changes (discontinuities) in the velocity
of seismic waves as they travel through the
geosphere mark the boundaries between its
physical subdivisions.
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Inner and Outer Core
• Physically different inner core is solid and outer core
is liquid
• Very dense inner core is solid in spite of extremely
high temperatures because of the very high
pressures at Earth’s center
• Outer core is liquid because pressures are slightly
lower in this region, but temps are still high,
exceeding melting point of iron. Outer core is liquid
because it drastically slows down fast seismic
waves (see Figure 2-6a) and other seismic waves
cannot pass through it at all
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Lower Mantle and Mantle
Transition
• Solid rock lower mantle occurs from 2900 km
(1800 mi) to 660 km (410 mi) depth, gradually
increases in density (and seismic wave velocity)
downward toward the boundary with molten outer
core
• Although upper and lower mantle rocks are similar
in composition, the higher P in the lower mantle
makes minerals there different (denser than) those
in the upper mantle
• Mantle transition zone is between 660 km (410 mi)
and 410 km (250 mi) deep
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Upper Mantle and the Moho
• Upper mantle lies above the transition
zone and extends upward to base of the
crust
• Defined by an abrupt increase in seismic
wave velocities as waves pass downward
through it; was one of the first internal
geosphere boundaries to be recognized
• Mohorovićić discontinuity or just Moho
marks the boundary
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Asthenosphere and
Lithosphere
• Brittle uppermost mantle and crust
comprise the lithosphere (tectonic plates)
• Weak zone (non-brittle) upper mantle just
below lithosphere is the asthenosphere
• Asthenosphere and lithosphere also
differentiated based on seismic velocities
(Fig. 2.6)
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Earth’s Atmosphere
• Gases that surround the geosphere (air)
comprise the atmosphere
• Atmosphere extends from Earth’s surface upward
for many hundreds of km; no distinct top,
gradually becomes less dense and merges with
empty space
• Also includes gases that fill voids in near-surface
geosphere
• The atmosphere is Earth’s most dynamic system
and is the vessel for life-dependent gases such
as oxygen and carbon dioxide
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Origin of the Atmosphere—the
First Atmosphere
• Gases were part of nebular cloud that condensed to form
our solar system
• Gases common in nebular clouds, and in outer gaseous
planets include hydrogen (H), helium (He), methane
(CH4), and ammonia (NH3); Earth’s first atmosphere
probably included such gases
• Prior to Earth’s complete differentiation of the geosphere
and while still very hot, perhaps before a liquid outer
core was fully developed and before complete
aggregation, gravity could not hold these gases closely
around Earth; the first atmosphere is therefore inferred to
have left Earth
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Earth’s Second Atmosphere
• Volatiles (easily vaporized elements or compounds) were
also present in nebular material that formed the initial
solid Earth. Such volatile components included hydrogen
(H), carbon (C), and nitrogen (N) as well as compounds
formed from these elements such as water (H2O), carbon
dioxide (CO2), and ammonia (NH3)
• These elements and compounds are characteristic of
primitive meteorites called carbonaceous chondrites
thought to be representative of the nebular cloud
composition that condensed to form the solar system
• Thus, materials in the early solid Earth that contained
volatile components if vaporized could have contributed
to the formation of Earth’s atmosphere
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Outgassing Forming the Second
Atmosphere
• What dominant process was active during the
first 1 billion years of Earth’s existence to form
the second atmosphere?
• Outgassing—via volcanism which transferred
volatile components from the interior geosphere
to Earth’s surface and atmosphere
• Volatiles in molten rock (magma) erupted from
volcanoes in the form of lava (surface)
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Volcanism—Clues to Outgassing
Forming Earth’s Second Atmosphere
• An estimate of what early
volcanism released to the
atmosphere can be obtained by
studying outgassing of modern
volcanoes like those in Hawaii
• Present-day volcanic eruptions
release water vapor (H2O),
carbon dioxide (CO2), nitrogen,
and sulfur compounds
(especially sulfur dioxide, SO2)
to the atmosphere
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Figure 2.8 Outgassing
Studying the release of gases by
modern volcanoes, such as Mt.
