You feel the earth unsteady beneath your feet, shifting and shaking. It is an earthquake! That is what happens when rocks in the lithosphere have been stressed too much and break. The lithosphere is the rocky layer that covers the entire earth, both continents and oceans. It has two parts: the crust and the upper mantle.

The crust varies in thickness. Under the oceans it is only 3 to 5 miles deep, but the continental crust extends about 25 miles. At the surface the crust is air temperature, but at its deepest parts it can reach 1,600 degrees Fahrenheit. The most common elements in the rocky layer are oxygen, silicon and aluminum.

Below the crust, the top layer of the upper mantle is also part of the lithosphere. With the crust and mantle section combined, the lithosphere is about 50 feet deep. In addition to oxygen and silicon, the upper mantle also contains significant amounts of iron and magnesium. This part of the lithosphere is denser than the crust.