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SGES 1302
INTRODUCTION
TO EARTH SYSTEM
LECTURE 3: Internal Structures of the Earth
Lecture 3
INTERNAL STRUCTURES OF THE EARTH
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Internal Structure
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The interior of the Earth, similar to the other terrestrial planets, is chemically
or compositionally divided into layers.
The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid
outer core that is much less viscous than the mantle, and a solid inner core.
Many of the rocks now making up the Earth's crust formed less than 100
million years ago (Ma); however the oldest known mineral grains are 4.4
billion years old, indicating that the Earth has had a solid crust for at least
that long.
Much of what is known about the interior of the Earth has been inferred.
The force exerted by Earth's gravity is one measurement of its mass.
After measuring the volume of the planet, its density can be calculated.
Calculation of the mass and volume of the surface rocks and bodies of
water allow estimation of the mass, volume and density of surface rocks.
The mass which is not in the atmosphere, oceans, and surface rocks must
be in deeper layers.
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Internal Structure
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Atmosphere, ocean and land surface can be studied directly.
Volcanic eruptions provided materials from depth (down to 200km)
Earth’s interior – inferred from its density, the way it transmit seismic waves
and the nature of its magnetic field.
The layering of the Earth has been inferred indirectly using the time of travel
of refracted and reflected seismic waves. The core does not allow shear
waves to pass through it, while the
seismic velocity is different in the
other layers. The changes in the
seismic velocity between the different
layers causes refraction, which is
described by Snell's law. Reflections
are caused by a large increase in
seismic velocity and are similar to light
reflecting from a mirror.
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Internal Structure
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The structure of the Earth can be
divided using 2 ways: based on
chemical composition and based on
physical properties.
Chemically, the Earth can be divided
into the crust, mantle, outer core, and
inner core.
By physical properties, the layering of
the earth is categorized as lithosphere,
asthenosphere, mesosphere, outer
core, and the inner core.
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Schematic view of the
interior of Earth.
1. continental crust
2. oceanic crust
3. upper mantle
4. lower mantle
5. outer core
6. inner core
A: Mohorovičić discontinuity
B: Gutenberg discontinuity
C: Lehmann discontinuity
0 35
70
2880
5160
6370 km
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Structural units based on composition:
crust, mantle & core
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The crust ranges from 5 to 70 km in depth.
The thin (~8km) parts are oceanic crust composed of dense iron
magnesium silicate (mafic) rocks and underlie the ocean basins.
The thicker (~40km) crust is continental crust, which is less dense
and composed of (felsic) sodium potassium aluminium silicate rocks.
The oceanic crust is relatively young and undeformed.
The crust-mantle boundary is marked by a discontinuity in the
seismic wave velocity, which is known as the Mohorovičić
discontinuity or Moho.
The cause of the Moho is thought to be a change in rock
composition from rocks containing plagioclase feldspar (above) to
rocks that contain no feldspars (below).
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Structural units based on composition:
crust, mantle & core
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Earth's mantle extends to a depth of 2890 km, making it the largest layer of
the Earth.
The pressure, at the bottom of the mantle, is ~140 GPa (1.4 Matm). The
mantle is composed of silicate rocks that are rich in iron and magnesium
relative to the overlying crust.
Although solid, the high temperatures within the mantle cause the silicate
material to be sufficiently ductile (behaves like plastic) that it can flow on
very long timescales.
Convection of the mantle is expressed at the surface through the motions of
tectonic plates.
The melting point and viscosity of a substance depends on the confining
pressure. As there is intense and increasing pressure as one travels deeper
into the mantle, the lower part of the mantle flows less easily than the upper
mantle.
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Structural units based on composition:
crust, mantle & core
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The average density of Earth is 5515 kg/m3, making it the densest planet in
the Solar system.
Since the average density of surface material is only around 3000 kg/m3, we
must conclude that denser materials exist within Earth's core.
Further evidence for the high density core comes from the study of
seismology.
In its earliest stages, about 4.5 billion years ago, melting would have caused
denser substances to sink toward the center in a process called planetary
differentiation, while less-dense materials would have migrated to the crust.
As a result, the core is largely composed of iron (80%), along with nickel
and one or more light elements, whereas other dense elements, such as
lead and uranium, either are too rare to be significant or tend to bind to
lighter elements and thus remain in the crust.
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Structural units based on composition:
crust, mantle & core
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Seismic measurements show that the core is divided into two parts, a
solid inner core with a radius of ~1220 km and a liquid outer core
extending beyond it to a radius of ~3400 km.
The solid inner core was discovered in 1936 by Inge Lehmann and is
generally believed to be composed primarily of iron and some nickel.
The liquid outer core surrounds the inner core and is believed to be
composed of iron mixed with nickel and trace amounts of lighter
elements.
It is generally believed that convection in the outer core, combined
with stirring caused by the Earth's rotation, gives rise to the Earth's
magnetic field through a process described by the dynamo theory.
The solid inner core is too hot to hold a permanent magnetic field but
probably acts to stabilise the magnetic field generated by the liquid
outer core.
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Structural units based on composition:
crust, mantle & core
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Why is the inner core solid, the outer core liquid, and the mantle
solid/plastic?
