Heat and pressure deep in the earth change shale to slate. Why does this happen?

Because of the ground above and the molten iron core.

Explanation:

As you go deeper into the earth you have more and more of the earth above you that applies pressure on you. The heat near the very top of the plate is relatively low and it decreases by a little as you go down. But once you start to go down into the earth by hundreds of meters then the temperature starts to rise and it rises pretty quickly due to the heat being transferred from the molten iron outer core of the earth.