Etna in Sicily, shown here, helps
us to understand how Earth’s
early atmosphere was formed.
Volcanism—Clues to Outgassing
Forming Earth’s Second Atmosphere
• Atmosphere created by outgassing in Earth’s early
history is thought to have contained mostly water vapor
and carbon dioxide along with nitrogen, sulfur
compounds, some hydrogen, and compounds formed
from the reactions of these gases, such as ammonia and
methane
• Carbon dioxide, methane, and water vapor are gases
that cause the atmosphere’s temperature to rise =
greenhouse gases. Climate during the time of the
second atmosphere was very warm—no polar ice caps,
no liquid water or free oxygen—life as we know it today
didn’t exist in the 1st BY (4.5 to 3.5 BYBP)
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The Third Atmosphere—Today’s Air
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Processes Dominant for Forming
Third Atmosphere
• Today’s atmosphere (Table 2–1) is called Earth’s third
atmosphere
• Big changes needed to transform Earth’s second
atmosphere to make the third atmosphere
• Large volumes of H20 and CO2 had to be removed and a
lot of N and O needed to be added to evolve into the third
atmosphere
• How did this come about?
• Excess H20 went to the hydrosphere—mostly in oceans,
but also as rivers, lakes, and streams. By 3.5 BYBP,
Earth’s oceans had formed from water that precipitated
from the second atmosphere as the geosphere began to
cool
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Third Atmosphere’s Link to Geosphere
and Oceans—Rock and Mineral Products
• Once the world ocean existed, CO2
from atmosphere began to dissolve in
its waters
• Dissolved CO2 in turn reacted with Ca
to form solid calcium carbonate
(CaCO3), which started to precipitate
and accumulate on the seafloor
• Because many organisms incorporate
CaCO3 into their shells and skeletons
that then accumulate on the seafloor,
the rate of precipitation increased
once living plants and animals came
upon the scene
Figure 2.9 Calcium Carbonate, an Important Carbon Sink
Many organisms incorporate calcium carbonate into their shells and skeletons (a), which then
accumulate on the ocean floor (b). During Earth’s history, much of this material has been compressed
into rock, like the famous white limestone cliffs that overlook the English Channel (c), thus becoming a
major geosphere sink for carbon.
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Third Atmosphere’s Link to Geosphere
and Oceans—Rock and Mineral Products
• Most seafloor CaCO3 accumulations gradually
turn into rocks and become part of the
geosphere (mostly CaCO3 rock as limestone)
• The geosphere is the world’s largest carbon
reservoir and a global sink for CO2;
concentration of CO2 in the atmosphere slowly
decreased as CO2 was transferred through the
oceans to the geosphere
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Nitrogen and Oxygen in the
Modern Atomosphere
• Nitrogen degassed significantly to provide
nearly 80% of our air as we now know it
• Even though nitrogen increased during
Earth’s early history there was still no free
oxygen. Origin of the atmosphere’s free
oxygen is tied to changes in the biosphere
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Biosphere’s Role in Production of Free
Oxygen for the Third Atmosphere
Figure 2.10a The Atmosphere’s Oxygen
Came from Photosynthetic Organisms
Earth’s first free oxygen, produced by
photosynthetic cyanobacteria (a), was
trapped in banded iron formations.
• Some of the earliest life forms on
Earth were microorganisms
called cyanobacteria that are
photosynthetic
• Photosynthetic organisms use
sunlight to convert CO2 and H20
into food and oxygen, a process
called photosynthesis
• Cyanobacteria started making
oxygen about 3.5 BYBP, but
oxygen didn’t increase in Earth’s
atmosphere for another billion
years
• Why was this the case?
