DEFINITION OF CONTINENTAL CRUST

Continental crust covers about 34.7 percent

of the Earth’s surface, whereas exposed continents cover

only 29.22 percent of the surface, with the discrepancy

accounted for in the portions of continents that lie underwater

on the continental shelves. Its lateral boundaries are

defined by the slope break between continental shelves and

slopes, and its vertical extent is defined by a jump in seismic

velocities to 4.7–5.0 miles per second (7.6–8.0 km/s) at the

Mohorovicic discontinuity. The continental crust ranges in

thickness from about 12.5 miles to about 37 miles (20–60

km), with an average thickness of 24 miles (39 km). The continents

are divided into orogens, made of linear belts of concentrated

deformation, and cratons, making the stable,

typically older interiors of the continents.

Most of the continental crust is now preserved in

Archean cratons, which form the cores of many continents.

They are composed of ancient rocks that have been stable for

billions of years since the Archean. Cratons generally have

low heat flow, few if any earthquakes, no volcanism, and

many are overlain by flat lying shallow water sedimentary

sequences. Continental shields are places where the cratonic

crust is exposed at the surface, whereas continental platforms

are places where the cratonic rocks are overlain by shallow

water sedimentary rocks, presently exposed at the surface.

Orogens and orogenic belts are elongate regions that are

eroded mountain ranges and typically have abundant folds

and faults. Young orogens are mountainous and include such

familiar mountain ranges as the Rockies, Alps, and the slightly

older Appalachians. Many Archean cratons are welded

together by Proterozoic and younger orogens. In fact, many

Archean cratons can be divided into smaller belts that represent

fragments of the planet’s oldest orogenic belts.

Orogens have been added to the edges of the continental

shield and cratons through processes of mountain building

related to plate tectonics. Mountain belts are of three basic

types, including fold and thrust belts, volcanic mountain

ranges, and fault-block ranges. Fold and thrust belts are contractional

mountain belts, formed where two tectonic plates

collided, forming great thrust faults, folds, metamorphic

rocks, and volcanic rocks. Detailed mapping of the structure

in the belts can enable geologists to reconstruct their history

and essentially pull them apart. It is found that many of the

rocks in fold and thrust belt types of mountain ranges were

deposited on the bottom of the ocean, continental rises,

slopes, shelves, or on ocean margin deltas. When the two

plates collide, many of the sediments get scraped off and

deformed, forming the mountain belts. Thus, fold and thrust

mountain belts mark places where oceans have closed.

Volcanic mountain ranges include places such as Japan’s

Fuji, and Mount St. Helens in the Cascades of the western

United States. These mountain ranges are not formed primarily

by deformation but by volcanism associated with subduction

and plate tectonics.

Fault-block mountains, such as the Basin and Range

Province of the western United States, are formed by the extension

or pulling apart of the continental crust, forming elongate

valleys separated by tilted fault-bounded mountain ranges.

Every rock type known on Earth is found on the continents,

so averaging techniques must be used to determine the

overall composition of the crust. It is estimated that continental

crust has a composition equivalent to andesite (or granodiorite)

and is enriched in incompatible trace elements,

which are the elements that do not easily fit into lattices of

most minerals and tend to get concentrated in magmas.

The continents exhibit a broadly layered seismic structure

that is different from place to place, and different in orogens,

cratons, and parts of the crust with different ages. In shields,

the upper layer typically may be made of a few hundred

meters of sedimentary rocks underlain by generally granitic

types of material with seismic velocities of 3.5 to 3.9 miles per

second (5.7–6.3 km/s) to depths of a few to 10 kilometers,

then a layer with seismic velocities of 3.9–4.2 miles per second

(6.4–6.7 km/s). The lower crust is thought to be made of layered

amphibolite and granulite with velocities of 4.2–4.5 miles

per second (6.8–7.2 km/s). Orogens tend to have thicker lowvelocity

upper layers, and a lower-velocity lower crust.

There is considerable debate and uncertainty about the

timing and processes responsible for the growth of the continental

crust from the mantle. Most scientists agree that most

of the growth occurred early in Earth history since more than

half of the continental crust is Archean in age, and about 80

percent is Precambrian. Some debate centers on whether early

tectonic processes were similar to those currently operating,

or if they were considerably different. The amount of current

growth and how much crust is being recycled back into the

mantle are currently constrained. Most petrological models

for the origin of the crust require that it be derived by a process

including partial melting from the mantle, but simple

mantle melting produces melts that are not as chemically

evolved as the crust. Therefore, the crust is probably derived

through a multistage process, most likely including early

melts derived from seafloor spreading and island arc magmatism,

with later melts derived during collision of the arcs with

other arcs and continents. Other models seek to explain the

difference by calling on early higher temperatures leading to

more evolved melts.

See also CRATONS; GREENSTONE BELTS; OROGENY.

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