Earth's Crust

The Earth's crust (also known as the outer crust or upper crust) is the outermost solid layer of the Earth. It rests on the mantle—the layer of rock beneath it. The boundary between the crust and mantle is marked by a change in seismic wave velocity; this boundary is known as the Moho (Mohorovičić discontinuity).

Relative to the Earth's total size, the crust is extremely thin: it represents less than one percent of the planet's total volume. Nevertheless, it is the part humans interact with directly—all extracted minerals, groundwater, and the foundations of buildings rest on or within the crust.

Types of crust

There are two types of crust. Continental crust is thicker, less dense by volume, and consists mostly of granite-like rocks. Oceanic crust is thinner, denser, and is composed mainly of basalt. Because of these differences, continental crust floats higher, while oceanic crust sinks when the two collide.

Composition

The most abundant elements in the crust are oxygen and silicon, followed by aluminum, iron, calcium, sodium, potassium, and magnesium. Most rocks are silicates—minerals composed of silicon and oxygen combined with other elements.

Movement and renewal

The crust and the upper mantle together form a rigid layer called the lithosphere, which is divided into plates that move. Where plates separate—as at mid-ocean ridges—new oceanic crust forms from rising magma. Where they collide, one plate descends (subduction) into the mantle. For this reason, oceanic crust is generally much younger than the oldest rocks of the continental crust.