Heat and pressure transform shale into metamorphic rocks. The specific type of metamorphic rock depends on the intensity of the heat and pressure:

* Low-grade metamorphism: Shale transforms into slate, a fine-grained, fissile rock.

* Medium-grade metamorphism: Slate changes into phyllite, a rock with a sheen caused by the alignment of mica minerals.

* High-grade metamorphism: Phyllite transforms into schist, a rock with larger mica crystals. Even higher temperatures and pressure can lead to the formation of gneiss, a banded metamorphic rock.

So, in essence, heat and pressure "bake" the shale into a harder, more crystalline rock. This process is called metamorphism.