Slate
Slate is a smooth, hard metamorphic rock formed from clay or fine-grained rocks like shale and basalt under low temperature and pressure conditions within the Earth. It is characterized by its ability to split into thin, strong slabs, making it a popular material for various applications, such as roofing, flooring, and tabletops. Slate typically appears in colors like red, green, gray, or black due to the presence of various minerals, which contribute to its mottled or streaked appearance. Major slate production regions include Pennsylvania, Vermont, and Wales, where slate is extracted through open-pit quarrying methods.
In addition to its use as cut slate, crushed slate serves as an aggregate material in construction and is utilized in roofing products. Historically, slate has been employed since colonial times in the United States and was even used in pencil production until the 1930s. The unique properties of true slate, which can be distinguished from other similar-looking rocks by its splitting angle, enhance its desirability in construction and decorative applications. Overall, slate's versatility and durability make it a valuable resource in various industries.
Slate
Where Found
Slate is found worldwide, generally in areas where older rocks from beneath the surface of the Earth have been exposed or in areas where younger rocks have been subjected to the pressure of mountain building. The most important areas of slate production are Pennsylvania, Vermont, and Wales.
Primary Uses
Cut slate is used for paving, roofing, electrical panels, blackboards, and tabletops. Crushed slate is used in aggregates (materials mixed with cement to make concrete).
Technical Definition
Slate is formed when clay or smooth, fine-grained rock such as shale or basalt is subjected to relatively low temperature and pressure beneath the Earth’s surface. This low-grade metamorphic process results in the formation of very small, often microscopic, crystals. The pressure inside the Earth causes these crystals to be flattened and elongated, resulting in the ability of slate to be split into thin slabs.
Description, Distribution, and Forms
Slate is a smooth, hard, metamorphic rock that easily splits into thin, strong slabs. Slate is usually red, green, gray, or black, but some slate is blue or purple. The color of slate is often mottled, streaked, or spotted. A few large grains of various minerals may be found within the otherwise homogeneous material. Thin lines may appear, which reveal the structure of the rock from which the slate was formed.
Slate varies widely in color because of the presence of small particles of various minerals within it. Red and purple slates contain hematite (iron oxide). Green slates contain chlorites (various green materials composed of aluminum silicates). Gray and black slates may contain carbon in the form of graphite, organic matter, or iron sulfide.
Other rocks that can be split into thin slabs are often loosely called slates. True slates can be distinguished by the fact that the angle at which they split is different from the angle at which they lie. Other rocks known as “slate” usually split into slabs parallel to the way they lie.
History
Slate has been used as a roofing material in the United States since early colonial times. Slate was also used in pencils until the 1930’s.
Obtaining Slate
Slate is obtained by quarrying from shallow open-pit mines. When an area containing slate is discovered, the material covering the slate is removed with power shovels and bulldozers. Large blocks of slate are then removed by drilling and by using the natural tendency of the rock to split. The blocks of slate, which are about 7.5 centimeters thick, are split with chisels and mallets into slabs about 0.5 centimeter thick. The slabs are then cut to the desired size by rotating blades.
Uses of Slate
Cut slate, known as dimension slate, is used to form panels for roofs, floors, and pavement. It is also used to provide a smooth, strong surface for electrical panels, blackboards, billiard tables, and laboratory tabletops. Slate that is not smooth enough to be used in the form of slabs may be mined using drills and explosives. The blasted slate is crushed and used in filler, in aggregate, or in roofing.
Bibliography
Natural Resources Canada.
U.S. Geological Survey.