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The Earth 1
Data

Location – The Earth is the third planet from the Sun, with an average distance of
approximately 93 million miles.

Size – The Earth is the fifth largest planet within the solar system, measuring 7909
miles in diameter.

Orbital Period – 1 year (365.25636 days).

Rotational Period – 1 day (23 hours 56 minutes 4 seconds).

Number of Satellites – 1.
Atmosphere

Composition – The Earth’s atmosphere is composed of about 78% nitrogen (N2) and
21% oxygen (O2). Most of the remaining 1% is composed of argon. However, there
are two other gases comprising this one percent that have significant importance:
1. Water Vapor (H2O) – Though the average water vapor content of the Earth’s
atmosphere is less than 1%. The amount varies from 0.1% in cold, dry regions to
3% in warm, wet regions. Water is an essential ingredient for life on Earth. In
cold, dry regions life is usually sparse, whereas in warm, wet regions life is
usually abundant.
2. Carbon Dioxide (CO2) – The amount of carbon dioxide in the Earth’s atmosphere
is about 0.04%, but it is increasing. This increase is due to the burning of fossil
fuels (oil, natural gas, coal). This causes a warming of the lower atmosphere,
referred to as a greenhouse effect, and produce global climatic changes. If
conditions are left unchanged, the amount of carbon dioxide in the Earth’s
atmosphere is predicted to double in the second half of this century.

Pressure – The force of the Earth’s gravity and the amount of gas cause a sea-level
pressure of approximately 14 lbs/ in2 or 1 bar. This pressure decreases rapidly with
increasing elevation; that is, at a higher elevation, the amount of gas is less, causing a
lower atmospheric pressure.

Temperature – The average temperature near the surface of the Earth is about 60º F.
This regulation of temperature is a result of the reflection of incoming solar radiation
by the atmosphere coupled with the blanketing effect, caused by greenhouse gases
(carbon dioxide and water vapor).
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
Layers – The atmosphere of the Earth is estimated to be about 600 miles thick.
However, because there is no set boundary where the atmosphere ends, this
estimation is somewhat arbitrary. Nevertheless, the Earth’s atmosphere can be
divided into several distinct layers identified using temperature changes, chemical
composition, movement, and density.

Troposphere – The lowest level of the Earth’s atmosphere is referred to as the
troposphere. It contains about 90% of the atmosphere’s mass. Moreover, this
layer is characterized by convection currents. These convection currents are
generated by the Sun’s radiation heating the ground and lower atmosphere. The
warmer air near the surface expands and rises, and is replaced by downdrafts of
cooler air from above. Convection currents in the troposphere generate nearly all
weather phenomena.

Stratosphere – The layer of the atmosphere directly above the troposphere is the
stratosphere. Unlike the troposphere, the stratosphere has very little vertical
mixing. The temperature is nearly uniform at approximately -75º F, and therefore
very dry (water is almost entirely frozen out).

Ozone Layer – In the upper part of the stratosphere, the Sun’s ultraviolet light
creates a layer of ozone through a process called photochemistry. Ozone is a
form of oxygen containing three atoms per molecule (O3).

Importance and Concern 2 – The ozone layer blocks ultraviolet light from
reaching the Earth’s surface, and therefore, protecting life from irreparable
harm. Since the early 1980’s, the ozone layer has shown signs of severe
damage by industrial chemicals called chlorofluorocarbons (CFC’s). A
large hole in the ozone layer has formed over the Antarctic and has been
depleted in the temperate latitudes.

Mesosphere – The atmosphere reaches its coldest temperature of around -170° F
in the mesosphere. This is also the layer in which a lot of meteors burn up while
entering the Earth's atmosphere.

Thermosphere – The thermosphere is a region in which temperatures again
increase with height. This temperature increase is caused by the absorption of
energetic ultraviolet and x-ray radiation from the Sun.

