I: Earth Layers The
size of the Earth -- about 12,750 kilometers (km)
in diameter-was known by the
ancient Greeks, but it was not until the turn
of the 20th century that scientists determined
that our planet is made up of three main layers:
crust, mantle, and core. The crust, the outermost
layer, is rigid and very thin compared with the
other two. Below the crust is the mantle, a dense,
hot layer of semi-solid rock approximately 2,900
km thick. The mantle, which contains more iron,
magnesium, and calcium than the crust, is hotter
and denser because temperature and pressure inside
the Earth increase with depth. At the center of
the Earth lies the core, which is nearly twice
as
dense as the mantle because its composition is
metallic (iron-nickel alloy) rather than stony.
The Earth's core is made up of two distinct parts:
a 2,200 km-thick
liquid outer core and a 1,250 km-thick solid inner
core. As the Earth rotates, the liquid outer core
spins, creating the Earth's magnetic field.
Not
surprisingly, the Earth's internal structure influences
plate tectonics. The upper part of the mantle
is cooler and more rigid than the deep mantle;
in many ways, it behaves like the overlying crust.
Together they form a rigid layer of rock called
the lithosphere (from lithos, Greek for stone).
Averaging at least 80 km in thickness over much
of the Earth, the lithosphere has been broken
up into the moving plates that contain the world's
continents and oceans. Scientists believe that
below the lithosphere is a relatively narrow,
mobile zone in the mantle called the asthenosphere
(from asthenes, Greek for weak). This zone is
composed of hot, semi-solid material, which can
soften and flow after being subjected to high
temperature and pressure over geologic time. The
rigid lithosphere is thought to "float"
or move about on the slowly flowing asthenosphere.
Inside Planet Earth
[
inside earth interactive graphic ]
[
outer earth layers graphic ]
[
general earth layers plus temperatures and depth]
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