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Teaching Plate Tectonics »
Earth's Internal Structure
Earth's Internal Structure
Crust - Mantle - Core
Three Parts of Earth's Interior:
A knowledge of earth's interior is essential for understanding
plate tectonics. A good analogy for teaching about earth's
interior is a piece of fruit with a large pit such as a peach
or a plum. Most students are familiar with these fruits and
have seen them cut in half. In addition the size of the features
are very similar.
If we cut a piece of fruit in half we will see that it is composed of
three parts: 1)
a very thin skin, 2) a seed of significant size located in
the center, and 3) most of the mass of the fruit being contained
within the flesh. Cutting the earth we would see: 1) a very
thin crust on the outside, 2) a core of significant size in
the center, and 3) most of the mass of the Earth contained
in the mantle.
Earth's Crust: There are two different types of crust: thin
oceanic crust that underlies the ocean basins and thicker
continental crust that underlies the continents. These two
different types of crust are made up of different types
of rock. The thin oceanic crust is composed of primarily
of basalt and the thicker continental crust is composed
primarily of granite. The low density of the thick continental
crust allows it to "float" in high relief on the much higher
density mantle below.
Earth's Mantle: Earth's mantle is thought
to be composed mainly of olivine-rich rock. It has different
temperatures at different depths. The temperature is lowest
immediately beneath the crust and increases with depth.
The highest temperatures occur where the mantle material
is in contact with the heat-producing core. This steady
increase of temperature with depth is known as the geothermal
gradient. The geothermal gradient is responsible for different
rock behaviors and the different rock behaviors are used
to divide the mantle into two different zones. Rocks in
the upper mantle are cool and brittle, while rocks in the
lower mantle are hot and soft (but not molten). Rocks in
the upper mantle are brittle enough to break under stress
and produce earthquakes. However, rocks in the lower mantle
are soft and flow when subjected to forces instead of breaking.
The lower limit of brittle behavior is the boundary between
the upper and lower mantle.
Earth's Core: Earth's Core is
thought to be composed mainly of an iron and nickel alloy.
This composition is assumed based upon calculations of its
density and upon the fact that many meteorites (which are
thought to be portions of the interior of a planetary body)
are iron-nickel alloys. The core is earth's source of internal
heat because it contains radioactive materials which release
heat as they break down into more stable substances.
The core is divided into two different zones. The
outer core is a liquid because the temperatures there are
adequate to melt the iron-nickel alloy. However, the inner
core is a solid even though its temperature is higher than
the outer core. Here, tremendous pressure, produced by the
weight of the overlying rocks is strong enough to crowd
the atoms tightly together and prevents the liquid state.
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