Earth Science 8.4 Earth’s Layered Structure
Earth’s
Layered
Structure
Earth Science 8.4 Earth’s Layered Structure
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Compared to the planets we
see in the sky, Earth’s
interior is close to us.
However, we can not reach
it.
The deepest well drilled by
modern man is only 12
kilometers below the Earth’s
surface.
We know what we know
about earth’s interior largely
from our study of seismic
waves that travel through
the earth.
Earth Science 8.4 Earth’s Layered Structure
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If Earth’s materials had the same
properties in each layer, seismic
waves would spread through it in
straight lines at constant speed.
This is not always the case.
Sometimes, seismic waves reach
seismographs located far away
from an epicenter faster than
locations close to an event.
Earth Science 8.4 Earth’s Layered Structure
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This increase in speed with depth
is due to an increase in pressure
deep down which changes the
elastic properties of the deeper
rock layers.
As a result, the paths of the
seismic waves through the earth
are refracted, or bent, as they
travel.
Earth’s interior consists of three
major layers defined by their
chemical composition: the crust,
the mantle, and the core.
Earth Science 8.4 Earth’s Layered Structure
Earth’s Crust:
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Earth’s crust, the thin rocky
outermost layer, is divided into
oceanic crust and continental
crust.
The oceanic crust is roughly 7
kilometers thick and composed of
igneous rocks basalt and gabbro.
The continental crust is 8 – 75
kilometers thick ( but averages at
40 kilometers thick) and is made
of several rock types.
Earth Science 8.4 Earth’s Layered Structure
Earth’s Crust:
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The average composition of the
continental crust is granitic rock
called granodiorite.
Continental rocks have an average
density of about 2.7 -2.8
Continental crust rocks are over 4
billion years old in some areas.
The rocks of the oceanic crust are
younger (180 million years or less)
Earth Science 8.4 Earth’s Layered Structure
Earth’s Mantle:
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Over 82 % of earth’s volume is
contained in the mantle; a rocky
shell that extends to a depth of
2890 kilometers.
The boundary between the crust
and mantle represents a change in
the chemical composition.
A common rock type in the
uppermost layers of the mantle is
peridotite, which has a density of
about 3.4
Earth Science 8.4 Earth’s Layered Structure
Earth’s Core:
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Earth’s core is a
sphere composed
mostly of iron-nickel
alloy. At the extreme
pressures found in the
center core, the ironrich material has an
average density of 13 (
13 times denser than
water)
Earth Science 8.4 Earth’s Layered Structure
Layer defined by
Physical properties:
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Earth’s interior has gradual
increases in temperature,
pressure, and density with
each increase in depth.
When a substance is
heated, the transfer of
energy increases the
vibrations of particles.
If the temperature
exceeds the melting point,
the forces between
particles are overcome and
melting begins.
Earth Science 8.4 Earth’s Layered Structure
Layer defined by
Physical properties:
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Earth can be divided into
layers based on the physical
properties of each layer:
 The lithosphere
 The asthenosphere
 The lower mantle
 The outer core
 The inner core
Earth Science 8.4 Earth’s Layered Structure
Layer defined by
Physical properties:
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Lithosphere:
 Earth’s outermost
layer consists of the
crust and the
uppermost mantle
and forms a cool,
rigid shell called the
lithosphere. This
layer averages about
100 kilometers in
thickness.
Earth Science 8.4 Earth’s Layered Structure
Layer defined by Physical
properties:
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Asthenosphere
 Beneath the lithosphere lies
a soft, comparatively weak
layer known as the
asthenosphere.
 Within the asthenosphere,
the rocks are close enough
to their melting point that
they are easily deformed.
 The asthenosphere is weaker
because the rock is near
melting point; just like hot
wax is weaker than cold wax.
 The lower lithosphere and
asthenosphere are both part
of the upper mantle.
Earth Science 8.4 Earth’s Layered Structure
Layer defined by
Physical properties:
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Lower Mantle:
 From a depth of about
660 kilometers down
to near the base of
mantle lies a more
rigid layer called the
lower mantle.
 Despite their
increased strength,
the rocks of the lower
mantle are still very
hot and capable of
gradual flow.
Earth Science 8.4 Earth’s Layered Structure
Layer defined by
Physical properties:
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Inner and Outer Core:
 The core, which is
composed of an iron-nickel
alloy, is divided into two
regions with different
physical properties.
