is a planet made up of three main shells:
A very thin crust, "the mantle" and the core.
The mantle and core are both divided into two parts. The core and
mantle are roughly equal in thickness. However the core is only 15
percent of the Earth's volume, while the mantle is 84 percent. (The
crust makes up the last 1 percent).
we have learned of the structure and composition of the Earth comes
from both laboratory and field work. We study the paths and characteristics
of earthquake waves. In laboratory we experiment on surface minerals
and rocks at high pressure and temperature. We have learned in great
detail about the materials of the planet's surface, it's motion in
the Solar System, its gravity and magnetic fields, and the flow of
heat from inside the Earth.
Because it is accessible the geology of
the crust is well studied and and therefor more is known about its
structure and composition than of the other shells. Patterns are created
when rocks are distributed and deposited in layers by eruption, solidification,
erosion, consolidation of rock particles and by crystallization of
porous rock. Roughly twelve plates (continents and ocean basins) have
moved around on the Earth's surface throughout much of the planet's
history. At the plate edges concentrations of earthquakes and volcanoes
occur. When plates collide mountains ranges like the Himalayas occur.
Plates include the crust and part of the upper mantle. They move
over the upper mantle very slowly, only a few centimeters per year.
The crust is thinner under the oceans than under continents.
The boundary between the crust and mantle is called the
Mohorovicic discontinuity (sometimes just Moho). This boundary
cannot be seen but it is detected by a sharp change in the speed of
earthquake waves there.
Our knowledge of the upper mantle is indirect.
We learn about it from analyses of earthquake waves, heat flow, magnetic
and gravity studies. We can also carry out laboratory experiments
on the rocks and minerals we believe to be there. Somewhere between
100 and 200 kilometers below the surface the temperature reaches the
melting point of rock. When molten rock eruptes from some volcanoes
this where it comes from. We believe that it is the upper mantle that
carries the tectonic plates. Below this is a transition zone. There
are two discontinuities caused by changes in density of minerals.
The forms of these minerals are shown by laboratory experiments at
high pressure and temperature. The Lower Mantle is believed to be
made up of simple iron and magnesium silicate minerals.
The core was the first
internal structure to be identified. It helped to explain the
Earth's density. The outer core is assumed to be liquid because it
does not transmit shear waves and the speed of compressional waves
slows sharply. The inner core is thought to be solid because of the
way those waves passing through it. Shear waves do not travel
through the core but can be converted to compressional waves or
they can be reflected. Together with rotations and inertia of the
whole Earth, laboratory experiments on iron under pressure and
magnetic-field dynamo theory we have built up a picture of the
inner and outer core. We believe that the core is composed
principally of iron with about 10 percent of some forms of oxygen,
sulfur and/or nickel.