The Layers of Earth (student reading)

Most of the Earth's rocky surface -the crust -- is just a thin layer on the Earth's surface. It makes up the continents and the ocean floors. It is thinnest under the oceans (6-11 km) and thickest under the largest mountain ranges (up to 70 km).

You can get a sense of how thin the rocky crust is by drawing a large circle; the Earth's crust is proportionately even thinner than the width of the circle's chalk line. You can also peel an apple - in this case the thin peel represents the crust.

Continental crust and oceanic crust differ. The continental crust is 33 km thick and light, so it floats on the lower layers. It is made up largely of granite and sedimentary layers. It is much deeper under mountains. The oceanic crust is thinner (10 km), more dense, and made of basalt, a darker volcanic mineral.

From USGS

The crust plates slide slowly about on a much thicker thick layer of rock called the mantle. The mantle is about 2,900 km thick and makes up about 80% of the planet's entire volume. How do scientists know when they have reached the mantle? The rock in the mantle is so hot that it is often partially molten.

In a volcanic explosion, or in a situation in which rocks crack, material comes from the mantle through the crust and cools on the surface.

Under the mantle in the Earth's core is a dense ball of elements such as nickel and iron. Temperatures within the core reach 3,700 degrees C.

lithos is Greek for "rock" or "stone," and sphairais Greek for "globe"

How Do Scientists Know? Listening to Waves as They Pass Through Earth Materials

When there is an earthquake, scientists have an opportunity to listen to the way the energy waves of the earthquake travel through different parts of the Earth's crust, mantle, and core. Adding waves together from many earthquakes, they generate a three-dimensional model of the Earth.

One type of seismic wave can only move through solid rock and not through liquid. It moves particles up and down or side to side.

Another type of wave generated by an earthquake moves in the direction it is being "pushed". This wave goes through both liquids and solids.

Sonar can generate waves through the Earth, too, for nearby materials.

http://www.geo.mtu.edu/UPSeis/making.html