Here's why:

* Sedimentary rocks form from the accumulation and cementation of sediments, like sand, mud, or shells.

* Igneous rocks form from the cooling and solidification of molten rock (magma or lava).

* Metamorphic rocks form when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemical reactions.

Slate is formed from the metamorphism of shale, a sedimentary rock. The heat and pressure cause the clay minerals in the shale to recrystallize, resulting in the characteristic layered structure of slate.