..... Click the link for more information. . The continents include Eurasia (conventionally regarded as two continents, Europe Europe (y
r`əp), 6th largest continent, c...... Click the link for more information. and Asia Asia (ā`zhə), the world's largest continent, 17,139,000 sq mi (44,390,000 sq km), with about 3.
..... Click the link for more information. ), Africa Africa (ăf`rĭkə), second largest continent (1997 est. pop. 743,000,000), c.
..... Click the link for more information. , North America North America, third largest continent (1990 est. pop. 365,000,000), c.9,400,000 sq mi (24,346,000 sq km), the northern of the two continents of the Western Hemisphere.
..... Click the link for more information. , South America South America, fourth largest continent (1991 est. pop. 299,150,000), c.6,880,000 sq mi (17,819,000 sq km), the southern of the two continents of the Western Hemisphere.
..... Click the link for more information. , Australia Australia (ôstrāl`yə), smallest continent, between the Indian and Pacific oceans.
..... Click the link for more information. , and Antarctica Antarctica (ăntärk`tĭkə, –är`tĭkə), the fifth largest continent, c.
..... Click the link for more information. .
Geographic Distribution of the Continents
More than two thirds of the continental regions are in the Northern Hemisphere, rimming the Arctic Ocean. South America and Africa project into the Southern Hemisphere as southward-pointing triangles, forming extensive peninsular regions separating the Atlantic, Pacific, and Indian oceans. In addition, the continents are generally antipodal to the ocean ocean, interconnected mass of saltwater covering 70.78% of the surface of the earth, often called the world ocean. It is subdivided into four (or five) major units that are separated from each other in most cases by the continental masses. See also oceanography .
..... Click the link for more information. basins (i.e., ocean basins are found on the opposite side of the earth from continental masses). For example, there is an antipodal relationship between the continental Antarctic region and the Arctic Ocean, and the Pacific Ocean lies opposite Africa and Europe. The continental areas above sea level comprise about 29% of the earth's surface. However, from a geological point of view, a submerged continental shelf is also part of a continent. Inclusion of the shelf area increases the extent of the continents to 35% of the globe. The earth's average land elevation is c.2,700 ft (820 m) above sea level; the highest point is the summit of Mt. Everest at 29,035 ft (8,850 m); and the lowest point is the shore of the Dead Sea at 1,345 ft (410 m) below sea level.
Geology and Topography of the Continents
Geologically and topographically the continents are exceedingly complex and variable in detail, yet certain large-scale structural and topographic features appear to be common to all. The continents are composed mainly of granitic rocks and measure an average of 25 mi (40 km) thick. Underlying the ocean are denser basaltic rocks measuring about 4 mi (7 km) thick. Basaltic rocks may also form the lower portions of the continental crust in many regions. The upper and lower crust zones deform by different mechanisms; the upper crust is brittle and deforms by faulting (see fault fault, in geology, fracture in the earth's crust in which the rock on one side of the fracture has measurable movement in relation to the rock on the other side. Faults on other planets and satellites of the solar system also have been recognized.
..... Click the link for more information. ) while the lower crust is ductile and capable of flow. The crust and the solid upper mantle form the lithosphere lithosphere (lĭth`əsfēr '), brittle uppermost shell of the earth, broken into a number of tectonic plates.
..... Click the link for more information. .
Plateaus, Shields, and Mountains
Generally, the continents contain vast interior plains or plateaus, underlain by a basement complex of igneous and metamorphic rocks of Precambrian Precambrian era, name of a major division of geologic time (see Geologic Timescale , table), from c.5 billion to 570 million years ago. It is often divided into the Archeozoic and Proterozoic; in other countries, the Precambrian is broken into other divisions,
..... Click the link for more information. age. In some places, the basement complex is exposed at the surface, where it is often called the shield, or craton. The interior of shield areas contain some of the oldest rocks known on the earth's surface. The Canadian Shield area of E Canada is the exposed basement complex of North America. Portions of shield areas are covered with veneers of flat-lying sedimentary rocks of younger age. The interior plains of the continents are frequently bounded on one or more sides by ranges of mountains. These mountains have been intricately folded and faulted. They also display abundant evidence of volcanic activity, large-scale igneous intrusions, and deformation structures associated with convergent plate movement. In the United States the folded Appalachian Mts. lie to the east of the interior plains and were caused mainly by the collision of two continents. The Rocky Mts. are to the west, formed by huge igneous masses that pushed upward through overlying sedimentary rocks, which were then eroded away.
Floating Continents and Isostasy
Evidence indicates that part of the mantle mantle, portion of the earth 's interior lying beneath the crust and above the core. No direct observation of the mantle, or its upper boundary, has been made; its boundaries have been determined solely by abrupt changes in the velocities and character of seismic
..... Click the link for more information. below the crust consists of semifluid rocks on which the continents and ocean basins, in effect, are floating. A condition of gravitational balance, called isostasy, exists between different parts of the earth's crust. The theory of isostasy claims that the continental crust floats higher than the oceanic crust because the former is composed of a thick layer of lower density rocks while the latter is composed of a thin layer of higher density rocks. Isostatic adjustments to changes in mass distribution on the earth's surface associated with plate interactions may occur through flow of semifluid materials deep in the earth. These materials cause a compensatory uplift of mountains and plateau areas as erosion wears them down. The mass of eroded material is added to and thus depresses the continental shelves and the ocean floor. Adjustments also accompany such changes as the growth and melting of continental ice sheets.
Theories of Continental Formation
The oldest continental rocks dated by radioactivity are 3.98 billion years old, which suggests that the continents and oceans are probably permanent features of the earth's surface. Although the continental regions have been periodically covered by shallow seas, they appear never to have been the sites of deep oceans. How the continents originated has been a major debate in geology. The 19th-century geologist J. D. Dana Dana, James Dwight, 1813–95, American geologist, mineralogist, and naturalist, b. Utica, N.Y., grad. Yale, 1833. His studies of the S Pacific, NW United States, Europe, and elsewhere led to changes in ideas on mountain building, volcanism, and the origin of the
..... Click the link for more information. proposed the continent accretion theory where the continents have always been stationary, with the gradual addition of new material around a central nucleus. Another theory was called the continental assimilation hypothesis, where the ocean areas accumulate the denser elements, then subside to form basins. In the late 19th cent., George Darwin proposed that the moon was gravitationally extracted from the Pacific Ocean, with the earth eventually redistributing into oceanic and continental crusts. In 1925, the expansion of the earth hypothesis stated that the present continents split apart as the earth expanded, noting that the continents could cover a sphere half the surface area of the present earth. Accepted theory now points to continental drift continental drift, geological theory that the relative positions of the continents on the earth's surface have changed considerably through geologic time. Though first proposed by American geologist Frank Bursley Taylor in a lecture in 1908, the first detailed theory
..... Click the link for more information. and seafloor spreading seafloor spreading, theory of lithospheric evolution that holds that the ocean floors are spreading outward from vast underwater ridges. First proposed in the early 1960s by the American geologist Harry H.
..... Click the link for more information. as a result of plate tectonics plate tectonics, theory that unifies many of the features and characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized geologists' understanding of continents, ocean basins, mountains, and earth history.
..... Click the link for more information. .
continent
One of seven large continuous masses of land: Asia, Africa, North America, South America, Antarctica, Europe, and Australia (listed in order of size). Europe and Asia are sometimes considered a single continent, Eurasia. The continents vary greatly in size and in ratio of coastline to total area. More than two-thirds of the world's continental land area lies north of the equator, and all the continents except Antarctica are wedge-shaped, wider in the north than in the south. See also continental drift.
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