Slate is a hard, fine-grained metamorphic rock that forms when sedimentary rocks, such as shale and mudstone, are subjected to relatively low temperature and pressure. It occurs chiefly among older rocks. Millions of years of geological compression force the flaky minerals (mica, chlorite, quartz) within clay sediments to shift perpendicular to the pressure. This pushing alters the material's fundamental structure and creates a new feature known as slaty cleavage. True slate splits easily along this plane into thin, but durable, sheets.

While slate's characteristic color is gray-blue, varieties range from dark gray to black. Organic materials present in the parent rock can create different tinges. Iron oxide creates a reddish purple tinge; chlorite turns slate green. The rock also varies greatly in surface texture and luster; some slates have a dull, matte finish while others can be as shiny as mica.

Better grades of the rock are widely used for roofing, flooring and sidewall cladding. Slate is also used to make blackboards and pool tables. Pennsylvania and Vermont serve as the principal slate producers for the United States, although slate mines can also be found in Maine, Georgia, Lake Superior, and the Rocky Mountains.

See Also

Bedding; Metamorphism; Sedimentation