Crust, in geology, the outermost solid layer of Earth’s lithosphere. It extends from the planet’s surface to a depth of 20–70 km (12–43 miles). The crust is underlain by Earth’s mantle. The thickness of the crust varies considerably, being thickest (35–40 km [22–25 miles]) under continental landmasses and thinnest (5–10 km [3–6 miles]) under oceans. The composition of the crust also differs according to location: it is composed of different types of rocks—granite, basalt, and so on—under continents; whereas oceanic crust consists primarily of dense iron- and magnesium-rich rocks such as basalt and gabbro.
Earth’s crust is not a single continuous shell but is broken into a number of large tectonic plates that move relative to one another on the planet’s surface. Earthquakes occur at locations where plates interact, such as at plate boundaries. At these boundaries three main types of interaction are possible: two plates may collide with each other; one plate may slide underneath another (a process known as subduction); or two plates may simply grind past each other (a process called strike-slip faulting). Most volcanoes also occur at plate boundaries, although some do occur within plates. Plate boundaries are often marked on the ground by features such as mountains or linear valleys known as faults.
The Earth’s crust is constantly being created, destroyed, and modified by geological processes. These include erosion caused by wind and water; weathering due to chemical reactions between rocks and water; metamorphism brought about by heat and pressure deep underground; melting triggered by heat from below or meteoritic impacts from above; and finally deposition—the laying down of sedimentary rock layers through compaction and cementation or the precipitation of minerals from solution in groundwater.