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Increases in Atmospheric
Oxygen
• Finally, about 2 billion years ago (about 1.5 billion years
after cyanobacteria started generating oxygen), most of
the material that readily reacted with the atmosphere’s
early oxygen was used up
• Oxygen concentration of the atmosphere began to slowly
increase as photosynthetic organisms increased in
abundance
• About 500 MYBP land plants first appeared and the
atmosphere’s oxygen level had become approximately
what it is today; almost all animals must breath in oxygen
to survive, thus oxygen levels are thought to have caused
a tremendous expansion of the biosphere’s diversity at this
time (called the Cambrian Period Explosion)
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BIFs—Storers of Early Produced
Oxygen
• So what was the sink for early produced oxygen?
• The answer is in the rocks. Newly formed oxygen reacted
with abundant Fe and S on Earth’s surface and dissolved into
the early ocean
• Chemical reactions formed solid minerals, especially iron
oxides that accumulated in sediments to become rocks
known as banded iron
formations (BIFs).
• BIFs are sinks where Earth’s
early oxygen is stored today
Figure 2.10b The Atmosphere’s
Oxygen Came from Photosynthetic
Organisms (b), a sink for early oxygen
in the geosphere.
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Compositional Structure of the
Atmosphere
• Atmosphere’s overall
thickness is typically 480
km (300 mi) or more
• Atmosphere has two layers
that are defined by
compositional variations—
homosphere and
heterosphere
Figure 2.11 The Structure of the Atmosphere
Variations in composition and temperature define the
atmosphere’s structure. Note that the heterosphere, from
80 km to about 480 km, is not shown in full. The region
above the heterosphere, where Earth’s atmosphere
merges with outer space, is sometimes called the
exosphere.
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Temperature Structure of the
Atmosphere
• Solar radiation interacting with Earth’s
surface and atmosphere produce
temperature variations that define four
atmospheric layers
• The four layers in ascending order:
troposphere, stratosphere, mesosphere,
and thermosphere
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Troposphere
• Troposphere’s thickness varies from 7 km (4 mi)
near poles to 17 km (11 mi) near the equator;
temp ∆ in the troposphere is related to the
heating of Earth’s surface by solar radiation—
cools upward until constant at tropopause
• Troposphere is most dynamic place within the
atmosphere
• Highly significant—where life lives, weather
occurs; about half of the mass atmosphere is in
the lower 5 km (3 mi) of the troposphere
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Stratosphere
• Temps  upward through the stratosphere
Higher temps of the stratosphere vs. those
of the underlying troposphere prevent air
from rising and crossing their boundary
zone, the tropopause
• Top of the stratosphere is marked by a
temp ; this boundary, called the
stratopause, is at an altitude of ~50 km (30
mi) from Earth’s surface
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Stratospheric Ozone—the Great
Protector of the Biosphere
• Temp ∆s through stratosphere are caused by the interaction of
incoming solar radiation and oxygen
• Common oxygen molecules (O2) absorb short-wavelength,
ultraviolet (UV) radiation in the upper stratosphere and split
apart into highly reactive oxygen atoms (O)
• These single O atoms then bond with O2 molecules to form
ozone (O3), oxygen molecules that contain three oxygen atoms
• Ozone forms slowly in the stratosphere and can be destroyed
by reactions with sunlight and other atmosphere components
• Ozone is concentrated (albeit in small amounts) in the lower
stratosphere where it is not destroyed as rapidly—part of the
stratosphere that is referred to as the ozone layer, a great
protector for screening UV and other harmful radiation for all life
on Earth
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Stratosphere (cont.)—
characteristics
Figure 2.12 The View from the
Stratosphere
This photo is taken from the cloudless
stratosphere, looking down on the
clouds in the troposphere below.