Ionosphere – Within the lower portion of the thermosphere is a region referred
to as the ionosphere. Here many atoms are ionized, that is broken apart into
positively charged ions and negatively charged electrons. The solar wind
brings charged particles to the upper levels of Earth’s atmosphere. They move
towards the north and south poles, and while the particles collide with the
ionized gas, colorful formations of light are produced – the Aurora Borealis
(the Northern Lights) 3 and the Aurora Australis (the Southern Lights).
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
Circulation 4 – The Earth is heated more near the equator than at the poles. Because
of this, the atmosphere (and the oceans) redistributes much of the solar energy from
warmer regions to cooler ones, which causes the formation of a global atmospheric
circulation. This circulation is usually represented in a simplified form, but in
actuality, the system is quite complicated. There are many factors that contribute to
the complexity of the system, including:
1. Landmasses (continents) obstructing circulation patterns.
2. The tilt of the Earth’s axis causing seasonal variations.
3. Coriolis Effect 5 – The Earth’s rotation generates the deflection of large air
masses. This phenomenon is known as the Coriolis Effect and is responsible for
rotating weather systems such as hurricanes 6.

Weather and Climate – Weather is the state of the atmospheric system at a given
place and time and its short-term variations, whereas climate is the average
atmospheric conditions within a geographic region over a long period of time.

Climate Changes – Climate on Earth does change. The most dramatic climatic
changes are those associated with the ice ages over the past million years. At
intervals of about 100,000 years, the average temperature of the Earth has
dropped by 2 or 3 degrees. This decrease in temperature is sufficient enough to
produce vast ice sheets (up to 2 miles thick) over much of the northern
hemisphere landmasses. There are number of causes associated with climatic
change:
1. Changes in the Earth’s orbit and tilt of its rotation axis (due to gravitational
forces of other planets) are thought to be the primary cause of the great ice
ages.
2. Variations in the amount of dust in the Earth’s atmosphere. The primary
source of atmospheric dust is large volcanic explosions 7. Though rare,
impacts of large asteroids or comets can also significantly increase the amount
of atmospheric dust 8.

Evolution – The Earth’s atmosphere did not always consist of the same relatively
stable mixture of gases that we breathe today. The present atmosphere is the result of
very gradual changes that began soon after the Earth formed.

Primordial Atmosphere – The first atmosphere of the Earth was composed of
hydrogen, helium, methane (CH4), ammonia (NH3), water vapor and low amounts
of oxygen. This atmosphere was swept away by intense solar winds – vast
streams of particles composed primarily of electrons and nuclei of hydrogen and
helium emitted by the Sun.
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
Secondary Atmosphere – The present atmosphere of the Earth is the result of the
release of gases dissolved in molten rock, a process called outgassing (similar to
gases observed in present-day volcanic eruptions). The principal components of
this “new” atmosphere were probably water vapor, carbon dioxide and nitrogen.
However, it did not contain oxygen. The oxygen of the Earth’s atmosphere was
slowly added, primarily by green plants through the process of photosynthesis.
As the Earth continued to cool, clouds formed and great rains occurred. These
torrential rains eventually filled the ocean basins. This event not only diminished
the amount of water vapor in the atmosphere, but also carried away most of the
carbon dioxide as well. Eventually, large amounts of the carbon dioxide within
the oceans were chemically incorporated into carbonate rock.
Surface

Hydrosphere – The hydrosphere covers approximately 75% of the Earth’s surface. It
includes freshwater lakes and streams, the polar ice caps, snow capped mountains,
groundwater, etc. However, the main component is the oceans.

Salt Content (Salinity) – Ocean water is not pure H2O. It contains “salts”, which
make up on the average 3.5% (35‰) of its mass. Six elements constitute more
than 90% of the salts dissolved in the oceans: chlorine, sodium, magnesium,
sulfur, calcium, and potassium. The concentration of each of these elements in
seawater is a balance between inflow (primarily from weathering and erosion of
continental rock) and precipitation into ocean sediment.

