Slate forms when shale, which consists of clay minerals, is put under pressure with temperatures of a few hundred degrees or so. Then the clays begin to revert to the mica minerals
from which they formed. This does two things: first, the rock grows hard enough to ring or "tink" under the hammer, and second, the rock gets a pronounced cleavage direction, so that it breaks along flat planes. Slaty cleavage
is not always in the same direction as the original sedimentary bedding planes, thus any fossils originally in the rock are usually erased, but sometimes they survive in smeared or stretched form.
With further metamorphism, slate turns to phyllite, then schist or gneiss.
Slate is usually dark, but it can be colorful too. High-quality slate is an excellent paving stone as well as the material of long-lasting slate roof tiles and, of course, the best billiard tables. Blackboards and handheld writing tablets were once made of slate, and the name of the rock has become the name of the tablets themselves.
See other pictures in the Slate Gallery. For more photos see the Metamorphic Rocks Gallery.
Geologic Features and Processes
Glaciers and Ice
Geology and Society