Earth's crust, the outermost shell, is made of a broad mixture of rock types which solidified billions of years ago, soon after the earth formed. There are two kinds of crusts. The first is continental crust, which lies above the water. The second is oceanic.
Oceanic crust is more active than continental, but it is thinner.
There are also smaller plates within these major plates. Plates drift about the globe at about 1 to 10 cm per year, and over long periods of time, they change in size and shape. They may be added to, crushed together, or pushed back into the mantle. Plates can be 50-250 miles thick. Sea levels also changes over time, exposing more or less crust.