There are two types of crust, oceanic and continental. As you might suspect from the names, oceanic crust is usually found under the oceans, and continental crust is found on the continents. Oceanic crust is made primarily of a rock called basalt . It contains almost as much iron and magnesium as the mantle rock. Oceanic crust also contains thin layers of limestone , serpentine , and clay .
Limestone is made of calcium and carbon dioxide.
Serpentine and clay are rocks like basalt that contain parts of water molecules. Continental crust is made mostly of granite , a rock that contains smaller amounts of iron and larger amounts of quartz . An important fact about continental crust is that it is less dense than basalt or mantle rocks. Specifically, parts of Earth bend and flow when pressure is applied, and other parts fracture and break. This way of looking at Earth helps us understand where earthquakes, volcanoes, and mountain ranges are found.