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Continents and ocean basins

The two major paleogeographic features are the continents and the ocean basins. Since early Precambrian time, Earth has been divided into deep ocean basins (average depth, 3.5 km, or 2.2 miles) and high-standing continents (average elevation, about 800 metres, or 2,600 feet). Continental lithosphere stands high above the ocean basins because it is less dense and is not easily subducted, or recycled back into Earth’s interior. Consequently, continents are made up of very old rocks, some dating back over 4 billion years. The amount of continental lithosphere has probably changed very little during the last 2.6 billion years—possibly increasing 10 to 15 percent. What has changed is the shape and the distribution of continents across the globe.

The ocean basins are also ancient paleogeographic features. Oceanic lithosphere is continuously created at oceanic ridges and then recycled back into Earth’s interior at subduction zones.

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