The difference between asthenosphere and
lithosphere is how the materials in these layers
can flow. Rocks in the lithosphere are "rigid",
meaning that they can bend but they cannot flow.
Rocks in the asthenosphere are "plastic",
meaning that they can flow in response to
deformation. Even though it can flow, the
asthenosphere is still made of solid (not
liquid) rock; you can think of it kind of like
Silly Putty. What determines whether rocks act
in a rigid manner (bending or breaking) or a
plastic manner (flowing) is temperature. Deep
in the Earth, hot rocks (above about 1300°C) can
flow, whereas cold rocks cannot. The lithosphere
is broken up into rigid plates that ride on top
of the flowing asthenosphere. In terms of
chemical composition, there is no difference
between the upper part of the asthenosphere and
the lower part of the lithosphere. In fact, if
the upper part of the asthenosphere cools down
it becomes part of the lithosphere.
In addition to layers with different
mechanical properties (lithosphere vs.
asthenosphere), we can talk about layers with
different chemical composition. The outer most
layer of the Earth is the crust, which varies in
thickness from about 7-70 km. Below that is the
mantle, which of made up of denser rocks than
the crust. At a depth of almost 3000 km, you
reach the core, which is made of iron and nickel
and is even denser than the mantle. The
lithosphere is made up the crust plus the very
upper part of the mantle, whereas the
asthenosphere is only upper mantle material.
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