What is Coal?

Coal is an organic sedimentary rock that is formed from compressed plant matter and debris accumulated in slow-moving bodies of water, such as swamps. It is predominantly composed of organic carbon, which makes it energy dense. Coal is an important economic deposit used for energy production and industrial applications.

How Does Coal Form?

Coal forms from peat, a spongy accumulation of organic material that has not fully decayed. Peat forms in fluvial, lacustrine, and coastal wetland settings, including swamps, bogs, and marshes. As the peat is buried by overlying sediment and subjected to increased temperatures and pressures, it undergoes a physical and chemical transformation into coal. The longer the peat is buried, the more likely it is to be exposed to higher temperatures and pressures. This causes the carbon in the coal to become more concentrated, and decreases the amount of water, oxygen, and hydrogen present.

Almond Formation coal
Almond Formation coal:  Outcrop of a coal bed and overlying sandstone in the Almond Formation. These rocks formed in a coastal plain along the western shore of the Western Interior Seaway. Photo was taken near Baggs in the south-central part of Wyoming. [Credit: WSGS]

Coal Ranks

Coals are categorized by rank depending on the degree of transformation they have experienced. The four coal ranks are lignite, subbituminous, bituminous, and anthracite. These different coals can be identified by their color, heat content, and shininess. Lignite is a soft, dull coal, often brown, with the least amount of stored energy because it had the least amount of pressure and temperature applied, and has the lowest carbon concentration. Subbituminous coal is black and harder than lignite, but is not very shiny. Bituminous coal is black, shiny, and has a substantial amount of heat value. Anthracite is the hardest, and this black shiny coal has the highest density of any coal type.

Most of Wyoming’s Paleogene coals are subbituminous, and most of the Cretaceous coal is subbituminous to bituminous. In Wyoming basins, generally the older and deeper coals are higher in rank than the younger coals. Consequently, the deeper the Powder River Basin coal mines mine, the higher the rank of the coal that is extracted.

What Makes Up Coal?

Coal differs from other rocks because it is predominantly composed of organic molecules containing carbon, hydrogen, nitrogen, oxygen, and sulfur. These compounds, called macerals, are the building blocks of coal, much in the same way that minerals make up rocks. Macerals are classified by the type of organic matter they originated from. Coals can contain fragments from trees, plants, and fungi, including seeds, bark, spores, roots, and stems. Macerals determine the internal chemistry of the coal.

Coal also contains an inorganic component, which can occur as discrete layers within the coal or dispersed as single grains or crystals throughout the organic matrix. These layers can consist of fine-grained sediment deposited during a flood event, or ash-rich layers deposited from volcanic ash fall. Minerals dispersed throughout coal can be primary, meaning they were deposited along with the organic matter via wind or water, or secondary, meaning they formed after the organic matter was deposited.


Kelsey Kehoe, kelsey.kehoe@wyo.gov