Coal is a combustible, black or brownish-black rock widely used as a major energy source. Geologically, it is classified as a biochemical sedimentary rock, sometimes referred to as an organic sedimentary rock. This classification is based on its origin, as coal forms from the accumulation and burial of once-living plant matter. The transformation into solid rock involves chemical and physical changes occurring over millions of years.
The Three Major Types of Sedimentary Rock
Sedimentary rocks are formed from the deposition and cementation of material derived from pre-existing rocks or organic remains. Geologists broadly divide these rocks into three main categories based on the source of their constituent material. Understanding these categories provides the context for coal’s unique classification.
Clastic sedimentary rocks are composed of fragments, or clasts, of older rocks and minerals that have been weathered, transported, and deposited. These fragments can range in size from microscopic clay to large boulders, and common examples include sandstone, shale, and conglomerate. The pieces are cemented together through a process called lithification, which involves compaction and the precipitation of mineral cements between the grains.
Chemical sedimentary rocks form when minerals precipitate out of water solutions, typically due to evaporation or changes in water chemistry. Rock salt (halite) and gypsum are common examples that form in arid environments where water bodies evaporate, leaving behind concentrated mineral deposits. Some types of limestone can also form chemically when calcium carbonate precipitates directly from seawater.
Organic, or biochemical, sedimentary rocks are characterized by being composed primarily of the remains of once-living organisms. These rocks include coquina, which is made of shell fragments, and certain limestones formed from marine organisms. Coal falls into this third category because its primary components are the carbon-rich remnants of ancient terrestrial plants that have been preserved and compacted.
Coal’s Classification as an Organic Rock
Coal is fundamentally an organic sedimentary rock because it originates from the accumulation and preservation of large quantities of plant material. The process begins in water-logged environments, such as swamps or bogs, where stagnant, low-oxygen conditions prevent the complete decay of dead vegetation. This anoxic environment slows the decomposition carried out by bacteria and fungi, allowing the organic material to accumulate.
The initial material that forms is peat, a soft, spongy, and porous deposit consisting of partially decomposed plant debris. Peat is not yet considered a rock, but rather the precursor sediment rich in water and volatile compounds. This accumulation must occur over long periods, with the rate of plant growth exceeding the rate of decay, to create thick layers of carbon-rich sediment.
The term “biochemical” applies because living organisms supplied the material, and biological processes, such as anaerobic decay, played a significant role in concentrating the carbon. This biological origin distinguishes coal from clastic rocks formed by physical erosion and chemical rocks formed by inorganic precipitation. The organic matter provides the high carbon content that defines the rock’s rank and energy potential.
The Process of Coalification
The transformation of peat into coal is known as coalification, a progressive process driven by burial, heat, and pressure. As more sediment and rock layers accumulate on top of the peat, the increasing weight compresses the material, squeezing out water and reducing its volume significantly. This compaction, along with the rising temperature from the Earth’s geothermal gradient, initiates chemical changes that increase the carbon concentration.
The first true coal rank to form is lignite, often called brown coal, which is the lowest grade with a carbon content typically ranging from 60 to 70%. Lignite is soft, contains high moisture, and yields the least amount of energy upon burning.
With deeper burial and higher temperatures, lignite transforms into sub-bituminous coal, which is darker, denser, and has a higher heating value. Further coalification leads to bituminous coal, a mid-rank coal that is harder, blacker, and much more common globally for electricity generation. Bituminous coal has a carbon content of approximately 76 to 90% and is characterized by its distinct banded layers.
The final stage of coalification results in anthracite, the highest rank of coal, which contains the highest fixed-carbon content, often exceeding 90%. Anthracite is a hard, brittle, and lustrous black rock that burns with a clean, hot flame and very little smoke. In some classifications, anthracite is considered a low-grade metamorphic rock because the extreme heat and pressure required for its formation often exceed typical sedimentary conditions. The entire sequence, from peat to anthracite, represents a continuum of increasing maturity, with carbon enrichment directly correlating to the energy density of the resulting rock.