 The outer core is a liquid
layer 2260 kilometers
thick. The flow of metallic
iron within this zone
generates the Earth’s
magnetic fields.
Earth Science 8.4 Earth’s Layered Structure
Layer defined by
Physical properties:
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Inner and Outer Core:
 The inner core is a
sphere having a radius
of about 1220
kilometers.
 Despite it’s high
temperatures, the
materials in the inner
core are compressed
into a solid state by the
immense pressure.
Earth Science 8.4 Earth’s Layered Structure
Discovering Earth’s
Layers:
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Recall that seismic waves
bend as they travel
through Earth.
During the 20th century,
studies of P waves and S
waves through earth
helped scientists identify
the boundaries of earth’s
layers and determine that
the outer core is liquid.
Earth Science 8.4 Earth’s Layered Structure
Discovering Earth’s
Layers:
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In 1909, Croatian seismologist
Andrija Mohorovicic first
presented evidence proving
Earth’s layering.
By studying seismic records, he
found that the velocity of
seismic waves increased
abruptly about 50 kilometers
below eastern Europe.
This boundary separates the
crust from the underlying
mantle and is called the
Mohorovicic discontinuity. The
name of this boundary is usually
shortened to the Moho.
Earth Science 8.4 Earth’s Layered Structure
Discovering Earth’s
Layers:
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Another boundary had been
discovered in 1906 between the
mantle and the core. Seismic
waves from even small
earthquakes can travel around
the world.
That is why a seismograph in
Antarctica can record
earthquakes that occurred
beneath California.
It was observed that P waves
were bent around the liquid
outer core. This zone where the
P waves can not reach with
direct waves is called the “P
wave shadow zone”
Earth Science 8.4 Earth’s Layered Structure
Discovering Earth’s
Layers:
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Where the “shadow zone”
exists the P waves still
reach but they are indirect
waves, bounced once
before arriving in the
shadow zone.
Earth Science 8.4 Earth’s Layered Structure
Discovering Earth’s
Layers:
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Like sound echoing off a
wall to reach around a
corner; the P waves are
refracted (bounced) off
the Earth’s crust before
they pass into the “shadow
zone” .
Inside the shadow zone,
only refracted P waves can
be sensed.
Earth Science 8.4 Earth’s Layered Structure
Discovering Earth’s
Layers:
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It was also shown that S
waves did not travel
through the core at all.
From this information, it
was concluded by
scientists that the outer
core was liquid.
Earth Science 8.4 Earth’s Layered Structure
Discovering Earth’s
Composition:
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To determine the
composition of earth’s
layers; scientists
studied
 seismic data,
 rock samples from the
crust and mantle,
 meteorites,
 and high pressure
experiments on earth
materials.
Earth Science 8.4 Earth’s Layered Structure
Discovering Earth’s
Composition:
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Scientists obtain data on
rocks by performing highpressure experiments. Small
samples of rock and metal
are heated and squeezed to
the same conditions found
deep within Earth’s interior.
Seismic data and rock
samples from drilling
indicate that the continental
crust is mostly made of low
density granitic rock.
Earth Science 8.4 Earth’s Layered Structure
Discovering Earth’s
Composition:
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Until the late 60s, scientists
had only seismic evidence to
determine the oceanic crust
makeup. Today, deep-sea
drilling platforms make it
possible to obtain rock
samples from beneath the
ocean floor.
The crust beneath the ocean
floor has a basalt
composition.
Earth Science 8.4 Earth’s Layered Structure
Discovering Earth’s
Composition:
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The composition of the
rocks of the mantle and core
is known from indirect data.
Some of the lava that
reaches Earth’s surface
comes from the partially
melted asthenosphere within
the mantle. In the
laboratory, experiments
show that partially melting
the rock called peridotite
produces something very
similar to the lava that
erupts from Hawaii.
Earth Science 8.4 Earth’s Layered Structure
Discovering Earth’s
Composition:
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Surprisingly, meteorites
that collide with Earth
provide evidence of earth’s
inner composition.
Meteorites are assumed to
be made of the original
material of which the Earth
was formed.
Their composition ranges
from metallic meteorites
made of iron and nickel to
stony meteorites made of
dense rock similar to
peridotite.
Iron meteorities