What Is Continental Crust?

Continental crust is the outermost solid layer of Earth that underlies the continents and their submerged margins, distinguished from oceanic crust by its greater thickness, lower density, and more silica-rich composition. It averages roughly 40 km thick, ranging from about 30 km beneath extended terrain such as the Basin and Range to more than 70 km beneath the Tibetan Plateau, whereas oceanic crust is typically 6 to 8 km thick. Its bulk composition approximates andesite, and because it is less dense than the mantle beneath it, it floats high enough to stand above sea level rather than being recycled at subduction zones as oceanic crust routinely is.

Continental crust is the subject of geology, geophysics, and geochemistry, and the primary evidence about it comes from three independent lines of measurement: seismic wave velocities, gravity and geodetic observation, and the chemistry of exposed and drilled rock. Nearly all of the accessible mineral, hydrocarbon, and groundwater resources on the planet sit within it, which is why its structure is mapped in far more detail than any other part of the solid Earth.

Composition and Layered Structure

The crust is conventionally divided into upper, middle, and lower layers, each defined by a characteristic range of compressional wave velocity. The upper crust is granitic and well sampled at the surface. The middle and lower crust are inferred mainly from seismology and from xenoliths carried up by volcanic eruptions, and they become progressively more mafic with depth. A global compilation of controlled-source seismic profiles published through the U.S. Geological Survey established an average compressional wave velocity of 6.45 km/s for the crust as a whole, a mean crustal density near 2830 kg/m3, and an average silica content of about 62 percent by weight. The velocity contrast between layers is subtle enough that composition and temperature cannot always be separated from a velocity profile alone.

The Mohorovicic Discontinuity

The base of the crust is marked by the Mohorovicic discontinuity, or Moho, the depth at which compressional wave velocity jumps to around 8.1 km/s as the rock becomes mantle peridotite. Andrija Mohorovicic identified it in 1909 from travel-time records of a Croatian earthquake. Seismic refraction and receiver-function methods remain the primary way of locating it, but coverage is uneven across continents, and satellite gravity has been used to fill the gaps. Data from the European Space Agency GOCE gravity gradiometry mission produced a global high-resolution Moho map derived from gravity inversion, resolving thick continental roots beneath the major mountain belts.

Formation and Crustal Growth

Continental crust is generated principally above subduction zones, where hydrated oceanic lithosphere descending into the mantle triggers partial melting and delivers magmas of intermediate composition to the overriding plate. Repeated cycles of arc magmatism, accretion, and collision build cratons that can persist for billions of years. Because buoyant crust resists subduction, the total volume of continental material has grown over Earth history, though the rate and continuity of that growth are debated. The argument that crustal velocity structure constrains bulk chemistry was made early in 1966 USGS work on the composition and evolution of the continental crust, which read the seismic profiles of the conterminous United States as evidence for a more mafic crust than was then assumed. Later global compilations revised that estimate toward an intermediate bulk composition, but the method of inferring chemistry from velocity remains standard.

Applications

Continental crust has applications in a range of fields, including:

• Mineral and hydrocarbon exploration
• Seismic hazard assessment and ground motion modeling
• Geothermal energy resource evaluation
• Satellite gravity and geodetic remote sensing
• Groundwater and aquifer characterization
• Geologic carbon storage site selection