Lecture 4: Basic Structure of the Earth
September 10, 2013
1) The Earth is approximately 4.5 billion years old. The Earth has changed enormously
during its history and today it is very different than it was when during its
"infancy."
2) Earth history has 2 basic periods, a period of formation followed by a period of
differentiation.
3) Earth formation occurred over a period of roughly 500 million years by the
gravitational accretion of cosmic material -- mostly through meteorite impacts.
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The Earth formed as an undifferentiated
mass whose composition reflected the meteorites that accreted to form
it.
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Impacts were frequent at first, and have
tapered off as the primordial matter of the solar system has clustered
into the planets and other bodies.
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Meteorite impacts still occur, but at a
very low rate compared to the early history of the Earth.
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The study of the earliest history of the
Earth, described above, is one of the realms of astronomers and
planetary geologists.
4) Starting about 4 billion years ago, Earth became differentiated into successive
envelopes of distinct compositional layers.
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This process of differentiation was driven
largely by heat from short-lived radioactive isotopes (e.g., 26Al), and from meteorite impacts.
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This process is ongoing - "The Living
Earth".
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The study of the period of Earth
differentiation is the realm of geologists and paleontologists.
5) The differentiation of Earth produced five basic compositional layers.
From outer-most to inner-most, these are:
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ATMOSPHERE - the outermost layer; the gaseous envelope
that surrounds the Earth; this is the least dense layer of the Earth; the maximum
thickness of the atmosphere is 96 km.
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HYDROSPHERE - includes the oceans, seas, lakes,
rivers, etc., which float on the denser solid crust and saturate many porous parts of the
crust; also the porous outer parts of the crust; the depth ranges up to 6 km.
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CRUST - the thin outer shell of the solid Earth,
approximately 5 to 40 km thick.
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MANTLE - surrounds the core; is less dense than the
core; its radius is approximately 2600 km.
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CORE - located at the center of the Earth; the densest
region of the Earth; composed largely of iron and other dense metals; its radius is about
3500 km.
6) The physical properties of the inner layers of the earth change at
different depths within the Earth, due to increases in heat and pressure.
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The LITHOSPHERE
consists of the crust and the uppermost mantle, and averages 100 km in
thickness.
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Below the lithosphere, the outer-most layer
of the mantle is known as the ASTHENOSPHERE. This is a molten or partly molten layer which churns
in slowly rolling convection currents whose motion is driven by heat moving outward from
the core.
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The MESOSPHERE is beneath the asthenosphere and is a
region of high temperature and rock strength.
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The metallic core is separated into two
parts. The OUTER CORE
is zone in which temperature and pressure are balanced in such a way that the iron is
molten and exists as a liquid. Currents of liquid movement within the outer core produce a
magnetic field whose flux reaches outwards beyond the Earth's surface.
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The INNER CORE
is under such great pressure that the iron is solid despite the enormous
temperature.
7) The lithosphere is not a smooth, continuous 'skin' around the Earth; it instead is
formed from a series of semi-rigid tectonic plates.
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The continuous flow of heat outward from
the core and inner mantle cause convection of the asthenosphere. This in turn causes movement and deformation of the overlying
tectonic plates.
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Plate movement is driven by the extrusion
of magma from spreading zones located in the floors of the oceans.
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In areas where crustal plates collide, the plate margins are deformed, often
producing mountains and deep oceanic trenches that are associated with volcanoes and
earthquakes.
8) The continued production of heat by radioactive decay in the core and inner mantle
has resulted in continuous evolution of the Earth and change in the structure and
composition of its layers.
9) Through tectonic movement, throughout the period of Earth differentiation, there has
been continual remodeling of the surface of the Earth.
10) TECTONICS is the study of the formation of Earth's
major structural features by thermal convection in the asthenosphere and subsequent
deformation of the lithosphere.
11) The appearance of Life about 4 billion years ago has also changed many aspects of
the crust, hydrosphere, and atmosphere.
12) Life history has been played out on the lithosphere, and in the hydrosphere and
atmosphere. It is profoundly influenced by processes occurring in the asthenosphere and
deeper layers of the mantle and core. Earth history and Life history are thus intricately
linked.
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