Earth's crust

Lore & Background

The crust is Earth’s solid outer shell, composed of rock and making up less than one percent of the planet’s radius and volume. It is the top component of the lithosphere, which is broken into tectonic plates whose motion allows heat to escape from Earth’s interior. The crust rests on the mantle; this configuration is stable because the upper mantle, made of peridotite, is significantly denser. The boundary between crust and mantle is defined by the Mohorovičić discontinuity, marked by a change in seismic velocity. Crustal temperature increases with depth, reaching roughly 700 to 1600 °C at the mantle boundary, with local increases of up to 30 °C per kilometer in the upper crust.

There are two distinct types of crust. Continental crust is 25–70 km thick, composed mostly of less dense, felsic rocks like granite, and is thicker in places such as the Tibetan Plateau and the eastern Baltic Shield (50–80 km). Oceanic crust is 5–10 km thick, composed of denser, mafic rocks like basalt, diabase, and gabbro. Average crustal thickness is about 15–20 km. Both types float on the mantle because they are less dense, but continental crust is more buoyant due to its greater thickness and lower density, forming high ground above deep ocean basins. Continental crust has an average composition similar to andesite, though the upper crust is more felsic (like dacite) and the lower crust more mafic (like basalt). The most abundant minerals are feldspars (about 41% by mass), quartz (12%), and pyroxenes (11%). Continental crust is enriched in incompatible elements by a factor of 50–100 relative to primitive mantle, while oceanic crust is enriched by about a factor of 10. The average density of continental crust is 2.835 g/cm³, increasing from 2.66 g/cm³ at the top to 3.1 g/cm³ at the base. Oceanic crust consists mainly of pillow lava, sheeted dikes of mid-ocean ridge basalt, a thin sediment layer, and lower gabbro.

Earth formed about 4.6 billion years ago, melting completely due to accretion heat. Its first crust, a primary or primordial crust, was repeatedly destroyed by impacts and reformed from magma oceans; none survives today. Secondary crust (oceanic) forms at mid-ocean spreading centers via partial melting of the mantle, producing basaltic magma. This ridge push drives plate tectonics, constantly creating new ocean crust while old crust is destroyed at subduction zones. T

Reader's Guide

The crust of Earth is of two distinct types: Continental (25–70 km thick, mostly composed of less dense, more felsic rocks such as granite; in a few places like the Tibetan Plateau, the Altiplano, and the eastern Baltic Shield, it is thicker at 50–80 km) and Oceanic (5–10 km thick, composed primarily of denser, more mafic rocks such as basalt, diabase, and gabbro). The average thickness of the crust is about 15–20 km. Both types of crust are less dense than the mantle below and float on it. The continental crust has an average composition similar to that of andesite, with the upper crust averaging a more felsic composition like dacite and the lower crust averaging a more mafic composition resembling basalt. The most abundant minerals in Earth's continental crust are feldspars (about 41% by mass), followed by quartz (12%), and pyroxenes (11%). All other constituents except water occur only in very small quantities and total less than 1%. Continental crust is enriched in incompatible elements compared to the basaltic ocean crust and much enriched compared to the underlying mantle. The estimated average density of the continental crust is 2.835 g/cm³, increasing with depth from 2.66 g/cm³ in the uppermost crust to 3.1 g/cm³ at the base. Oceanic crust is composed predominantly of pillow lava and sheeted dikes with the composition of mid-ocean ridge basalt, with a thin upper layer of sediments and a lower layer of gabbro.

Did You Know?

• Earth formed approximately 4.6 billion years ago from a disk of dust and gas orbiting the newly formed Sun.
• None of Earth's primary crust has survived to today; all was destroyed by erosion, impacts, and plate tectonics.
• The oldest ocean crust on Earth today is only about 200 million years old.
• The oldest continental crustal rocks have ages from about 3.7 to 4.28 billion years.
• The average age of Earth's current continental crust has been estimated to be about 2.0 billion years.

