Coal is a dark, combustible sedimentary rock originating from the compressed organic remains of ancient plant life. The quality of a coal deposit is measured by its rank, reflecting the degree of physical and chemical alteration it has undergone. Bituminous coal, often called “soft coal,” is a high-grade coal positioned between sub-bituminous coal and the highest rank, anthracite. It is characterized by a high energy content, typically yielding over 10,500 British Thermal Units per pound, and a fixed carbon content ranging from 77 to 87 percent. Bituminous coal forms under geological conditions involving heat and pressure, which transform the initial plant matter into a dense, carbon-rich fuel source.
The Starting Material: Peat Formation
The coalification process begins with the massive accumulation of plant material, the precursor substance known as peat. This accumulation occurs in environments like swamps, bogs, and marshlands, where constant waterlogging creates an anaerobic, low-oxygen environment. This environment prevents the complete decomposition of dead vegetation, including wood, leaves, and mosses. The resulting biochemical degradation transforms the material into a soft, water-saturated, spongy organic deposit. Over thousands of years, this incomplete breakdown forms thick layers of peat, which is high in moisture and volatile compounds.
Initial Compression and Chemical Change
The transition from peat to the first true coal requires geological burial. As sediments like sand, silt, and clay are deposited atop the peat, the increasing weight creates an overburden, applying vertical pressure. This compaction squeezes out significant water trapped within the peat structure. The pressure and resulting moderate temperature increase (below 100 degrees Celsius) drive the first chemical alteration, converting peat into lignite, or brown coal. During this stage, the material loses moisture, oxygen, and hydrogen, moderately increasing the carbon concentration.
With continued, shallow burial, lignite transitions into sub-bituminous coal. This low-rank coal is darker and denser than lignite, showing further consolidation. However, these conditions are not yet sufficient to drive off the volatile compounds needed to achieve the higher fixed carbon content of bituminous coal.
The Critical Transition to Bituminous Coal
The formation of bituminous coal requires an escalation in heat and pressure, shifting the process to organic metamorphism. This transition occurs when sub-bituminous coal is buried deep enough—often thousands of feet—to encounter elevated temperatures. Temperature is the primary factor determining the final rank of the deposit.
The geothermal gradient subjects the buried material to sustained temperatures ranging from 85 degrees Celsius (185 degrees Fahrenheit) up to 235 degrees Celsius (455 degrees Fahrenheit). This heat provides the energy necessary to break down complex organic molecules, causing the final phase of chemical alteration known as carbonization.
Carbonization results in the expulsion of remaining volatile matter, including gases like methane and carbon dioxide, and residual moisture. As these elements are driven off, the proportion of fixed carbon in the remaining rock dramatically increases. This high concentration of stable carbon defines bituminous coal, giving it a much higher energy value than its lower-rank precursors.
Although heat is the main driver of chemical change, the immense vertical pressure from the deep overburden physically compacts the material to a dense, hard state. Lateral tectonic stress, associated with mountain-building events, can also contribute to the final compaction and heating. While reaching the bituminous stage takes millions of years, high temperatures accelerate the chemical reactions.