Earths crust is the thin, rigid outermost solid layer of the planet. It forms the outer surface on which continents and ocean floors occur and is chemically and physically distinct from the mantle beneath it. Although the crust surrounds the entire planet, it represents only a very small fraction of Earths total volume and mass. Its thickness varies substantially according to whether it is continental or oceanic. Oceanic crust is generally about 5 to 10 kilometres thick, whereas continental crust commonly ranges from roughly 30 to 40 kilometres and can exceed 70 kilometres beneath major mountain belts. The crust is therefore not a uniform shell of equal thickness.
The crust is composed predominantly of silicate minerals and rocks containing oxygen, silicon, aluminium, calcium, sodium, potassium, magnesium and iron in varying proportions. Continental crust is generally more chemically diverse and contains abundant granitic and related rocks, while oceanic crust is dominated by basaltic and gabbroic rocks that contain comparatively more iron and magnesium. The crust is separated compositionally from the mantle by the Mohorovicic discontinuity, commonly called the Moho. The Moho is identified primarily through a marked increase in seismic wave velocities because mantle rocks have different composition and physical properties from crustal rocks.
Earths crust formed through long term geological processes involving melting, crystallisation, volcanic activity, differentiation and recycling. Early Earth was much hotter and underwent extensive separation of materials according to density and chemical behaviour. Over geological time, crust has continually been produced, altered, uplifted, eroded, metamorphosed and recycled. Oceanic crust is generated mainly at mid ocean ridges where mantle material partially melts and magma rises and solidifies. Continental crust has a more complex history involving repeated magmatic, metamorphic and sedimentary processes, and portions of continental crust are billions of years old.
The crust is mechanically important because it forms part of the lithosphere, which also includes the uppermost rigid mantle. Tectonic plates consist of lithosphere and move over the mechanically weaker asthenosphere. Consequently, the crust itself does not normally move independently as a global tectonic layer. Instead, continental and oceanic crust are embedded within plates. A single tectonic plate may contain both types of crust, as occurs beneath many continental margins where continental crust transitions into oceanic crust.
Earthquakes provide one of the principal ways scientists investigate the crust. Seismic waves travel through the planet at velocities determined by density, composition, temperature and physical state. By measuring the arrival times and behaviour of seismic waves, geophysicists can estimate crustal thickness and identify internal boundaries. Seismic reflection and refraction, gravity measurements, magnetic surveys, drilling and analysis of exposed rocks provide additional evidence about crustal structure. The crust therefore represents a directly observable but also geophysically investigated component of Earths interior.
Continental crust and oceanic crust differ in thickness, density, composition, age and recycling behaviour. Continental crust is generally thicker and less dense, allowing continents to stand at higher average elevations. Oceanic crust is thinner and denser and is continually created and destroyed through plate tectonic processes. Most oceanic crust is younger than about 200 million years because old oceanic lithosphere is commonly consumed at subduction zones. Continental crust can survive for billions of years because its lower density makes it comparatively resistant to being subducted wholesale into the mantle.
The crust should not be confused with the lithosphere. The crust is defined primarily by composition, whereas the lithosphere is defined mainly by mechanical behaviour. The lithosphere includes the crust plus the rigid uppermost mantle. The crust should also not be confused with the entire solid Earth or with tectonic plates. Plates are pieces of lithosphere that can contain crust and upper mantle. Similarly, the crust is not simply the solid surface visible at land level because oceanic crust lies beneath seawater and much continental crust is buried beneath sediments or ice.
The crust is fundamental to life and human activity because it contains soils, mineral resources, groundwater systems and the geological environments in which terrestrial ecosystems develop. Human societies obtain metals, construction materials, fossil fuels and many industrial minerals from crustal rocks and sediments. Mountains, plains, volcanoes, ocean basins and many other familiar landforms ultimately depend on interactions involving crustal rocks and tectonic processes. Understanding the crust therefore connects geology with natural hazards, resource management, landscape development, climate history and the long term evolution of the planet.
