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IGNEOUS ROCKS
Lesson 5 - Page 4

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The formation of granite is a complex process. It can be explained by plate tectonics. Almost all granite forms at convergent plate boundaries where subduction is taking place. Subduction is the downward plunging of one plate under another. Granite forms in two steps. In the first step, the subducting plate dives into the Earth’s mantle, heats up and begins to melt. This creates magma. Magma rises, and causes melting in the rock above it, making more magma. When this magma cools, it makes igneous rocks like diorite, not granite.

In the second step, subduction, melting, and magma formation continue. However, when the magmas formed in this step rise upward, they cause part of the diorite formed in the first step to begin to melt. This, combined with melting of surrounding sedimentary and igneous rocks, makes magma which cools to become granite. 


Areas of granite formation

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