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the Mid-Atlantic Ridge (Original)

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Author Topic: the Mid-Atlantic Ridge (Original)  (Read 9780 times)
Carolyn Silver
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« Reply #60 on: July 28, 2008, 11:40:22 pm »

The continents

It is a striking fact that some nine tenths of all the sedimentary rocks composing the continents were laid down under the sea [6]. The continents have suffered repeated marine inundations, but because the seas were mostly shallow (less than 250 m), they are described as 'epicontinental'. Marine transgressions and regressions are usually attributed mainly to eustatic changes of sea level caused by alterations in the volume of midocean ridges. T.H. Van Andel points out that this explanation cannot account for the 100 or so briefer cycles of sea-level changes, especially since transgressions and regressions are not always simultaneous all over the globe. He proposes that large regions or whole continents must undergo slow vertical movements. He admits that such movements 'fit poorly into plate tectonics', and are therefore largely ignored [7].





Figure 8. Maximum degree of marine inundation for each Phanerozoic geological period for the former USSR and North America. The older the geological period, the greater the probability of the degree of inundation being underestimated due to the sediments having been eroded or deeply buried beneath younger sediments. (Reprinted with permission from Hallam [8]. Copyright by Nature.)
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