(Courtesy of USGS, found online at http://pubs.usgs.gov/publications/text/inside.html)
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. This layered structure can be compared to that of a
boiled egg. The crust, the outermost layer, is rigid and very thin compared
with the other two. Beneath the oceans, the crust varies little in thickness,
generally extending only to about 5 km.
The thickness of the crust beneath continents is much more variable
but averages about 30 km; under large mountain ranges, such as the Alps or the
Sierra Nevada, however, the base of the crust can be as deep as 100 km. Like
the shell of an egg, the Earth's crust is brittle and can break.
Cutaway views showing the internal structure of the Earth. Below:
This view drawn to scale demonstrates that the Earth's crust literally is only
skin deep.
Below right: A view not drawn to scale to show the Earth's three main layers
(crust, mantle, and core) in more detail (see text).
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. As a comparison, the mantle might be thought
of as the white of a boiled egg. 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. Unlike the yolk of an egg, however, the Earth's core
is actually 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). The lithosphere
tends to be thinnest under the oceans and in volcanically active continental
areas, such as the Western United States. 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.