• Upward temp  through
stratosphere prevents air
from rising within it and
causes it to be internally
stratified
• Small quantities of water
vapor and stratified character
of the stratosphere make it a
generally cloudless part of
the atmosphere; winds ~
parallel to Earth’s surface
• Observable as clear area
above clouds in troposphere
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The Mesosphere
• Above the stratosphere, from ~ 50 to 80 km (30 to 50 mi) in
altitude
• Temps  upward through the mesosphere to a boundary zone
(mesopause) where the lowest temps in the atmosphere are
present
• Temps  upward through the mesosphere reflect less
concentration of gas molecules that absorb UV radiation; also
related to presence of small amounts of CO2; CO2 absorbs
and reradiates solar energy, but there are so few other gas
molecules around to absorb this energy it is lost to space; this
cools the mesosphere, the opposite of the effect CO2 has in
the troposphere; there, CO2 acts as a greenhouse gas by
trapping heat energy from Earth’s surface and transferring it
to other gas molecules in the atmosphere
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The Mesosphere (cont.)
• Although the concentration of gas molecules in
the mesosphere is low, there are still enough of
them to have some significant effects—friction
with the few gas molecules present causes
meteoroids to heat up, observable as shooting
stars, and most burn up in the mesophere
• Exosphere—beyond mesophere (where
satellites travel)—atmosphere/space boundary
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The Thermosphere
• Outer atmospheric layer above the mesosphere.
Temps  and air molecules  upward through the
thermosphere as it gradually merges into space at
altitudes of 480 km (300 mi) or higher
• Compositionally defined the same as the
heterosphere; temperature  upward in
thermosphere is due to interaction of intense solar
radiation with the increasingly sparse gas molecules
and atoms (mostly nitrogen and oxygen)
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The Thermosphere (cont.)
• In cases where solar radiation is intense, it can change gas molecules
into charged particles (ions); because a lot of the ionization of the
atmosphere happens in the thermosphere =the ionosphere
• Charged particles colliding in the thermosphere produce the northern
lights (aurora borealis) and southern lights (aurora australis)—moving
sheets and wisps of colored light visible at high latitudes on clear
nights
Figure 2.13 Northern Lights on a
Clear Winter Night
Ions, formed by the collision of fastmoving charged particles ejected from
the Sun with atoms of gas in the
atmosphere, create the polar auroras.
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Earth’s Hydrosphere
• Directly connected between solar energy and
Earth system processes
• Consists of all water in oceans, on land in
streams and lakes, in glaciers and other
accumulations of ice, in the atmosphere and
underground
• “The Water Planet”—Water covers 71% of
Earth’s surface
• Earth uniquely positioned in solar system so that
water exists in all three phases —solid (ice),
liquid, and gas (water vapor)
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Origin of Earth’s Water
• How did Earth become so dominated by water on its surface?
• Water was part of the original nebular debris that coalesced,
forming Earth. Other contributions were from volcanic
outgassing that released volatiles including water vapor, during
formation of Earth’s second atmosphere, and possibly from
comets slamming into Earth
• Upon early Earth’s more complete cooling, water vapor in the
second atmosphere condensed and was able to remain liquid
on Earth’s surface
• Oldest known rocks that formed from ocean sediments are 3.8
BY old. These rocks confirm some compositional characteristics
of the second atmosphere and convey to us that oceans were
present, meaning the hydrosphere was complete at that time
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The Hydrosphere’s Reservoirs
• Unlike the atmosphere or
geosphere, the hydrosphere
lacks an internal structure
but possesses distinctive
reservoirs
• The world’s oceans are the
largest reservoir in the
hydrosphere (97% of
Earth’s water)
Figure 2.14 The Hydrosphere’s Reservoirs
More than 97% of the hydrosphere consists of saltwater in the oceans. Of
the fresh (nonsalty) water, almost 69% is frozen in glaciers, ice caps, and ice
sheets, and 30% is underground. The rest—only 1.3%—can be found in
many small reservoirs such as streams, rivers, and lakes. This fresh, liquid
surface water thus makes up only about 0.036% of all the water on Earth.
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The Hydrosphere’s Reservoirs (cont.)
