I | INTRODUCTION |
Continent, one of the earth's largest continuous units
of landmass.
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 earth's 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 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 earth's 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 earth's 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 earth's outermost shell, the crust, a layer of
relatively cool rock ranging in thickness from an average of 5-10 km (3-6 mi)
beneath the oceans to 40 km (25 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 origins of continental drift is called plate tectonics because, in charting
the directions that the continents have taken, geologists discovered that the
earth's crust and upper mantle are divided into a number of semirigid plates,
each of which has recognizable boundaries and moves as a unit. Some of these
tectonic plates (the Pacific plate, for example) consist almost entirely of
oceanic crust; others, such as the North American and Eurasian plates, are made
up of mostly continental crust. Plate boundaries are generally located in
midocean or close offshore, but in a few places rise from the seabottom and
extend across dry land. Western California, where the earthquake-prone San
Andreas fault marks the boundary between the Pacific and North American plates,
is one such place.
The land-sea patterns of today have evolved
over the course of hundreds of millions of years, during which time continental
landmasses drifted, were united by collisions, then torn apart and recombined.
These movements show no sign of slackening or abating, so the distribution of
sea and dry land will continue to change for as long as the planet contains the
heat energy required to drive the movement of its crustal plates.
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