Metamorphic Rock Formation from igneous rocks is caused by heat and pressure. This is most often the result of mountain building.
Plate Tectonics and Mountain Building
When the plates of the earth collide, they squeeze the rocks at the borders with unbelievable force. This force increases the pressure in this and surrounding areas. Friction is also created by the plates grinding together. This friction generates enough heat to melt the rocks at the point of contact. It also causes mountains to be built.
This pressure can actually squeeze the spaces out of the minerals within the rock. This makes the rocks denser. The heat and pressure together cause the rock to flow instead of break or fracture. The mineral grains become realigned. They flatten out and get longer.
For metamorphism to occur energy is needed to fuel the chemical reactions. Heat is the primary source of this energy.
Igneous rocks exposed to this kind of heat and pressure are transformed into new kinds of rocks. Granite changes to gneiss.
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