The answer depends both on the relative melting points of the different
layers (nickel-iron core, silicate crust and mantle) and on the increase in
temperature and pressure as one moves deeper into the Earth.
At the surface both nickel-iron alloys and silicates are sufficiently cool to be
solid.
In the upper mantle, the silicates are generally solid (localised regions with
small amounts of melt exist); however, as the upper mantle is both hot and
under relatively less pressure, the rock in the upper mantle has a relatively
low viscosity (behaves like plastic).
In contrast, the lower mantle is under very high pressure and therefore has
a higher viscosity than the upper mantle.
The metallic nickel-iron outer core is liquid despite the enormous pressure
as it has a melting point that is lower than the mantle silicates.
The inner core is solid due to the overwhelming pressure found at the
center of the planet.
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Layers based on physical properties:
lithosphere, asthenosphere, mesosphere & core
Tectonic plates of the lithosphere
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Layers based on physical properties:
lithosphere, asthenosphere, mesosphere & core
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The lithosphere is the solid outermost shell of a rocky planet.
On the Earth, the lithosphere includes the crust and the uppermost mantle
which is joined to the crust across the Mohorovičić discontinuity.
The base of lithosphere is defined as the 1280°C isotherm in the mantle:
lithosphere-asthenosphere boundary is a thermal boundary that vary in
space and time (at that temperature, olivine the dominant mineral in the
mantle becomes very weak)
The 1280°C isotherm is only a few km deep below mid ocean ridge,
beneath oceanic plains is about 100 km, and beneath continents may be
more than 150 km.
As the Earth's surface cools, the lithosphere thickens over time. It is
fragmented into tectonic plates, which move independently relative to one
another.
All crust is in the lithosphere, but lithosphere generally contains more
mantle than crust.
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Layers based on physical properties:
lithosphere, asthenosphere, mesosphere & core
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The concept of the lithosphere as Earth’s strong outer layer was developed by Barrell
(1914). The concept was based on the presence of significant gravity anomalies over
continental crust, from which he inferred that there must be a strong upper layer
(which he called the lithosphere) above a weaker layer which could flow (which he
called the asthenosphere).
These ideas were enlarged by Daly (1940), and have been broadly accepted by
geologists and geophysicists.
Although these ideas about lithosphere and asthenosphere were developed long
before plate tectonic theory was formulated in the 1960's, the concepts that strong
lithosphere exists and that the lithosphere rests on weak asthenosphere are essential
to the plate tectonic theory.
Another distinguishing characteristic of the lithosphere is its flow properties. Under
the influence of the low-intensity, long-term stresses that drive plate tectonic motions,
the lithosphere responds essentially as a rigid shell and thus deforms primarily
through brittle failure, whereas the asthenosphere is heat-softened and deforms
plastically.
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Layers based on physical properties:
lithosphere, asthenosphere, mesosphere & core
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There are two types of lithosphere:
 Oceanic lithosphere, which is associated with Oceanic crust; &
 Continental lithosphere, which is associated with Continental crust
Oceanic lithosphere is typically about 50-100 km thick (but beneath the midocean ridges is much thinner), while continental lithosphere is about 150 km
thick, consisting ~50 km of crust and 100 km or more of uppermost mantle.
Oceanic lithosphere is denser than continental lithosphere. The upper
mantle portion of both types of lthosphere has the same density, however,
the oceanic crust has higher density than the continental crust.
New oceanic lithosphere is constantly being produced at mid-ocean ridges
and is recycled back to the mantle at subduction zones. As a result, oceanic
lithosphere is much younger than continental lithosphere: the oldest oceanic
lithosphere is about 170 million years old, while parts of the continental
lithosphere are billions of years old.
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Layers based on physical properties:
lithosphere, asthenosphere, mesosphere & core
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The asthenosphere is the region of the Earth between 100-200 km
below the surface (1280°C isotherm) and extending to as deep as
400 km - that is the weak or "soft" zone in the upper mantle.
It lies just below the lithosphere, which is involved in plate
movements and isostatic adjustments. In spite of its heat, pressures
keep it plastic, and it has a relatively low density.
Seismic waves, the speed of which decrease with the softness of a
medium, pass relatively slowly through the asthenosphere, thus it
has been given the name low-velocity zone.
Under the thin oceanic plates the asthenosphere is usually much
nearer the seafloor surface, and at mid-ocean ridges it rises to within
a few kilometres of the ocean floor.
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Layers based on physical properties:
lithosphere, asthenosphere, mesosphere & core
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The upper part of the asthenosphere is believed to be the zone upon which
the great rigid and brittle lithospheric plates of the Earth's crust move about.
Due to the temperature and pressure conditions in the asthenosphere, rock
becomes ductile, moving at rates of deformation measured in cm/yr over
distances eventually measuring thousands of kilometers.
The mesosphere refers to the lower mantle in the region between the
asthenosphere and the outer core.
It is the largest layer of the earth. This region, also called the lower mantle,
is named in order to differentiate from the lithosphere and the
asthenosphere portions of the mantle.
It is more solid or rigid than the asthenosphere due to higher pressures.
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