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II CONTINENTAL GEOGRAPHY A continent is distinguished from an island or a peninsula not merely by greater size but also by geological structure and development (see below). The continents, in order of size, are Eurasia (conventionally regarded as the two continents of Europe, individually the second smallest, and Asia), Africa, North America, South America, Antarctica, and Australia.
The continental area-all land rising above sea level-amounts to about 29% of the earths total area. More than two-thirds of the continental land area lies north of the equator. In addition, the continental masses include the submerged continental shelves, which slope gently from the ocean shores of the continents to depths of about 100 fathoms (about 183 m/600 ft); at approximately this point begins the more abrupt plunge to the oceanic depression known as the continental slope. If the continental shelves are taken into account, the total continental area increases to 35% of the earths surface. Islands standing on the continental shelf of a given continent are considered part of that continent. Prominent examples are Great Britain and Ireland in Europe; the Malay Archipelago and Japan in Asia; New Guinea, Tasmania, and New Zealand in Australasia; and Greenland in North America.
III CONTINENTAL GEOLOGY In geology, continents are defined in terms of the earths crustal structure and constituency, rather than land-surface areas. Geophysicists have studied these features by using seismography records of shock waves produced by earthquakes. Their data suggest that the center of the earth is a hot, dense, partly molten nickel-iron core more than 6000 km (more than 4000 mi) in diameter. Surrounding this core is a mantle of hot, solid rock, 3000 km (1800 mi) thick, a portion of which is semiplastic. This is enclosed, in turn, by the earths outermost shell, the crust, a layer of relatively cool rock ranging in thickness from about 5 km (about 3 mi) beneath the oceans to 70 km (44 mi), on the average, beneath the continents.
Beneath the oceans the crust consists of a single layer of dense, dark basaltic rock made up, in large part, of iron-magnesium minerals. On the continents, this layer is buried beneath a much thicker layer of lighter colored, less dense rocks made up of aluminosilicate minerals. Because of the difference in density, the lighter rocks float on the basaltic ones. By a principle known as isostasy, in those areas where the lighter rocks rise highest-such as the great mountain ranges-they also extend downward to greater depths; beneath these ranges, roots of light rocks extend downward into the dark rocks of the crust to depths that are appreciably greater than under the vast, flat plains that occupy the interior regions of most continents.
In the 1960s geologists began to uncover proof that the continents not only float-that is, move up and down within the crust-but that they also travel, or drift, laterally. The study of the history and ...

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