How coal was formed-3 types

What once had been plants gradually turned into coal.

Millions of years ago, dead plant matter fell into swampy water and over time, a thick layer of dead plants lay decaying at the bottom of the swamps.
Over time, the surface and climate of the Earth changed, and more water and dirt washed in, halting the decay process, forming peat.
The weight of the top layers of water and dirt packed down the lower layers of plant matter.
Under heat and pressure, this plant matter underwent chemical and physical changes, pushing out oxygen and leaving rich hydrocarbon deposits.

Coal can be found deep underground (as shown in this graphic), or it can be found near the surface.

As we said, coal formed millions of years ago when the earth was covered with huge swampy forests where plants – giant ferns, reeds and mosses – grew. As the plants grew, some died and fell into the swamp waters. New plants grew up to take their places and when these died still more grew. In time, there was a thick layer of dead plants rotting in the swamp.

The surface of the earth changed and water and dirt washed in, stopping to decaying process. More plants grew up, but they too died and fell, forming separate layers. After millions of years, many layers had formed, one on top of the other. The weight of the top layers and the water and dirt packed down the lower layers of plant matter. Heat and pressure produced chemical and physical changes in the plant layers which forced out oxygen and left rich carbon deposits. In time, material that had been planted became coal.

Coals are classified into three main ranks, or types: lignite, bituminous coal, and anthracite. These classifications are based on the amount of carbon, oxygen, and hydrogen present in the coal. Coal is defined as a readily combustible rock containing more than 50% by weight of carbon. Coals other constituents include hydrogen, oxygen, nitrogen, ash, and sulfur. Some of the undesirable chemical constituents include chlorine and sodium. In the process of transformation (coalification), peat is altered to lignite, lignite is altered to sub-bituminous, sub-bituminous coal is altered to bituminous coal, and bituminous coal is altered to anthracite.

Prolonged heat and pressure help form different types of coal

Lignite – is the lowest rank of coal – which means that it has the lowest heating value and lowest carbon content. Although lignite is more solid than peat it crumbles when shipped long distances. Most lignite in the U.S. is in North and South Dakota, Montana, and Texas. Lignite is used to generate electricity. Other uses include generating synthetic natural gas and producing fertilizer products.


Bituminous – is intermediate in rank and sometimes called soft coal. It appears smooth when you first see it but look closer and you’ll find it has many layers. It is the most abundant kind of coal. It has a high heating value, but it also has a high sulfur content. More than 80% of the bituminous coal produce in the U.S. is burned to generate electricity. Other major coal users are the cement, food, paper, automobile, textile and plastic industries. Another important industrial use is to provide coke for iron and steel industries. Bituminous coal derivatives or by-products can be changed into many different chemicals form which we can make paint, nylon, aspirin and many other items.

Bituminous coal
Bituminous coal

Anthracite – is the highest rank of coal which means that it has the highest heating value and highest carbon content. It is very hard, deep black, and looks almost metallic because it is brilliantly glossy. Anthracite burns longer, with more heat and with less dust and soot than other types of coal. The primary market for anthracite is for heating homes. Nearly all of the anthracite in the U.S. is in Pennsylvania, but there are some small beds in other states.


Featured image: University of Kentucky; Adapted from Kentucky coal education

One thought on “How coal was formed-3 types

Leave a Reply

Your email address will not be published. Required fields are marked *