The Theory of Plate Tectonics

The idea that continents moved during the past goes back?to the time when people first noticed that the margins of eastern South America and?western Africa looked as if they fit together. Geologists also noticed that similar or?identical fossils occur on widely separated continents, that the same types of rocks from?the same time period are found on different continents, and that ancient rock formations?indicating former glacial conditions occur in today’s tropical areas. In 1912 Alfred Wegener, a German?

meteorologist, amassed a tremendous amount of this geological,?paleontological, and climatological data and proposed the?hypothesis of continental drift to explain this collection of facts.?

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According to this theory, all of the continent were at one time??united in one single supercontinent that Wegener named Pangaea.?Pangaea later broke apart and the individual continents drifted?to their current locations. The continental drift hypothesis?

explained why shorelines of different continents fit together, why the?same species of prehistoric animals and plants are found on?different continents, and why rock formations indicating glacial?conditions are found on continents located in the tropics.?

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To test the hypothesis of continental drift, all researchers had??to do was to go into the field and examine a continent’s rocks and?fossils from a particular time period to see if they conformed to what?the hypothesis proposed about that continent at the time in question.?In almost all cases the data fit the hypothesis. However,?Wegener’s hypothesis had one shortcoming: It did not explain how?or why the continents moved.?

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During the 1950s and 1960s, new data emerged that enabled?geologists to propose a mechanism of continental drift. According to this theory, Earth’s??surface consists of a number of rocky tectonic plates. The plates fit together at their edges?but are not attached to each other. The part of plates that make up the seafloor is called?oceanic crust. The continents are just the parts of tectonic plates that rise above sea level.? ?Beneath the plates is the molten rock of Earth’s mantle.?The rock of the plates is less dense than this molten rock, so the tectonic plates,?including those carrying continents, float on the mantle.?

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An essential part of the tectonic plate explanation?of continental drift is the idea that molten rock from the mantle is forced up through?the seafloor at mid-ocean mountain ridges where two plates meet.?This molten material adds new oceanic crust to the plates on either side of the ridge.?The seafloor spreads out in both directions from the mid-ocean ridges,?and the tectonic plates on either side of the ridge move apart from each other as?new oceanic crust is added between them. Simultaneously, at the edge of a plate furthest?from the mid-oceanic ridge, the oceanic crust is being continuously destroyed by being?pushed down into the mantle in regions called trenches. As the oceanic crust of the plates?grows on one edge and is destroyed on the opposite edge, the plates and their attached?continents move across Earth’s surface.?

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According to this hypothesis, the European and North American?plates should be steadily moving away from each other at a rate of up to several centimeters per year.?

Precise measurements of continental positions by satellites have verified this, thus?

?confirming the validity of the plate movement hypothesis. Furthermore, if plates are moving?away from mid-oceanic ridges as predicted by the tectonic plate hypothesis, then oceanic?crust should become progressively older with increasing distance from these ridges. To test?this prediction, deep-sea sediment and oceanic crust were sampled. The younger the oceanic?crust, the less sediment would have been deposited on it. Analysis of the oceanic crust and?the layer of sediment above it showed that the age of oceanic crust does?indeed increase with distance from the mid-oceanic ridges, and that the oldest oceanic?crust was furthest from the ridges.?

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?In view of the confirmation of these and other predictions?of the tectonic plate hypothesis, most geologists accept the hypothesis. Its acceptance??has been widespread because of the overwhelming evidence supporting it?and also because it explains the underlying relationship?between many seemingly unrelated geologic features and events.?

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