Dalziel, Ian W. O. Department of Geological Sciences, University of Texas, Austin, Texas.
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The six major continents are Africa, Antarctica, Australia, Eurasia, North America, and South America. The high elevation of these discrete masses of rock, relative to the floor of the oceans, results from their low density. Each continent is embedded within denser rock forming one of the seven rigid 100-km-thick (60-mi) lithospheric plates making up the cold surface layer of the planet. The Pacific plate is purely oceanic. The plates, and therefore the continents, are moving relative to one another. Prior to the formation of the Atlantic, Indian, and Southern ocean basins over the past 180 million years by the process known as sea-floor spreading, the continents were assembled in one supercontinent called Pangea (literally “all Earth”; Fig. 1). Pangea came together by the collision, about 300 million years ago (MYA), of two smaller masses of continental rock, Laurasia and Gondwanaland. Laurasia comprised the combined continents of ancient North America (known as Laurentia), Europe, and Asia. Africa, Antarctica, Australia, India, and South America made up Gondwanaland (this name comes from a region in southern India). The term “supercontinent” is also applied to Laurasia and Gondwanaland; hence it is used in referring to a continental mass significantly bigger than any of today's continents. A supercontinent may therefore incorporate almost all of the Earth's continental rocks, as did Pangea, but that is not implied by the word. See also: Continent; Continents, evolution of; Plate tectonics
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