• Freshwater—only 2.8% of Earth’s water; however,
only a small amount of that water is readily
available for human use
• Freshwater distributed in lakes, streams, rivers,
underground, and in the atmosphere but these
are small compared to Earth’s accumulations of
solid freshwater in glaciers, ice caps, and ice
sheets (up to 68.6% of Earth’s freshwater is ice)
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The World Ocean
• Contains 97.2% of all the water in the
hydrosphere—a continuous body of water
that covers 71% of Earth’s surface. Over 3
kilometers (1.9 mi) deep in over half this area
• Internal structure, defined by variations in
salinity and temperature but as one big
reservoir it has only two parts
• Upper layer—about 200 meters (700 ft) thick,
that is warmed by the sun and mixed by the
waves and currents created by surface winds
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The World Ocean (cont.)
• Lower layer—at depths below about 1000 meters (3000 ft), solar
radiation has little effect, and water temps are low, commonly in the 0o
to 4o C (32o to 39o F) range. Water can still be in motion because
salinity and temp differences change the water’s density. Denser
(colder and saltier) water sinks and slowly flows through the deep
ocean and back to the surface in a global circulation
• The world ocean is a major influence on global climate. Because of
water’s great specific heat capacity compared to air of the atmosphere
Figure 2.15 Movement of Water
in the World Ocean
Currents, driven by winds and
density differences, move water in
the hydrosphere’s largest
reservoir, the oceans.
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Glaciers, Ice Caps, and Ice
Sheets
• If snow accumulation > melting
of snow then glaciers can
form
• Seasonal conditions that
enable glaciers to form exist at
high elevations in mountains
and at high latitudes near the
north and south poles
• Mountain (alpine) glaciers vary
from small patches to large
rivers of ice that slowly flow
downslope; ice movement
carves many landforms that
are distinctive of glacial
regions
Figure 2.16 Glaciers in Southern
Alaska
Glaciers carve deep valleys, move
broken and ground-up rock along their
base and margins, and coalesce into
large masses of ice. These glaciers
flow from an ice cap in the coastal
mountains of southern Alaska.
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Glaciers, Ice Caps, and Ice
Sheets (cont.)
• Where glaciers coalesce and cover larger areas
they become ice caps (less than 50,000 square
km, or 19,000 mi2) and ice sheets (greater than
square 50,000 square kilometers)
• Modern world—ice covers about 10% of Earth’s
land area; most in ice sheets on Greenland and
one on Antarctica
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The Water Cycle
Figure 2.17 The Water Cycle
The water cycle transfers matter and
energy among Earth systems.
• Hydrosphere interacts with other Earth
systems in the water cycle; cycle
transfers a prized resource among its
reservoirs (oceans, atmosphere, on
and under land)
• Atmosphere over the oceans is a key
part of the water cycle; ~ 86% of its
water vapor obtained through
evaporation from seas
• When air rises, it cools, causing water
vapor to turn into tiny droplets to form
clouds. Where air temperature falls, as
when air rises along mountains, the
liquid water droplets condense and fall
to Earth’s surface. Precipitation of rain,
snow, or ice (hail) transfers water to the
land
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Water Cycle (cont.)
• Precipitation transfers water to 3 reservoirs on land—
(1) ice; (2) surface water and (3) groundwater
• Rivers and streams carry water back to the oceans;
groundwater migrates back to the oceans in many
coastal settings
• Water is used by plants, animals, and people, but it is
stored only temporarily—it cycles back to the
atmosphere through respiration and transpiration
• Earth system processes and interactions in the water
cycle involve energy and matter transfer between
atmosphere, biosphere, and geosphere
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Earth’s Biosphere
• Consists of all life on Earth—large and small
• From molecular level to vast, complex
ecosystems, significant interaction with various
Earth spheres
• Produces food, role in carbon cycle, a pollution
filter, a capturer of energy, aids in soil
development, etc.