Gas Content – A number of gases are dissolved in the oceans. The most
important of these are oxygen and carbon dioxide.

Oxygen – The oxygen in the oceans allows for the existence of advance forms
of life, such as fish, which use gills to extract the oxygen they require from the
water.

Carbon Dioxide – The amount of carbon dioxide in the oceans is about 60
times greater than within the atmosphere. As a result, the oceans regulate the
amount of carbon dioxide in the atmosphere, and therefore influence the
climate of the Earth.

Temperatures – The average temperature of the oceans is about 39º F. The
surface temperature varies from about 85º F near the equator to below freezing
within the polar ice caps. However, the temperature a half-mile or more below
the surface changes very little with location and season.

Currents 9 – Like the atmosphere, areas of oceans are heated unequally and
produce circulation patterns referred to as currents. These currents are affected in
similar ways as the atmosphere: location of landmasses, seasonal variations and
the Coriolis Effect. Moreover, currents heat and cool adjacent landmasses. For
example, the Gulf Stream is responsible for England having a warmer climate
than is normal for a landmass of its latitude.
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
Lithosphere 10 – The Earth’s rigid outer shell, which comprises the continental crust,
oceanic crust and upper mantle (about 60 miles in thickness), is referred to as the
lithosphere.

Composition – Respectively, oxygen and silicon are the two most abundant
elements comprising the lithosphere, and therefore most minerals and rocks
contain these elements. As a whole, these minerals and rocks are called silicates.

Continents and Ocean Basins – The Earth’s lithosphere is divided into two
principal regions, continents and ocean basins. These divisions differ in
elevation, geologic history, rock types (chemical composition), density and age.

Continents – Geologically and topographically, the continents are exceedingly
complex and variable in detail, yet certain large-scale structural and
topographic features are common to all:
1. They have two basic components 11: (1) Cratons – areas that are stable and
relatively inactive and (2) Orogenic Belts – areas that are unstable and
active. Earthquakes, mountain belts and volcanoes are found within
orogenic belts.
2. Major Rock Type – Granite
3. Average Density – 2.7 g/cm3. As a result, continental rock “floats” higher
than the denser oceanic rock.
4. Age – Continental rocks are old, some as old as 3.8 billion years. They
are found in the areas of cratons.

Ocean Basins – Ocean basins cover 70% of the Earth’s surface, with the
following common properties:
1. Major Rock Type – Basalt
2. Average Density – 3.0 g/cm3. Oceanic rock is denser than continental
rock due to the higher abundance of ferromagnesium minerals.
3. Age – The rocks of the ocean floor are young in a geologic time frame;
most are less than 150 million years old.

Affect of Internal Processes – In the 1960s, the science of geology underwent a
dramatic change that altered the basic assumption of the field. Until then, almost all
geological science has been based on the idea that the major features of the Earth (the
continents and ocean basins) remained in a fixed location. However, the Earth is an
active planet. It exhibits dynamic interaction between its inner and outer parts, which
puts the Earth’s surface into a state of lateral and vertical motion.

Continental Drift – The term continental drift applied to early theories supporting
the possibility that the continents are in motion over the Earth’s surface. In the
early 1900s, German meteorologist Alfred Wegener was the first to collect
substantial evidence in support of the theory of continental drift, though this data
did little to convince his colleagues:
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1. Jigsaw Puzzle Fit 12 – The shapes of continents fit together like pieces of a
jigsaw puzzle, suggesting that they were once joined together, and have split
and moved apart from one another. For example, the east coast of South
America fits well with the west coast of Africa.

Pangea 13, 14 – Alfred Wegener presented evidence for all the continents
having been joined in a single super-continent referred to as Pangea about
225 million years ago.

