shale, sedimentary rock formed by the consolidation of mud or clay, having the property of splitting into thin layers parallel to its bedding planes. Shale tends to be fissile, i.e., it tends to split along planar surfaces between the layers of stratified rock. Shales comprise an estimated 55% of all sedimentary rocks. The composition of shale varies widely. Shales with very high silica content may have been formed when large quantities of diatoms and volcanic ash were present in the original sediment. Large numbers of fossils in shales may give them a high calcium content; such shales may grade into limestones. Shales that contain a large percentage of alumina are used as a source of that mineral in the manufacture of cement. Shales containing abundant carbonaceous matter grade into bituminous coal. Oil shales are widely distributed in the W United States and may be a future source of petroleum.

Synthetic crude oil that is extracted from oil shale by pyrolysis, or destructive distillation. The oil obtained from oil shale cannot be refined by the methods that have been developed for crude oil, however, because shale oil is low in hydrogen and contains large amounts of nitrogen and sulfur compounds. To be made usable, shale oil must be hydrogenated and then chemically treated to remove the nitrogen and sulfur, a process too expensive to make shale oil commercially competitive with crude oil. Seealso kerogen, petroleum.

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Any of a group of fine-grained, laminated sedimentary rocks consisting of silt- and clay-sized particles. Shale constitutes roughly 60percnt of the sedimentary rock in the Earth's crust. Shales are commercially important, particularly in the ceramics industry. They are a valuable raw material for tile, brick, and pottery and constitute a major source of alumina for portland cement. In addition, advances in recovery methods may one day make oil shale a practical source for liquid petroleum.

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Any fine-grained sedimentary rock that contains solid organic matter (kerogen) and yields significant quantities of oil when heated. This shale oil is a potentially valuable fossil fuel, but the present methods of mining and refining it are expensive, damage the land, pollute the water, and produce carcinogenic wastes. Thus, oil shale will probably not be exploited on a wide scale until other petroleum resources have been nearly depleted. Estonia, China, and Brazil have facilities for producing relatively limited quantities, and the U.S. government operates an experimental plant in Colorado.

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Shale (also called mudstone) is a fine-grained sedimentary rock whose original constituents were clay minerals or muds. It is characterized by thin laminae breaking with an irregular curving fracture, often splintery and usually parallel to the often-indistinguishable bedding plane. This property is called fissility. Non-fissile rocks of similar composition but made of particles smaller than 1/16 mm are described as mudstones. Rocks with similar particle sizes but with less clay and therefore grittier are siltstones. Shale is the most common sedimentary rock.


The process in the rock cycle which forms shale is compaction. The fine particles that compose shale can remain in water long after the larger and denser particles of sand have deposited. Shales are typically deposited in very slow moving water and are often found in lake and lagoonal deposits, in river deltas, on floodplains and offshore of beach sands. They can also be deposited on the continental shelf, in relatively deep, quiet water.

'Black shales' are dark, as a result of being especially rich in unoxidized carbon. Common in some Paleozoic and Mesozoic strata, black shales were deposited in anoxic, reducing environments, such as in stagnant water columns.

Fossils, animal tracks/burrows and even raindrop impact craters are sometimes preserved on shale bedding surfaces. Shales may also contain concretions.

Shales that are subject to heat and pressure alter into a hard, fissile, metamorphic rock known as slate, which is often used in building construction.

See also



  • Blatt, Harvey and Robert J. Tracy, 1996, Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., Freeman, ISBN 0-7167-2438-3

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