DEFINITION OF CRUST

Thin, low-density rock material making up the outer

layer of the solid Earth, ranging in thickness from about 3

miles (5 km) and less near the mid-ocean ridges, to more

than 50 miles (70 km) beneath the tallest mountain ranges.

This is followed inward by the mantle, a solid rocky layer

extending to 1,802 miles (2,900 km). The outer core is a

molten metallic layer extending to 3,169-mile (5,100-km)

depth, and the inner core is a solid metallic layer extending

to 3,958 miles (6,370 km).

The temperature increases with depth with a gradient of

55°F (30°C) per kilometer in the crust and upper mantle, and

with a much smaller gradient deeper within the Earth. The

heat of the Earth comes from residual heat trapped from initial

accretion, radioactive decay, latent heat of crystallization

of outer core, and dissipation of tidal energy of the Sun-

Earth-Moon system. Heat flows out of the interior of the

Earth toward the surface, through convection cells in the

outer core and mantle. The top of the mantle and the crust is

a relatively cold and rigid boundary layer called the lithosphere

and is about 65 miles (100 km) thick. Heat escapes

through the lithosphere largely by conduction, transport of

heat in igneous melts, and in convection cells of water

through mid-ocean ridges.

The Earth’s crust is divisible broadly into continental

crust of granodioritic composition and oceanic crust of

basaltic composition. Continents comprise 29.22 percent of

the surface, whereas 34.7 percent is underlain by continental

crust, with continental crust under continental shelves

accounting for the difference. The continents are in turn

divided into orogens, made of linear belts of concentrated

deformation, and cratons, the stable, typically older interiors

of the continents.

The distribution of surface elevation is strongly bimodal,

as reflected in the hypsometric diagrams. Continental freeboard

is the difference in elevation between the continents

and ocean floor and results from difference in thickness and

density between continental and oceanic crust, tectonic activity,

erosion, sea level, and strength of continental rocks.

See also CONTINENTAL CRUST; CRATONS; LITHOSPHERE;

OCEAN BASIN.

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