Another Pangea 15 – Computer models show that in approximately 250
million years in the future, the continents will be again in close
proximity to each other.
2. Fossil Evidence – The remains of particular species of animals and plants.
For example, fossil remains of Mesosaurus, a freshwater reptile, are found in
South America and Africa. Because of its bone structure, it would not have
been able to swim a large ocean, therefore suggesting that these two
continents were once joined as one landmass.
3. Rock Matches 16 – Rock matches between continents. For example, layers of
sandstone, shale and clay with coal seams located in Brazil, match with layers
of identical composition and structure in South Africa.
4. Paleoclimatic Evidence – Evidence from ancient climates. Glacial deposits
and other data indicate that in the Devonian period (360 to 408 million years
ago), a polar ice cap covered the Sahara, while eastern North America lay near
the equator.
5. Polar Wandering – The apparent movement of the Earth’s magnetic poles
over time with respect to the continents. Wegener found evidence indicating
that ancient poles were in different positions than the present poles. He
believed that this was due to the changing positions of the continents.


Mechanism – Alfred Wegener describe the drift of continents as rigid bodies
moving through a yielding seafloor. He believed that the forces causing
continental drift were a combination of centrifugal force from the Earth’s
rotation and the gravitational forces that causes tides. Geophysicists correctly
objected. The sea floor is rigid and not viscous and that the forces involved in
Wegener’s mechanism were insufficient to cause continental drift.
Sea-Floor Spreading – In 1962, Harry Hess proposed a new hypothesis called seafloor spreading. This hypothesis was able to explain a mechanism for continental
drift by proposing that the sea floor is moving, too 17. One theory suggests that
convection within the mantle of the Earth (behaving similar to a conveyor belt)
may be responsible for continental drift. Convection is a circulation pattern
driven by temperature differences. Hot material being less dense rises and cold
material being denser sinks 18. There is considerable evidence to support the
convection theory:
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1. Geologic Features 18 – Hot molten rock from the mantle rises and cools to
form new oceanic crust and what is referred to as an oceanic ridge. As molten
rock continues to rise beneath the ridge, the convective circulation pattern
splits and diverges near the surface. The newly formed crust moves
horizontally away from the ridge on both sides (sea-floor spreading). As the
newly formed oceanic crust continues to move horizontally away from the
oceanic ridge, it gradually cools and contracts, becoming denser. Eventually,
it becomes dense enough to sink back into the mantle (where it is melted)
forming an oceanic trench (a deep, narrow trough).
2. Paleomagnetism – Paleomagnetism is studying the Earth’s magnetic field in
the past. Small crystals of magnetic minerals in cooling molten rock act like
tiny compass needles, preserving a record of the Earth’s magnetic field 19.
Moreover, the Earth’s magnetic field has at times reversed its polarity. Such a
change in the polarity of the Earth’s magnetic field is referred to as a magnetic
reversal. (Magnetic reversals may be due to variations in circulation patterns
in the liquid outer core where the Earth’s magnetic field originates.) For this
reason, the magnetism of old rock can be measured to determine the strength
and direction of the Earth’s magnetic field in the past. In the mid 1960’s,
deviations from an average reading of the Earth’s present magnetic field were
noted within rock of the sea floor. These deviations are called magnetic
anomalies. They showed to be arranged in bands that lie parallel to an
oceanic ridge 20. Moreover, the pattern on one side of the ridge is a mirror
image of the pattern on the other side. This banded pattern of magnetic
anomalies originates from the following process: (1) There is constant
opening of tensional cracks within an oceanic ridge. (2) These cracks are
filled by molten rock from the mantle. (3) As the molten rock cools, the
magnetic minerals within the rock preserve the Earth’s magnetism at the time.
When the Earth’s magnetic field has a normal polarity (the present
orientation), the cooling rock is normally magnetized. Rock that cools when
the Earth’s field is reversed is reversely magnetized 21. (4) Sea-floor
spreading then cracks the rock in two, and the two halves are carried away in
opposite directions down the sides of the ridge. In this way, a system of
normally magnetized rock and reversely magnetized rock forms parallel to the
oceanic ridge 22.
3. Oceanic Sediments – Dated by fossil plankton they contain, sediments can be
no older than the surface on which they came to rest. Studies show that
sediment near an oceanic ridge is relatively young and becomes progressively
older moving away from the ridge. More specifically, there are no sediments
older than 200 million years on the sea floor, indicating that this sediment is
conveyed to oceanic trenches, and dragged back down into the mantle.
4. Benioff Seismic Zones 23 – A Benioff seismic zone is a distinct area of
earthquake activity that begins at an oceanic trench and slopes landward and
downward into the Earth at an angle of about 30 to 60, indicating where the
lithosphere is plunging back into the mantle.
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5. Negative Gravity Anomalies – Gravity measured over oceanic trenches is
lower than in areas adjacent to the trenches. These lower gravity values are
caused by the low-density rock making up the plunging lithosphere.
6. Direct Measurements – Lithospheric motion has been directly measured using
satellites, radar and lasers.