• What started evolution to make life so diverse?
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Earth’s Biosphere (cont.)
• Chemical and fossil (paleontological) evidence in ancient
rocks provides insight into the origin of life on Earth;
fossils are remains or evidence of former life preserved
in rocks
• Example of fossils (Figure 2-18)—fossils can be actual
remains, or just imprints of an organism or its hard parts,
such as shells or dinosaur bones
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Earth’s Biosphere (cont.)
Figure 2.18 Fossilized Remains of Former Life
Fossils are remains or imprints of organisms, preserved
in rocks. (a) Ammonites, members of a group of shelled
animals related to modern squid and octopi, survived for
some 350 million years. The one who left these remains
floated in the ocean about 200 million years ago. (b)
These fossilized bones belonged to a relative of the
famous T. rex. Dinosaurs of this group lived from 170
to 65 million years ago. (c) Seed plants like the ones that
left these impressions grew in Europe and North America
some 280–300 million years ago.
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Some of the Oldest Fossils
• Bacteria—oldest fossils on Earth. Traces of thread-like, singlecelled organisms have been preserved in 3.2-BY old sedimentary
rocks in Australia
• Stromatolites are fossils found in even older rocks (as much as 3.5
BY). These are structures built from layer after layer of muddy
sediment trapped by mats of bacteria (Figure 2-19a). Descendants
of stromatolite-forming bacteria still inhabit some coastal
environments.
• Cyanobacteria, the first photosynthetic organisms, live in
stromatolites today, but it is unknown if bacteria that lived in very
old stromatolites were photosynthetic
Figure 2.19a Stromatolites
Stromatolites—layers of muddy sediment trapped by
films of microorganisms—are some of Earth’s oldest
fossils (a)
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Early Evolutionary Thought and
Fossils
• In 17th century Danish anatomist Nicolas Steno
(also known as Neils Stenson)—recognized
“tongue stones” were fossilized shark teeth as he
compared modern shark anatomy to the rock
record—established that that fossil represented
former life in former seas
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Early Evolutionary Thought and
Fossils (cont.)
By the end of 18th century, fossils became focus of naturalist studies.
Vertebrate fossils were the specialty of Georges Cuvier and
invertebrate fossils were the specialty of Jean Baptiste Lamarck, both
members of the French Museum of Natural History. By 1798 Cuvier
showed that vertebrate fossils represented species that no longer
existed—for example, that mammoths differed from living elephants
and that mammoths were extinct. Lamarck recognized life’s
complexity and change through time.
Figure 2.20 An Extinct Vertebrate
and a Modern Relative
From the study of their fossil
remains, Cuvier demonstrated that
mammoths (a), shown here in an
artist’s reconstruction, were an
extinct species, different from
modern elephants (b).
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Darwin and Wallace—Evolutionary
Giants
• Charles Darwin and Alfred Russel Wallace—
investigated how life changed over time by the
process of natural selection
• Wallace studied the distribution of species
(biogeography), developed his own ideas about
natural selection
• Darwin and Wallace communicated, shared
specimens, and jointly presented their ideas on
the subject at a scientific meeting in 1858
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Darwin and Wallace—Evolutionary
Giants (cont.)
Darwin published Origin of the Species in 1859, while Wallace continued
his biogeography studies. Darwin’s best examples came from his early
studies as a naturalist on a cruise of the H.MS. Beagle.
Figure 2.21 Darwin’s Journey
As a young man, Charles Darwin sailed
around the world as naturalist on H.M.S.
Beagle. Much of the voyage, which lasted
nearly five years, was spent exploring the
coasts of South America. Darwin made many
of the observations that inspired his theory of
evolution by natural selection in the
Galápagos Islands. In that isolated region, a
unique and distinctive array of species
evolved as plants and animals have adapted
to the varied environments on the different
islands.
