The outermost layer of the earth is called the crust. It is much thinner than all other layers and is made up of two parts -- the oceanic crust and the continental crust. The continental crust is composed of granitic rocks that contain a large amount of aluminum silicates. The denser oceanic crust found beneath the earth's oceans is composed of basaltic rocks that contain less silica and aluminum and more iron and magnesium than the continental crust.
The mantle is just below the crust and is the thickest layer, nearly 3,000 kilometers deep. It is composed mostly of iron and magnesium silicates. The upper mantle is made up of dense rocks that are very hot. These rocks flow under stress as a result of the large temperature difference -- 1,600 degrees Fahrenheit to 4,000 degrees Fahrenheit -- between the top and bottom parts of the mantle. This flow is the reason continental plates move.
Earth's outer core, which lies just below the mantle, is composed of liquid Iron, nickel and other materials. The metals are in a molten state due to relatively low pressures -- compared to the inner core -- and very high temperatures that vary from 4,000 degrees Fahrenheit to 9,000 degrees Fahrenheit. The outer core is about 1,400 miles deep and lies about 1,800 miles below the earth's crust.
The inner core, like the outer core, is also composed of iron and nickel. Even though it is hotter than the outer core, the metals remain solid. This is because of the extremely high pressures in the inner core that keep the metal molecules confined within a very small volume where they cannot move about like the molecules in a liquid. The pressure in the inner core is three million times that of atmospheric air pressure.