Heat: Heating is the primary control on coalification and rank increases in coal. On average, heat in the earth rises 1 degree Fahrenheit per 70 to 100 feet of depth. The deeper a coal is buried in a subsiding basin, the higher its rank. Heating during burial can also occur through interaction with hydrothermal fluids (hot fluids passing through rocks in the earth), or through contact metamorphism (igneous intrusions contacting a coal) (see, for example, Hower and Gayer, 2002; Ruppert and others, 2010). Heating causes hydrocarbon compounds (compounds composed of hydrogen, carbon, and oxygen) in the peat to break down and alter in a variety of ways, resulting in coal. In general, moisture and gases (for example, methane, carbon dioxide) are systematically expelled from the peat and resulting coal with increasing burial and heat. This results in increasingly carbon-rich coals, which we record as different ranks of coal.
Time: Time is another influence on coalification and rank. It is well established that the geologic age of a coal does not control rank, so time is not considered a major influence on coal rank. Historically, time’s primary influence was thought to be providing more chances for heating (see, for example, Stach and others, 1982). Modern research has shown, however, that the amount of time organic compounds and coals are influenced by increased heating can also influence rank (see, for example, Bostick, 1971, 1973). Moreover, different rates of chemical and physical changes occur during coalification and organic maturation (see, for example, Burnham and Sweeney, 1989), which can be considered time-influenced parameters.
Pressure: Pressure is generally considered a minor influence on coalification and rank (van Krevlin, 1961; Teichmüller and Teichmüller, 1979; Stach and others, 1982). During peatification and lignite stages of coalification, moisture is squeezed out of the peat and coal through compaction by the weight of overlying sediments. This vertical load force could be considered a pressure influence. In many cases, however, when geologists consider pressure, they are considering pressures from deep in the earth or pressures associated with fault movement and mountain-building. These types of tectonic pressures seem to have less influence than temperature on coalification, except when increased pressure is associated with increased heating. In some cases, laboratory research indicates high pressure may also suppress rates of chemical reactions for certain organic chemical compounds, which might influence vitrinite particles used for vitrinite reflectance and rank studies (see, for example, Dalla-Torre and others, 1997; Sajgo and others, 1986), although results of research are varied (see, for example, Goodarzi, 1985)(Heat, Time, Pressure, and Coalification, Coal, Kentucky Geological Survey, University of Kentucky, n.d.).
From ancient swamps to energy-rich rock, the journey of coal is a testament to immense geological forces! How do heat, time, and pressure specifically transform organic matter into coal, and what impact do these factors have on its quality? Share your insights!
References:
Heat, time, pressure, and coalification, Coal, Kentucky Geological Survey, University of Kentucky. (n.d.). Retrieved June 30, 2025, from https://www.uky.edu/KGS/coal/coal-heat-time-pressure.php
