The Scoop on Divergent Plate Boundaries

Divergent plate boundaries are where two plates move away from each other. The mid-ocean ridges are places where mantle asthenosphere rises slowly upward. As it rises, some of the rock melts to form magma. The melting temperature of rock decreases as the pressure on the rock decreases. As the mantle rock rises, its temperature stays about the same, because cooling takes a long time. However, the pressure from the overlying rock is less, so some of the rock melts. The magma then rises up, because it is less dense than the rock. It forms volcanoes in the central valley of the mid-ocean ridge. Geoscientists in deep-diving submersibles can watch these undersea volcanoes! Because of the great water pressure in the deep ocean, and also the cooling effect of the water, these volcanoes behave differently from volcanoes on land. The lava oozes out of cracks in the rocks, like toothpaste out of a tube. Some of the magma stays below the sea floor and crystallizes into rock there. All of these new igneous rocks, at the sea floor and below, make new oceanic crust, which then moves away from the ridge crest.

In both the North Atlantic Ocean and the South Atlantic Ocean, there is no plate boundary along the coastlines on either side of the ocean. That tells you that the Atlantic Ocean is getting wider as time goes on. Why? Because new lithosphere is being created all the time at the mid-ocean ridge but is not being consumed at the edges of the continents. Does that make you wonder what would happen if you could make time run backward and watch the ocean shrink? At some time in the past, there was no Atlantic Ocean!

A new ocean begins when hot mantle material begins to move upward beneath a continent. Geoscientists are still not certain about why that happens. The lithosphere of the continent bulges upward and is stretched sideways. Eventually it breaks along a long crack, called a rift. Magma rises up to feed volcanoes in the rift. As the rift widens, the ocean invades the rift. A new ocean basin has now been formed, and it gets wider as time goes on. 

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