The time of onset of plate tectonics is controversial. Many sources believe it began 1 to 2 billion years after Earth's formation, whereas others view plate tectonics as being far more ancient, its inception dating back over 4 billion years. Much of the controversy involves the rate of heat flow from the early Earth and how this would have affected the composition and rigidity of the planet's surface.
By the time the crust had solidified, more than half of the heat that could result from planetary accretion, core formation, and decay of radioactive isotopes (such as uranium-235) had already been lost from Earth. During the 2 billion years of the Archaean era, heat flow slowed. Some workers believe that the early crust was still too hot and thin to act as the rigid plate necessary for plate tectonics; according to this hypothesis, plate tectonics may not have commenced until 2.5 billion years ago. There is evidence in much older rocks, however, of fault lines and movements consistent with plate tectonics.
The rate at which plate tectonics built continental surfaces on Earth was not constant. If we plot the size of the continents through time relative to the present area, we do see not a linear increase but a logistic curve—a curve that started slowly, picked up speed in its middle, and then slowed near the end. We have spoken in another context of the "Cambrian Explosion." Here, Earth underwent a "continental explosion" that resulted in a rapid formation of land area. Many lines of evidence suggest that by far the greatest growth took place rather rapidly, during a period between about 2 and 3 billion years ago. This rapid growth completely changed Earth from a planet dominated by oceans to one dominated (at least in terms of its global temperatures and chemistry) by continents.
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