The low-density outermost layer of the Earth above the Mohoroviić discontinuity (the Moho), a global boundary that is defined as the depth in the Earth where the compressional-wave seismic velocity increases rapidly or discontinuously to a value in excess of 4.7 mi/s (7.6 km/s; the upper mantle). The crust is also the cold, upper portion of the Earth's  lithosphere, which in terms of plate tectonics is the mobile, outer layer that is underlain by the hot, convecting asthenosphere.

The Earth's continental crust has evolved over the past 4 billion years, and is highly variable in geologic composition and internal structure. The worldwide mean thickness of continental crust is 24 mi (40 km), with a standard deviation of 5.4 mi (9 km). The thinnest continental crust (found in the Afar Triangle, northeast Africa) is about 9 mi (15 km) thick, and the thickest crust (the Himalayan Mountains in China) is about 47 mi (75 km) thick. Ninety-five percent of all continental crust has a thickness within two standard deviations of the mean thickness, between 13 mi (22 km) and 37 mi (58 km). The Antarctic continent has a crustal thickness of 24 mi (40 km) in the ancient, stable (cratonic) region of East Antarctica, and about 12 mi (20 km) in the recently stretched (extended) crust of West Antarctica. Continental margins, which mark the transition from oceanic to continental crust, range in thickness from about 9 mi (15 km) to 18 mi (30 km).Continental margin

Despite its geologic complexity, the continental crust may generally be divided into four layers: an uppermost sedimentary layer, and an upper, middle, and lower crust composed of crystalline rocks. The sedimentary cover of the continental crust is an important source of natural resources. This cover averages 0.6 mi (1 km) in thickness, and varies in thickness from zero (for example, on shields) to more than 9 mi (15 km) in deep basins. In stable continental crust of average thickness (25 mi or 40 km), the crystalline upper crust is commonly 6–9 mi (10–15 km) thick and has an average composition equivalent to a granite. The middle crust is 3–9 mi (5–15 km) thick and has a composition equivalent to a diorite; and the lower crust is 3–12 mi (5–20 km) thick and has a composition equivalent to a gabbro. Due to increasing temperature and pressure with depth, the metamorphic grade of rocks increases with depth, and the rocks within the deep continental crust generally are metamorphic rocks, even if they originated as sedimentary or igneous rocks.

Crustal properties vary systematically with geologic setting, which may be divided into six groups: orogens (mountain belts), shields and platforms, island arcs (volcanic arcs), continental magmatic arcs, rifts, extended (stretched) crust, and forearcs. Orogens are typified by thick crust [average thickness is 29 mi (46 km), but the maximum thickness is as much as 47 mi (75 km) in the Himalayas]. Shields and platforms, such as the Canadian Shield and the Russian Platform, commonly have an approximately 26-mi-thick (42-km) crust, including a 3--6 mi-thick (5--10 km) lower crust. In comparison with shields, island arcs (such as Japan) have thinner crusts and significantly shallower middle and lower crustal layers due to the intrusion of mafic (that is, low silica content) plutons. Continental magmatic arcs, such as the Cascades volcanoes of the northwestern United States, intrude preexisting continental crust, and therefore they are generally 3–9 mi (10–15 km) thicker than island arcs. Continental rifts, such as the East African and Rio Grande rifts, have an average crustal thickness of about 22 mi (36 km). Extended continental crust, such as the Basin and Range Province of the western United States, averages 18 mi (30 km) in thickness. Forearcs are regions that were formed oceanward of volcanic arcs, such as much of the west coast of North America. They typically have thin crust, about 15 mi (25 km), and have a thick (9 mi or 15 km) upper crustal section that consists of relatively low-density metasedimentary rocks.North America Oceanic islands Rift valley Sedimentary rocks Volcano

At least three processes provide new continental crust. The first is the accretion and consolidation of island arcs, such as Japan or the Aleutian Islands, onto a continental margin. The second process is the tectonic underplating of oceanic crust at active subduction zones. In this process, the continental crust grows from below as oceanic crust is welded to the base of the continental margin, either when subduction stops or when subduction steps oceanward and a new trench is formed. This process has been identified in western Canada and southern Alaska. The third process is the magmatic inflation of the crust at continental arcs, rifts, and regions of crustal extension. This process has been identified in many regions.

The surface of the ocean crust, except for some locally high volcanoes and plateaus, resides some 1–3 mi (2–5 km) below sea level, and about another kilometer below the average level of the continents. The ocean crust represents the youngest and geologically most dynamic portion of the Earth's surface. Most of it was produced at mid-ocean ridges during the process of sea-floor spreading. The ridges define the trailing edges, or accreting boundaries, of the major lithospheric plates that are moving about the surface of the Earth at present. Thus, the oldest rocks of the ocean crust date back no earlier than the rifting episodes that created most of these plates and initiated the most recent phase of continental drift, the Pangaean breakup, in Late Jurassic times. Mid-Oceanic Ridge

There are fault slices of types of ocean crust on land, known as ophiolites, where nearly or entirely complete cross sections through the crust can be mapped and sampled. These strongly indicate that the ocean crust consists in downward sequence of submarine extrusives (usually pillow basalts), feeder dikes (often vertically sheeted), or sills, gabbros, and peridotites. There is much uncertainty, however, about the extent to which typical ophiolites, most of which formed in island-arc or backarc environments, can represent abyssal ocean crust, which is produced at the major accreting plate boundaries. Moreover, the physical correspondence of the rocks in ophiolites to ocean crust is often complicated by their complex structure and extent of alteration and metamorphism, particularly in the ultramafic sections.