Shale

A piece of shale

Shale is the most common sedimentary rock. It is a fine-grained rock that is formed from clays or muds. Shales contain fossils and often form reservoirs of petroleum and other hydrocarbons. These rocks are also used as construction material.

Characteristics

Shale 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 .0625 millimeters are described as mudstones. Rocks with similar particle sizes but less clay (and therefore grittier) are known as siltstones.

Formation

Weathering shale at a road cut in southeastern Kentucky

Limey shale overlaid by limestone in Tennessee

The process in the rock cycle that 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 (such as 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.

See also

• Rock
• Sand
• Sedimentary rock
• Silt
• Slate

References

ISBN links support NWE through referral fees

• Blatt, Harvey, and Robert J. Tracy. 1995. Petrology: Igneous, Sedimentary, and Metamorphic, 2nd ed. New York: W.H. Freeman. ISBN 0716724383
• Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412
• Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060
• Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals, Rev. ed. New York: St. Martin's Press. ISBN 1582381321
• Skinner, Brian J., Stephen C. Porter, and Jeffrey Park. 2004. Dynamic Earth: An Introduction to Physical Geology, 5th ed. Hoboken, NJ: John Wiley. ISBN 0471152285
• Tucker, Maurice E. 2001. Sedimentary Petrology. 3rd ed. Oxford: Blackwell Publishing. ISBN 0632057351

External links

All links retrieved January 27, 2023.

• Shale – About.com: Geology, by Andrew Alden
• The Burgess Shale – University of California Museum of Paleontology

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