The crust is Earth's outermost major compositional layer and the thin rocky shell upon which the continents and ocean floors are situated. It forms only a small fraction of Earth's total volume and mass compared with the mantle and core, yet it is the part of the solid Earth most directly accessible to observation and human activity. The crust is composed predominantly of silicate minerals and rocks, but its composition varies considerably between continental and oceanic regions. It forms the upper part of the lithosphere together with the uppermost rigid mantle. The crust is not a single uniform material or continuous rock type; it contains enormous geological diversity produced by volcanism, sedimentation, metamorphism, tectonic deformation, erosion and biological activity.
Continental crust is generally thicker and less dense than oceanic crust. Its thickness varies considerably but can exceed several tens of kilometres and is greatest beneath major mountain belts. Continental crust commonly contains rocks of relatively silica rich compositions, including granite and related lithologies, as well as large quantities of metamorphic and sedimentary rocks. Oceanic crust is generally much thinner and denser and is dominated by basaltic rocks and their intrusive equivalents, such as gabbro. Oceanic crust is continually produced at divergent plate boundaries and is commonly recycled into the mantle at subduction zones. Continental crust can survive for much longer periods because it is more buoyant and is not normally subducted as readily as oceanic crust.
The crust is formed and modified through numerous geological processes. Magma that cools and solidifies creates igneous rocks, either below the surface or after eruption as lava. Weathering breaks existing rocks down, while erosion transports the resulting particles. Sediments accumulate in basins and can become sedimentary rocks through compaction and cementation. Heat, pressure and chemically active fluids can transform existing rocks into metamorphic rocks. Tectonic forces fracture, fold and uplift crustal material, while erosion gradually removes exposed material. These processes continuously alter the crust and contribute to the rock cycle.
The boundary between the crust and mantle is called the Mohorovicic discontinuity, commonly shortened to the Moho. It is identified primarily through changes in seismic wave velocities associated with the transition from crustal to mantle materials. The Moho is not at one universal depth. It tends to be deeper beneath continents and shallower beneath oceans, although its depth varies regionally. The discovery of the Moho was an important development in geophysics because it demonstrated that Earth has a distinct compositional transition beneath the crust. Seismic observations remain essential for mapping this boundary beneath regions where direct geological observations are impossible.
The crust participates directly in plate tectonics because it is incorporated into tectonic plates. Oceanic crust forms at mid ocean ridges where mantle material rises, undergoes partial melting and produces basaltic magma. As newly formed oceanic lithosphere moves away from the ridge, it cools, becomes denser and accumulates sediments. At subduction zones, old oceanic lithosphere can descend into the mantle. Continental crust participates in collision zones, where compression can thicken and deform it, producing mountain ranges. Transform faults can offset crust laterally without necessarily creating or destroying large amounts of crust.
The crust contains many economically important resources. Metallic ore deposits, industrial minerals, building stone, aggregates, fossil fuels and groundwater occur within or are associated with crustal rocks. The distribution of these resources reflects geological history, including ancient volcanic systems, hydrothermal activity, sedimentary basin development, weathering and tectonic deformation. Geological mapping and geophysical methods help identify structures and rock types that may host resources. Human settlements and infrastructure are also strongly dependent on crustal conditions because foundations, tunnels, roads, mines and reservoirs interact directly with the uppermost geological environment.
The crust is also strongly connected to the biosphere, atmosphere and hydrosphere. Soil develops largely through interactions among weathered crustal material, water, air, organisms and organic matter. Rivers erode and transport crustal material into lakes and oceans, while groundwater moves through pores and fractures. Carbon dioxide can react with minerals during chemical weathering, influencing the carbon cycle. Biological organisms can accelerate weathering and contribute to sediment production. These interactions mean that the crust is not isolated from Earth's surface systems but continuously exchanges materials and energy with them.
The crust is frequently confused with the lithosphere, but the terms are not synonymous. The crust is a compositional layer defined primarily by its distinctive chemical and mineralogical characteristics, whereas the lithosphere is a mechanical layer defined by relative rigidity and includes the crust plus part of the upper mantle. Likewise, the crust should not be imagined as a thin shell of identical thickness surrounding the planet. It varies in composition, thickness, age and geological structure. Studying the crust provides information about Earth's formation, plate tectonics, continental evolution, natural resources, geological hazards and the long history of interactions among Earth's solid, liquid, gaseous and biological systems.
