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Continent

Crusts Compared, Continental Margins, Crustal Origins, Growing Pains, Primeval ContinentsStructure of a continent

A continent is a large land mass and its surrounding shallow continental shelf. Both are composed of felsic crust. Continents, as by-products of plate tectonic activity, have grown to cover about one-third of Earth's surface over the last four billion years. Continents are unique to Earth, as plate tectonics does not occur on the other planets of our solar system.


Horizontal crustal structure

Continent interiors consist of several structural segments. A rock foundation called the craton composes most of every continent. This consists of several large masses, called terranes, composed of ancient igneous and metamorphic rocks joined together into a rigid, stable unit. Each terrane may be quite different in structure, rock type and age from adjoining terranes. Where exposed at the earth's surface, cratons are called shields. These typically are ancient surfaces planed flat by erosion. In areas where younger sedimentary rocks cover the craton, it is called the stable platform. The craton below these sedimentary layers is usually called basement rock. Ancient mountain chains, or orogenic belts, occur within the craton where two smaller cratons became sutured together in the distant past. Some of the world's highest mountain ranges, such as the Himalayas, developed when two cratons (continents) collided, and erosion has not yet leveled them.

The margins of continents host younger orogenic belts than their interiors. These belts usually form due to plate convergence along active continental margins. Younger orogenic belts, with their steep slopes, tend to shed large volumes of sediment. In coastal regions, these sediments form a seaward-facing, shallowly-sloping land surface called a coastal plain. Within the continental interior, sediments eroded from mountains to form an area of interior lowlands, such as the United States' Great Plains region.

Divergence within a continent's interior leads to rifting. Initially, a steep-sided rift valley forms accompanied by small- to moderate-sized volcanic eruptions. Eventually this rift valley extends to the coast and the valley floor drops below sea level. A small inland sea develops, like the Red Sea between Africa and the Arabian subcontinent, with an oceanic ridge at its center. Given sufficient time and continued sea floor spreading, this sea becomes an ocean, similar to the Atlantic, with passive margins and wide continental shelves along both its shores.

An unusual continental feature develops when a continental rift fails. For whatever reason, rather than divergence producing an inland sea, rifting ends and the structure that remains is called an aulocogen. Depending on the rift's degree of development when failure occurs, the aulocogen may range from an insignificant crustal flaw to a major crustal weakness. Geologists attribute many powerful mid-plate earthquakes, such as the three 1811-1812 New Madrid, Missouri earthquakes, to fault movements associated with failed rifts.

Other common, large-scale continental structures include basins and domes. Basins are circular areas where the crust has subsided, usually under the load of accumulated sediments or due to crustal weakness such as an aulocogen. Domes occur when the crust is uplifted, perhaps due to compression of the continental interior during plate convergence at a nearby active margin.


Vertical crustal structure

Continental crust is heterogenous; however, general trends in structure, composition, and rock type are known. Our knowledge of the subsurface character of the crust comes from two main sources. The crustal interior is observed directly in areas where uplift and erosion expose the cores of ancient mountain belts and other structures. In addition, seismic waves produced during earthquakes change speed and character when moving through the crust. These changes allow geophysicists to infer crustal structure and density.

Continents reach their greatest thickness (up to 45 mi; 70 km) below mountain ranges and are thinnest (10-15 mi; 16-24 km) beneath rifts, aulocogens, shields, and continental margins. Density tends to increase downwards, in part due to an increase in mafic content. The upper crust has an average composition similar to granite, while the lower crust is a mixture of felsic and mafic rocks. Therefore, the average composition of the continents is slightly more mafic than granite. Granite contains an average of 70-75% silica; basalt about 50%. The continental crust is composed of 65% silica, the composition of the igneous rock granodiorite. The intensity of metamorphism and volume of metamorphic rock both increase downward in the crust as well.


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