Plate Tectonics – The concept of plate tectonics was introduced in the late 1960s
by combining two preexisting theories, continental drift and sea-floor spreading.

Tectonics – Tectonics refers to forces within the Earth, which cause the
Earth’s crust to move. This movement may cause the Earth’s crust to deform
(change shape).

Plate 24 – Plate refers to the Earth’s lithosphere being divided into segments.
There are eight large plates plus a few dozen smaller plates. A plate may be:
(1) entirely sea floor, like the Nazca Plate, (2) entirely continental rock, such
as the small plate that roughly coincides with the country of Turkey, and (3)
both sea floor and continental rock, like the North American plate, South
American plate and African plate.

Plate Boundary Interaction – Plates are in continuous motion and their
interactions are responsible for most of the geologic activity. Though plate
interiors are relatively geologically inactive, plate boundaries can be very
active. There are three main types of plate boundary interactions 25:
1. Divergent Boundary – Where two plates are moving apart. This results in
the formation of an oceanic ridge 26.

Oceanic Divergence – Where two oceanic plates are moving apart.
For example, the Nazca and Pacific Plates. This area of divergence is
called the East Pacific Rise 27.

Continental Divergence 28 – A divergent boundary can form in the
middle of a continent in the following manner: (1) The crust thins and
a rift valley forms due to tension 29. For example, the East African Rift
Valleys 30. (2) The continent tears in two 31. Volcanic activity forms
oceanic crust. Seawater may flood into the linear basin. For example,
the Red Sea 32. (3) The new ocean widens and an oceanic ridge
develops 33. For example, the Mid-Atlantic Ocean Ridge 34.
2. Convergent Boundary – Where two plates move toward each other.

Ocean-Ocean Convergence 35 – Where two oceanic plates come
together, with one plate sliding (subducting) beneath the other. The
subduction leads to the formation of an oceanic trench. For example,
the Mariana trench 36. Magma, generated by the subduction, works its
way upward to erupt as an island arc – a curved line of volcanoes that
form a string of islands parallel to the oceanic trench.
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
Ocean-Continent Convergence 37 – Where an oceanic plate and a
continental plate come together, followed by the subduction of the
oceanic plate under the continental plate, thus forming an oceanic
trench. For example, the Peru-Chile Trench 38. Magma, generated by
the subduction can form: (1) island arcs at sea, such as the Aleutian
Islands, (2) continental volcanoes, such as the cascade volcanoes in the
Pacific Northwest, and/or (3) mountain belts, such as the Andes
Mountains in South America.