Structure and Place in Earth's Architecture

The crust is the outermost rocky shell of our planet, accounting for less than one percent of Earth's total radius and volume. It sits as the uppermost component of the lithosphere, a solidified division that also encompasses the upper portion of the mantle. This lithosphere is fractured into tectonic plates, and the movement of those plates serves as the mechanism by which Earth's internal heat escapes into space. The crust rests atop the mantle in a configuration that remains gravitationally stable because the upper mantle, composed of peridotite, is substantially denser than the crustal material above it. The dividing line between these two layers is conventionally marked at the Mohorovičić discontinuity, a boundary identified not by a change in rock type but by a measurable contrast in how seismic waves travel through the material. This seismic-velocity signature is what allows geologists to map the boundary even though it lies far beyond the reach of any drill.

Two Faces of the Crust

Earth's crust is not a single uniform layer but comes in two fundamentally different flavors. Continental crust ranges from 25 to 70 kilometers thick, with exceptional thicknesses of 50 to 80 kilometers beneath the Tibetan Plateau, the Altiplano, and the eastern Baltic Shield. It is built primarily from less dense, more felsic rocks like granite, and its average composition resembles andesite, with the upper portion trending toward dacite and the lower portion toward basalt. Oceanic crust, by contrast, is a mere 5 to 10 kilometers thick and is dominated by denser, more mafic materials such as basalt, diabase, and gabbro. Because both crustal types are less dense than the peridotite mantle beneath them, they effectively float. The thicker, lighter continental crust rides higher, producing the continents that rise above sea level, while the thinner oceanic crust forms the deep basins below—a phenomenon known as isostasy. The average crustal thickness across the whole planet falls in the 15 to 20 kilometer range.

A Story Written in Deep Time

Earth assembled 4.6 billion years ago through the accretion of planetesimals and smaller rocky bodies orbiting the young Sun. That violent assembly melted the proto-planet entirely, and as it cooled, a primary or primordial crust first appeared. Yet none of that original material survives; it was obliterated by repeated large impacts, erosion, and the later onset of plate tectonics. The crust we see today is a secondary and tertiary product. Oceanic crust is born at mid-ocean spreading centers, where partial melting of the mantle generates basaltic magma that solidifies into new seafloor. This ridge push drives plate motion, and because new crust is constantly created, old crust must be consumed at subduction zones, meaning the oldest oceanic crust is only about 200 million years old. Continental crust, however, is far older, with the oldest known rocks dating to 3.7 to 4.28 billion years in the Narryer Gneiss terrane of Western Australia, the Acasta Gneiss in Canada's Northwest Territories, and the Fennoscandian Shield. Some zircon crystals from the Narryer terrane push as far back as 4.3 billion years. The average age of current continental crust is about 2 billion years, with most rocks older than 2.5 billion years residing in cratons.

Heat, Minerals, and Chemical Fingerprint

Temperature within the crust climbs steadily with depth, rising by as much as 30 degrees Celsius per kilometer in the upper crust and reaching between 700 and 1,600 degrees Celsius at the base where it meets the mantle. The mineral makeup of continental crust is dominated by feldspars, which account for roughly 41 percent of its mass, followed by quartz at 12 percent and pyroxenes at 11 percent. All other constituents, aside from water, are present in trace amounts totaling less than one percent. The average density of continental crust is about 2.835 grams per cubic centimeter, increasing from 2.66 in the uppermost layers to 3.1 at the base. Chemically, continental crust is notably enriched in incompatible elements compared to both oceanic crust and the underlying mantle. The most incompatible elements are concentrated by a factor of 50 to 100 relative to primitive mantle rock, whereas oceanic crust shows enrichment of only about a factor of 10. Oceanic crust, meanwhile, is structured as pillow lava and sheeted dikes of mid-ocean ridge basalt composition, capped by a thin sediment layer and underlain by gabbro.

Gallery