<P> As the tectonic plates migrate, oceanic crust is subducted under the leading edges of the plates at convergent boundaries . At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges . The combination of these processes recycles the oceanic crust back into the mantle . Due to this recycling, most of the ocean floor is less than 7015315576000000000 ♠ 100 Ma old . The oldest oceanic crust is located in the Western Pacific and is estimated to be 7015631152000000000 ♠ 200 Ma old . By comparison, the oldest dated continental crust is 7017127177128000000 ♠ 4,030 Ma . </P> <P> The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo - Australian, and South American . Other notable plates include the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean . The Australian Plate fused with the Indian Plate between 7001500000000000000 ♠ 50 and 55 Mya . The fastest - moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm / a (3.0 in / year) and the Pacific Plate moving 52--69 mm / a (2.0--2.7 in / year). At the other extreme, the slowest - moving plate is the Eurasian Plate, progressing at a typical rate of 21 mm / a (0.83 in / year). </P> <P> The total surface area of Earth is about 510 million km (197 million sq mi). Of this, 70.8%, or 361.13 million km (139.43 million sq mi), is below sea level and covered by ocean water . Below the ocean's surface are much of the continental shelf, mountains, volcanoes, oceanic trenches, submarine canyons, oceanic plateaus, abyssal plains, and a globe - spanning mid-ocean ridge system . The remaining 29.2%, or 148.94 million km (57.51 million sq mi), not covered by water has terrain that varies greatly from place to place and consists of mountains, deserts, plains, plateaus, and other landforms . Tectonics and erosion, volcanic eruptions, flooding, weathering, glaciation, the growth of coral reefs, and meteorite impacts are among the processes that constantly reshape the Earth's surface over geological time . </P> <P> The continental crust consists of lower density material such as the igneous rocks granite and andesite . Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors . Sedimentary rock is formed from the accumulation of sediment that becomes buried and compacted together . Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form about 5% of the crust . The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both . The most abundant silicate minerals on Earth's surface include quartz, feldspars, amphibole, mica, pyroxene and olivine . Common carbonate minerals include calcite (found in limestone) and dolomite . </P>

What is the surface area of the earth