Continent-Continent Convergence 39 – Where two continental plates
come together, causing the subduction of the ocean basin between
them. After collision the denser oceanic lithosphere breaks off the
continental lithosphere and continues to sink, leaving the continent
behind. The two continents are welded together along a dipping suture
zone that marks the old site of subduction. The convergence gives rise
to the formation of a mountain belt in the interior of a continent. For
example, the Himalaya Mountains in Central Asia.
3. Transform Boundary – Where one plate slides horizontally past another
plate. The boundary is marked by a single fault or on a group of parallel
faults. For example, The San Andreas fault in California 40, 41.

Earthquakes (Seismicity) 42 – Earthquake activity (the result from the release
of stresses associated with rock breakage) has provided important evidence in
support of plate tectonics. Earthquakes are concentrated along plate
boundaries where rock is being created (oceanic ridges) or destroyed (oceanic
trenches), compressed (orogenic belts) or extended (rifts), and masses are
sliding past each other (fault lines).

Volcanism 43 – Most volcanic activity occurs along plate boundaries. There
are thousands of known volcanoes scattered over the Earth’s surface.
However, only about 800 are either active or known to be historically active.
Volcanism occurs in three tectonic settings:
1. Convergent Boundaries – Volcanism of this setting are located in the
Circum-Pacific region. This region is commonly known as the ring of
fire.
2. Oceanic Ridges – Approximately 80% of volcanic rocks are produced at
the oceanic ridges. Iceland is the most active volcanic region on Earth. It
is in a continuous state of tension as the eastern and western portions are
moved apart by sea-floor spreading.
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3. Intraplate Volcanism – Volcanic activity also occurs at locations
associated with the interior regions of lithospheric plates. Intraplate
volcanoes are immense features that may rise to about 6 miles above the
ocean floor, dimensions greater than the largest mountains on the
continents. Magmas that built these volcanoes are derived from a
relatively fixed magma source in the upper mantle called a hot spot.
Because the magma source lies beneath the plate and because the plate is
moving laterally, seamounts and/or volcanic islands are created in a linear
pattern known as a chain of seamounts and/or islands 44. For example, the
Hawaiian-Emperor chain in the Pacific 45.

Affect of External Processes – External processes are also responsible for shaping the
surface of the Earth (rounded hills and valleys).

Weathering – The natural breakdown of rocks and their minerals. The are two
main types of weathering:
1. Mechanical Weathering – The physical breakdown of rock into smaller
pieces. For example, frost action is the mechanical effects of freezing water
on rocks.
2. Chemical Weathering – The breakdown of rock resulting from exposure to
water and atmospheric gases. For example, limestone goes into solution when
exposed to carbonic acid (formed by carbon dioxide dissolving in water),
which can result in the formation of caves.

Erosion – The picking up or physical removal of rock particles over the surface of
the Earth. Erosional agents include: running water (the primary erosional agent),
glacial ice, waves, wind and/or gravity.

Deposition – The settling or coming to rest of transported material. Sediment is
deposited when running water, glacial ice, waves or wind loses energy and can no
longer transport its load.

Impacts – Like all other solid planetary bodies, the Earth was once pockmarked
with thousands of craters. The period of intense bombardment of the planets by
meteorites was an early event, and the number of impacts has decreased
exponentially during the last 4 billion years. Because of the Earth’s atmospheric
and geologic processes, most craters have been erased. However, over 100
craters have been identified, which show how impact processes modify the
Earth’s surface even now.

Barringer Crater 46 – Located near Winslow, Arizona, Barringer crater is
nearly 1 mile across and 600 feet deep. It was formed by the impact of
100,000 tons of iron about 50,000 years ago, making it the youngest crater of
its size.

Manicougan Crater 47 – Manicougan crater in northern Quebec is over 40
miles in diameter and was caused by an impact about 200 million years ago.
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Interior

Direct Study – The Earth’s interior is very inaccessible to direct study. The deepest
that people have traveled beneath the Earth’s surface is approximately 2.5 miles
(TauTona gold mine in South Africa). The deepest well ever drilled (the Kola
scientific well in Russia) has penetrated a depth of over 7.5 miles.

