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Sunken Continents versus Continental Drift

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Author Topic: Sunken Continents versus Continental Drift  (Read 5782 times)
Carolyn Silver
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Posts: 4611

« on: July 28, 2008, 11:20:25 pm »

The oceans

In the past, sediments have been transported to today's continents from the direction of the present-day oceans, where there must have been considerable areas of land that underwent erosion. For instance, the Paleozoic geosyncline along the seaboard of eastern North America, an area now occupied by the Appalachian mountains, was fed by sediments from a borderland ('Appalachia') in the adjacent Atlantic. Other submerged borderlands include the North Atlantic Continent or Scandia (west of Spitsbergen and Scotland), Cascadia (west of the Sierra Nevada), and Melanesia (southeast of Asia and east of Australia). A million cubic kilometers of Devonian sediments from Bolivia to Argentina imply an extensive continental source to the west where there is now the deep Pacific Ocean. During Paleozoic-Mesozoic-Paleogene times, the Japanese geosyncline was supplied with sediments from land areas in the Pacific.
    When trying to explain sediment sources, plate tectonicists sometimes argue that sediments were derived from the existing continents during periods when they were supposedly closer together. Where necessary, they postulate small former land areas (microcontinents or island arcs), which have since been either subducted or accreted against continental margins as 'exotic terranes'. However, mounting evidence is being uncovered that favors the foundering of sizable continental landmasses, whose remnants are still present under the ocean floor.
    Oceanic crust is regarded as much thinner and denser than continental crust: the crust beneath oceans is said to average about 7 km thick and to be composed largely of basalt and gabbro, whereas continental crust averages about 35 km thick and consists chiefly of granitic rock capped by sedimentary rocks. However, ancient continental rocks and crustal types intermediate between standard 'continental' and 'oceanic' crust are increasingly being discovered in the oceans, and this is a serious embarrassment for plate tectonics. The traditional picture of the crust beneath oceans being universally thin and graniteless may well be further undermined in the future, as seismic research and ocean drilling continue
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