When you hear the word “coal,” what do you think of? An industry that has more than doubled in productivity since 1980? A fuel whose use has increased by 80 percent, while the nation’s air quality has steadily shown improvement since 1971? Or, an energy resource that provides over half of all the electricity used in the United States each day and over 80 percent of the electricity in Utah? Coal is America’s most readily available, least expensive, major fossil fuel resource, and it plays a vital role in our nation’s economy.

The Formation of Coal

Coal is classified by geologists as a mineral. But most minerals, like salt or iron ore, were formed by inorganic matter. Coal, on the other hand, came from organic matter-plants, that lived about 300 million years ago.

During the Pennsylvanian Period in Earth’s history, the Earth was covered with huge swampy forests of giant ferns, reeds and mosses, which grew taller than our tallest trees today. As these plants died and fell into the swamp water, new plants grew to take their place and when these plants died, still others grew. In time, there was a thick layer of dead, decaying plants in the water.

The surface of the earth also changed and dirt washed into the water covering the dead plants, preventing them from completely decomposing. More plants grew, but they too died and fell into the water, forming a separate layer of dead decaying plants which over time were also covered by sediments, preventing their complete decomposition. After millions of years, many layers had formed one on top of the other.

The weight of the overlying layers compressed the lower layers of plant matter, forming peat. Heat and pressure caused by the overlying sediments produced chemical changes in the peat, forcing out oxygen and hydrogen, leaving behind rich carbon deposits-coal. Geologists estimate that a layer of plants 20 feet thick may have been required to form a coal seam one foot thick. Coal seams vary in thickness, ranging from only a few inches thick to more than 100 feet in thickness.


Types of Coal

Coal is a very complex and diverse energy resource that can vary greatly, even within the same deposit. In general, there are four basic varieties of coal, which are the result of geologic forces having altered plant material in different ways. These varieties descended from the first stage in the formation of coal: the creation of peat or partially decomposed plant material.


Increased pressures and heat from overlying strata caused buried peat to dry and harden into lignite. Lignite is a brownish-black coal with generally high moisture and ash content and lower heating value. Geologically, is the youngest and the lowest ranked coal, containing 25 to 35 percent carbon and the lowest heating value -4,000 to 8,300 Btus per pound. However, it is an important form of energy for generating electricity, particularly in the American Southwest, and to produce synthetic natural gas and liquids. Lignite is found in the Gulf Coast and Northern Plains of the United States. Lignite reserves account for 9 percent of the United States’ coal reserves.


Under still more pressure, some lignite was changed into the next rank of coal: subbituminous. This coal is a combustible mineral formed from the remains of trees, ferns and other plants that existed and died during the time of the dinosaurs. A dull black coal with a higher heating value than lignite that is used primarily for generating electricity and for space heating. It contains about 35 to 45 percent carbon and has a heating value between 8,300 to 11,500 Btus per pound. Subbituminous coal is predominately found in Utah, Montana, Wyoming, Colorado, New Mexico, Washington, and Alaska. Subbituminous coal accounts for about 37 percent of the coal reserves in the United States.


Sometimes called “soft coal” it is the most common type of coal found in the United States. It is 45 to 86 percent carbon, softer than anthracite, and has a heat content between 10,500 and 14,000 Btus per pound. This is the type most commonly used for electric power generation in the U.S. and for producing coke for the steel industry. About 52 percent of the United States’ coal reserves are bituminous coal.


Sometimes called ‘hard coal,” anthracite was formed from bituminous coal when great pressures developed in folded rock strata during the creation of mountain ranges. Anthracite has the highest energy content of all coals. It contains 86 to 97 percent carbon, and has a heat content of nearly 15,000 Btus (British thermal units) per pound. It is used for space heating and generating electricity. There are about 7.3 billion tons of anthracite reserves, located in 11 counties of northeastern Pennsylvania. Anthracite accounts for about 2 percent of the coal reserves in the United States. All coal has carbon and sulfur to some degree. Coal that was formed in swamps covered by seawater contains a higher sulfur content; low sulfur coal was generally formed under freshwater conditions.