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) while the lower crust is ductile and capable of flow. The crust and the solid upper mantle form the lithosphere.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 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.
Evidence indicates that part of the mantle 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.
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