Slate usually forms in a tectonic environment when a convergent plate boundary meets a basin that was once made of sedimentary rock. Heat and pressure convert the substances in the basin, specifically shale, to slate. It can be hard to tell when shale becomes slate, according to Geology.com.
Slate is composed of many of the same minerals that make up shale. When shale is heated and put under heavy pressure from tectonic plate activity, the clay minerals inside the rock revert to mica. This gives the new rock the ability to break apart, or cleave, along flat planes. These planes sometimes run in a different direction from the original planes in the shale, which can ruin fossils inside the slate.
Slate is a metamorphic rock, meaning it was created when a pre-existing rock transformed. During this process, the original rock, which is called a protolith, transforms after being put under extreme pressure and high heat. Igneous, sedimentary and metamorphic rocks can all serve as protoliths. Metamorphic rocks such as slate can also form when protoliths are exposed to magma. Low-grade metamorphism produces slate, meaning extreme pressures and temperatures are not required. Gneiss and schist are two examples of high-grade metamorphic rocks.