Shale

A piece of shale.

Shale is a fine-grained sedimentary rock whose original constituents were clays 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.[1]

Formation

The process in the rock cycle which forms shale is compaction. The fine particles that compose shale can remain suspended in air long after the larger and denser particles of sand have deposited out. 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 (see oil shale).

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, and metamorphic material known as slate, which is often used in building construction.

Limey shale overlaid by limestone, Cumberland Plateau, Tennessee

Weathering shale at a road cut in southeastern Kentucky

See also

• Bituminous shale
• Oil shale
• Burgess shale
• List of minerals
• Barnett Shale
• Slate
• Siltstone
• Mudstone

References

ISBN links support NWE through referral fees

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

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