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Darwin’s Galapagos Island
Studies
Figure 2.22 Seven of Darwin’s Finches
Darwin discovered an amazing variety of
closely related species on the Galápagos
Islands in 1835. Thirteen species of finches
had evolved different characteristics,
especially of their beaks, to adapt to
available food on the different islands.
• Via anatomical
comparisions Darwin
studied related but
different species of
finches on the Galápagos
Islands
• He proposed that natural
selection explained how
these and other species
evolved
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Mass Extinctions through
Geologic Time
• Difficult times for the
biosphere are times when
a large number of species
became extinct. These are
called mass extinctions
• At least 5 major extinctions
during the last 540 million
years
• Most recent and most
famous mass extinction ~
65 MYBP when dinosaurs
perished
Figure 2.23 Mass Extinctions
Mass extinctions indicate very
stressful times for the biosphere.
There have been five major mass
extinctions during the last 540 million
years.
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Death of the Dinosaurs
• Scientists think this mass
extinction resulted from the
impact of a large asteroid on
what is now the Yucatan
Peninsula of Mexico
• Earth was not a closed system
65 million years ago!
• Impact of this large asteroid
darkened the skies with dust
and caused severe global
Figure 2.24 The End of the Dinosaurs
climate changes that were just
The dinosaurs died out in a mass extinction that occurred 65 million
years ago. The cause of this event is thought to have been the
too much for the dinosaurs—
impact of an asteroid, shown here in an artist’s depiction (a), in the
Gulf of Mexico just off the coast of Mexico’s Yucatan Peninsula (b).
and many other species—to
The asteroid was probably at least 10 kilometers (6 mi) in diameter.
Its impact released more energy than a million hydrogen bombs
handle
and blasted a crater greater than 160 kilometers (100 mi) in
diameter, now buried under thick layers of sediment.
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The Passenger Pigeon—a
Recent Extinction Example
Figure 2.25 Martha: The Last
Passenger Pigeon
• The passenger pigeon is
probably the only species for
which we know the exact date
of its extinction. The last
known passenger pigeon,
Martha, died at 1:00 p.m.,
September 1, 1914, in the
Cincinnati Zoological Garden
• Fate of the passenger pigeon
was determined years before;
passenger pigeons had a
problem; they were easy to
catch and good to eat
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Understanding Geologic Time
and Earth History
• Earth’s history to be understood must be placed
into the context of geologic time
• Scientists determine both relative and absolute
ages of events in Earth’s history
• Examples: age of glacial advance, a volcano
erupting, an ocean basin forming, a mountain
belt forming
• Age determination aids in our understanding
rates of geologic processes and Earth system
interactions
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Relative Time Tools
• Being able to tell that one strata or event is
older or younger in comparison to another
is referred to as relative dating
• A number of early scientists made
observations and developed relative time
dating tools or established principles that
we still use today to aid in discerning Earth
history
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Relative Time Tools—Basic
Principles
• Nicholas Steno’s Law of Superposition
• William “Strata” Smith’s fossil
succession idea
• Combination of relative and eventually
absolute time concepts helped to establish
the geologic time scale
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Geologic
Time Scale
Figure 2-27 The Geologic Time Scale
The geologic time scale summarizes
Earth’s geologic history as determined
from studies of rocks, fossils, and their
relative ages. Radiometric dating has
enabled the absolute ages of geologic
events to be determined and added to the
time scale. (Note that the time intervals
are not drawn to scale.)
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SUMMARY
• Earth’s position in the solar system is perfect for
evolution of three physical systems—permits
wide range of life forms to be sustained and
evolve
• Globally defined physical systems include the:
• Geosphere
• Hydrosphere, and
• Atmosphere
• Physical systems interact to provide niches for
all of the biosphere, including humans
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SUMMARY (cont.)
• Geosphere—differentiated into core,
mantle, and two types of crust—
continental and oceanic
• State of matter of a differentiated Earth—
detected indirectly by seismic waves as
solid inner core, liquid outer core, mantle,
asthenosphere, and crust
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