Shale Rock: Geology, Composition, 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 is found all over the world in areas where water exists or once flowed.

How Shale Forms

Sandstone and shale can preserve animal tracks, like these dinosaur footprints near Denver, Colorado.
Sandstone and shale can preserve animal tracks, like these dinosaur footprints near Denver, Colorado. milehightraveler / Getty Images

Shale forms via compaction, typically from particles in slow or quiet water, such as river deltas, lakes, swamps, or the ocean flood. Heavier particles sink, ultimately forming sandstone and limestone, while clay and fine silt remain suspended in water. Over time, these fine particles settle and build upon each other, forming rock. Shale typically occurs in a broad sheet, 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

The clay clasts or particles in shale are less than 0.004 millimeters in diameter, which means 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 tends to be classified based on its mineral content, so 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 the composition of the minerals. 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.
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.

Shale is crushed and heated with limestone to make cement for the construction industry. Heating 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 hardens when mixed with water and allowed to dry.

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. Typically high temperatures and special solvents are needed to 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.
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" was often used to refer 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.

Key Takeaways

  • 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 fissility. In other words, shale readily breaks into thin layers.
  • 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.

Sources

  • 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 Metallurigical Terms, American Institute of Mining Engineers. p. 78.