Key Facts to Know about Shale Rock

Geology, Composition, and Uses

Shale is a common sedimentary rock, known for flaking into sheets.
Shale is a common sedimentary rock, known for flaking into sheets. Gary Ombler / Getty Images

Shale is the most common sedimentary rock, accounting for about 70 percent of the rock found in the Earth's crust. It is a fine-grained clastic sedimentary rock made of compacted mud consisting of clay and tiny particles of quartz, calcite, mica, pyrite, other minerals, and organic compounds. Shale occurs worldwide wherever water exists or once flowed.

Key Takeaways: Shale

  • Shale is the most common sedimentary rock, accounting for about 70 percent of the rock in the Earth's crust.
  • Shale is a fine-grained rock made from compacted mud and clay.
  • The defining characteristic of shale is its ability to break into layers or fissility.
  • Black and gray shale are common, but the rock can occur in any color.
  • Shale is commercially important. It is used to make brick, pottery, tile, and Portland cement. Natural gas and petroleum may be extracted from oil shale.

How Shale Forms

Layers - Estratos
siur / Getty Images

Shale forms via compaction from particles in slow or quiet water, such as river deltas, lakes, swamps, or the ocean floor. Heavier particles sink and form sandstone and limestone, while clay and fine silt remain suspended in water. Over time, compressed sandstone and limestone become shale. Shale typically occurs in a broadsheet, several meters thick. Depending on the geography, lenticular formations may also form. Sometimes animal tracks, fossils, or even imprints of raindrops are preserved in shale layers.

Composition and Properties

Colorful shale in Kings Cove, Newfoundland
Kristin Piljay / Getty Images

The clay clasts or particles in shale are less than 0.004 millimeters in diameter, so the structure of the rock only becomes visible under magnification. The clay comes from decomposition of feldspar. Shale consists of at least 30 percent clay, with varying amounts of quartz, feldspar, carbonates, iron oxides, and organic matter. Oil shale or bituminous also contains kerogen, a mixture of hydrocarbons from deceased plants and animals. Shale is classified based on its mineral content. There is siliceous shale (silica), calcareous shale (calcite or dolomite), limonitic or hematitic shale (iron minerals), carbonaceous or bituminous shale (carbon compounds), and phospatic shale (phosphate).

The color of shale depends on its composition. Shale with a higher organic (carbon) content tends to be darker in color and may be black or gray. The presence of ferric iron compounds yields red, brown, or purple shale. Ferrous iron yields black, blue, and green shale. Shale containing a lot of calcite tends to be pale gray or yellow.

The grain size and composition of minerals in shale determine its permeability, hardness, and plasticity. In general, shale is fissile and readily splits into layers parallel to the bedding plane, which is the plane of clay flake deposition. Shale is laminated, meaning the rock consists of many thin layers that are bound together.

Commercial Uses

Fracking can extract petroleum and natural gas from oil shale.
grandriver / Getty Images

Shale has many commercial uses. It is a source material in the ceramics industry to make brick, tile, and pottery. Shale used to make pottery and building materials requires little processing besides crushing and mixing with water.

Crushing shale and heating it with limestone makes cement for the construction industry. Heat drives off water and breaks limestone into calcium oxide and carbon dioxide. Carbon dioxide is lost as a gas, leaving calcium oxide and clay, which harden when mixed with water and dried.

The petroleum industry uses fracking to extract oil and natural gas from oil shale. Fracking involves injection of liquid at high pressure into the rock to force out the organic molecules. High temperatures and special solvents extract the hydrocarbons, leading to waste products that raise concerns about environmental impact.

Shale, Slate, and Schist

Increasing pressure and temperature changes shale into slate, which in turn may become phyllite, schist, and gneiss.
versh / Getty Images

Up to the mid-19th century, the term "slate" often referred to shale, slate, and schist. Underground coal miners may still refer to shale as slate, per tradition. These sedimentary rocks have the same chemical composition and may occur together. The initial sedimentation of particles forms sandstone and mudstone. Shale forms when the mudstone becomes laminated and fissile. If shale is subjected to heat and pressure, it can metamorphose into slate. Slate can become phyllite, then schist, and eventually gneiss.


  • Blatt, Harvey and Robert J. Tracy (1996) Petrology: Igneous, Sedimentary and Metamorphic (2nd ed.). Freeman, pp. 281–292.
  • H.D. Holland (1979). "Metals in black shales – A reassessment". Economic Geology. 70 (7): 1676–1680.
  • J.D. Vine and E.B. Tourtelot (1970). "Geochemistry of black shale deposits – A summary report". Economic Geology. 65 (3): 253–273.
  • R. W. Raymond (1881) "Slate" in .A Glossary of Mining and Metallurgical Terms American Institute of Mining Engineers.
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Helmenstine, Anne Marie, Ph.D. "Key Facts to Know about Shale Rock." ThoughtCo, Aug. 31, 2021, Helmenstine, Anne Marie, Ph.D. (2021, August 31). Key Facts to Know about Shale Rock. Retrieved from Helmenstine, Anne Marie, Ph.D. "Key Facts to Know about Shale Rock." ThoughtCo. (accessed May 29, 2023).