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Metamorphic Rock Formation from sedimentary rocks is caused by heat and pressure. Where does this heat and pressure come from?

The heat and pressure comes from inside the earth. From the upper mantle up to within a few kilometers of the surface of the earth there is a tremendous amount of heat and pressure. This heat and pressure increase with depth. It is estimated that the temperature increases about 20o to 30o C per kilometer of depth.

Plate Tectonics Adds Heat And Pressure
There is something else that adds to both the heat and pressure, and that is plate tectonics. 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.

For metamorphism to occur energy is needed to fuel the chemical reactions. Heat is the primary source of this energy.

The pressure within the earth is the result of gravity pulling the crust of the earth downward.Like heat, pressure increases with depth. 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.

Sedimentary rocks exposed to this kind of heat and pressure are transformed into new kinds of rocks. Limestone changes to marble. Shale changes to slate.

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