The Earth's crust is solid matter. Both oceanic and continental crusts are made entirely of rock. The former is primarily composed of denser material such as basalt, and it is, on average, 4 miles thick.
In contrast, the continental crust is generally made up of lighter rock such as granite. It varies from 6 to 47 miles in thickness. When the two types of crust meet, the continental crust, being the lighter of the two, always forces the heavier oceanic crust down; this process is known as subduction. Under the continental crust, the oceanic crust enters the mantle of the planet, liquefying slightly as it turns into a semisolid form known as magma.