Remote Study 48 – Vibrational waves associated with earthquakes are our main source
of information about the Earth’s interior. The shaken Earth responds like a giant bell,
and its interior structure determines the “tones” that will be detected at different
places on the surface. By measuring the vibrations, scientists can reconstruct
different aspects of the Earth’s interior (pressure, temperature, phase differences,
various layers).

Density – The average density of the Earth is 5.52 g/cm3. This density is derived by
averaging the densities of the layers that comprise the Earth’s interior, from 3.0 g/cm3
at the surface to 12.0 g/cm3 at the core. The density of a material suggests it
composition. A low density indicates a composition of lighter elements, while a
higher density indicates a composition of heavier elements.

Differentiation 49 – Because the Earth’s interior is segregated according to
changes in density it is referred to as being differentiated. Differentiation is a
process by which the denser portions of a planetary object will sink to the center,
while less dense materials rise to the surface.

Pressure – The internal pressure of the Earth increases with increasing depth. This is
caused by the increasing amount, and therefore weight, of overlying rock. At the
center of the Earth, the pressure reaches nearly 4 million bars.

Temperature – The internal temperature of the Earth increases with increasing depth,
reaching a maximum value of greater than 9,000 F at its center. The Earth’s internal
heat is quite evident in deep mines and molten rock (lava) from volcanoes and
fissures. This heat is generated by three main sources: (1) gravitational energy
converted to heat during formation and accretion of the Earth, (2) frictional energy
from the Earth’s tides, and (3) the decay of radioactive elements of potassium,
uranium and thorium (the primary heat source of the Earth’s interior).

Layers 50 – The Earth’s interior is comprised of four primary layers:
1. Crust – The crust ranges in thickness from an average of approximately 4 miles
beneath the oceans to more than 24 miles beneath the continents.
2. Mantle – The mantle is about 1800 miles in thickness, making it the largest layer
of the Earth’s interior. The rock comprising the mantle is similar to the crust, but
of higher density. Most of the tectonic activity seen on the Earth’s surface is
caused by convection currents in the upper mantle (asthenosphere).
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3. Outer Core – The outer core is about 1200 miles in thickness. The boundary
between the mantle and core marks a sharp change in density (5.0 g/cm3 to 10.0
g/cm3). The sudden increase in density is related to a change in composition. The
outer core is believed to be composed of iron-nickel.

Magnetic Field – The outer core is thought to be the source of the Earth’s
magnetic field. Scientists believe for a planetary object to generate a
magnetic field it must possess an internal layer with three properties: (1) metal
or a material with metallic properties, (2) in a liquid phase, and (3) convective
flows caused by the planetary object’s rotation (referred to as a dynamo). The
Earth’s magnetic field forms a shield around the Earth referred to as the
magnetosphere 51.
4. Inner Core – The inner core is about 900 miles in thickness, and is associated with
the center of the Earth. Like the outer core, it is composed of iron-nickel, but due
to the higher pressure it is in a solid phase.
Life

Biosphere – The biosphere is that part of the Earth that consists of self-replicating
molecules, a property referred to as life. The biosphere continues to evolve.

Propagation – It is still not clear how life was created. However, there are some
definite criteria for the origin and continued existence of life on Earth. They
include a relatively stable surface temperature, an atmosphere of a suitable
composition, and the availability of liquid water.

Beginnings – The oldest sedimentary rocks known on Earth contain chemical
hints that life made its appearance 3.8 billion years ago. Rocks 3.5 billion years
old exhibit visible structures thought to have been formed by primitive forms of
life called stromatolites 52.

Niches – The biosphere is not distributed evenly over the Earth, but is
concentrated in various environmental niches. Most life exists in a range
extending from the depth to which sunlight penetrates the oceans (about 600 feet)
to the snow line of mountain ranges about 3.5 miles above sea level.

Diversity – The biosphere consists of about 8.7 ± 1.3 million species.
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