Dictionary:

shale

  (shāl)

A fissile rock composed of layers of claylike, fine-grained sediments.

[Probably from Middle English, shell, from Old English scealu.]

A class of fine-grained clastic sedimentary rocks with a mean grain size of less than 0.0625 mm (0.0025 in.), including siltstone, mudstone, and claystone. One-half to two-thirds of all sedimentary rocks are shales. See also Sedimentary rocks.

Shale is deposited as mud, that is, small particles of silt and clay. The particles are deposited when fluid turbulence caused by currents or waves is no longer adequate to counteract the force of gravity, or if the water evaporates or infiltrates into the ground. Clay particles often form larger aggregates which settle from suspension more rapidly than individual particles. Silt particles and clay aggregates are often deposited as thin layers less than 10 mm (0.4 in.) thick called laminae. See also Depositional systems and environments.

Mineralogically, most shales are made up of clay minerals, silt-sized quartz and feldspar grains, carbonate cements, accessory minerals such as pyrite and apatite, and amorphous material such as volcanic glass, iron and aluminum oxides, silica, and organic matter. The most common clay minerals in shales are smectite, illite, kaolinite, and chlorite. The type of clay particles deposited is dependent on the mineralogy, climate, and tectonics of the source area. See also Clay minerals; Chlorite; Illite; Kaolinite.

Shales are usually classified or described according to the amount of silt, the presence and type of lamination, mineralogy, chemical composition, and color. Variations in these properties are related to the type of environment in which the shale was deposited and to postdepositional changes caused by diagenesis and compaction. See also Diagenesis.

The small size of pores in shale relative to those in sandstone causes shale permeability to be much lower than sand permeability. Although fracturing due to compaction stresses or to tectonic movements can create deviations from this general trend, shales often form permeability barriers to fluid movement; this has important bearing on the occurrence of subsurface water and hydrocarbons. Ground-water aquifers are commonly confined by an underlying low-permeability shale bed or aquiclude, which prevents further downward movement of the water. Hydrocarbon reservoirs are often capped by low-permeability shale which forms an effective seal to prevent hydrocarbons from escaping. See also Aquifer.

A fine-grained sedimentary rock formed when layers of clay are compressed by the weight of overlying rocks. Shales have a layered structure and are easily split along the bedding planes.

For more information on shale, visit Britannica.com.

Argillaceous sedimentary rock derived from clays or silts; typically thinly laminated and weak along planes; may be undesirable as a concrete aggregate.

A sedimentary rock formed from layers of clay.

Shale

Shale is a fine-grained sedimentary rock whose original constituents were clays or muds. It is characterized by thin laminae^[1] 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.^[2]

Formation

Limey shale overlaid by limestone, Cumberland Plateau, Tennessee

The process in the rock cycle which forms shale is compaction. The fine particles that compose shale can remain suspended in water 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. Also this rock is easy to cut into because it is formed by the compression of small molecules

'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, metamorphic rock known as slate, which is often used in building construction.

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
• Bearpaw Shale
• Wianamatta shale

Footnotes

1. ^ "shale". Chambers Dictionary of Science and Technology. (1999). Chambers Harrap Publishers Ltd. 
2. ^ Rocks: Materials of the Lithosphere - Summary. Retrieved on 2007-07-31.

References

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

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Dansk (Danish)
n. - skiferler, skiffer

Nederlands (Dutch)
(klei)schalie

Français (French)
n. - schiste, shale

Deutsch (German)
n. - Schiefer

Ελληνική (Greek)
n. - (γεωλ.) αργιλικός σχιστόλιθος

Italiano (Italian)
scisto

Português (Portuguese)
n. - xisto (m)

Русский (Russian)
глинистый сланец

Español (Spanish)
n. - esquisto, pizarra

Svenska (Swedish)
n. - skifferlera, skiffer

中文（简体） (Chinese (Simplified))
页岩, 泥板岩

中文（繁體） (Chinese (Traditional))
n. - 葉岩, 泥板岩

한국어 (Korean)
n. - 혈암, 셰일, 이판암

日本語 (Japanese)
n. - 頁岩, 泥板岩

العربيه (Arabic)
‏(الاسم) ألزيت ألحجري‏

עברית (Hebrew)
n. - ‮צפחה, אבן פצלתית‬

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