Lessons From the Sea Page Grade 5 Unit 4
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| Volcanoes are created by this collision of plates too, as well as all the world’s mountain ranges! How do you think this happens?
When the two giant masses of rock come together, they have to go somewhere, right? Usually the heavier rock, which is the lithosphere from the ocean bottom, grinds its way under the lighter rock of the continental crust. As the leading edge of the ocean plate sinks beneath the continental crust, the friction of the two great rock masses pushing past each other creates so much heat that it melts the rock into a thick liquid called magma. Some of this magma shoots up through cracks in the rock as lava, forming volcanoes. That is the reason why anywhere there are convergent plates, there are lots of volcanoes. Where this sinking area — or subduction zone — occurs, deep trenches are formed beneath the sea. Some of these deep ocean trenches, like the Mariana trench — where the Pacific plate dives beneath the Eurasia plate — are the deepest places on Earth. At over 11 kilometers (over 6.8 miles deep) the Mariana trench is 1 mile deeper than the tallest mountain, almost 7 times as deep as the Grand Canyon in Arizona, and 10 times deeper than Waimea Canyon in Kaua‘i, which Mark Twain called the “Grand Canyon of the Pacific.” In some places, the oceanic crust is particularly thin, or the cracks in it particularly large, so that magma has a direct route to the surface. These areas are called hot spots. Most hot spots are found near the plate boundaries, but some, like the hot spot that created the Hawaiian Islands, are in the middle of a plate. These hot spots may be associated with a peculiarity in the asthenosphere of an unusually hot bubble of magma in that area. Frequently, the plates stick against each other for awhile as they try to move past, and then suddenly break free. This is what causes earthquakes to happen. Great chunks of rock are pushed up to the surface by these earthquakes, gradually growing into tall mountain ranges. 
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