How Coal is Formed

Coal is a fossil fuel that has been used for several hundred years. Most people have heard of coal and know that the Earths coal supplies are limited but they may not know how coal was formed. There are some mistaken beliefs that coal comes from the remains of long extinct dinosaurs but this is not true. The deposits of coal that people use today are the result of a very long formation process that began several hundred million years ago.

Where coal began

What most people may not know is that most coal began as peat which is a basic term for photosynthesis using plants which were covered over by new growth and sank below the surface. This could include living or already dead plants and all parts of the plant, not just the leaves but bark, roots, branches, basically everything. While the term peat is similar to peat moss, which is found in many places of the world today, peat refers to a variety of plants and not just the sphagnum moss that peat moss refers to. The peat that formed coal is around 300 to 400 million years old. During that period of Earth’s past, the world was a large swampy with plenty of photosynthesis plants which all contained carbon. Through a series of chemical and physical processes, which are referred to as coalification, the plant material changed to become the coal that it is today.


As part of the coalification process the peat material is subjected to compaction/pressure, heat, and decay due to bacteria which ultimately results in the formation of coal. In addition to just layers of peat, over the course of time the peat was likely covered by sediments or even former oceans and seas which further compact the peat. The water is squeezed out due to the pressure and the peat material will likely become heated as it is further pushed down into the Earth’s surface, sometimes to depths of thousands of feet. Under these conditions, the material continues to decay by way of anaerobic bacteria which do not require oxygen. The plant material is altered which leaves the remaining the carbon material as gases are lost and other materials are consumed. The coalification process will result in several overall stages of coal and therefore lead to several distinctive types of coal.

Types of coal

There are four basic types of coal which are part of the coalifaction process. As the process proceeds over millions of years, the carbon content will continue to increase and this is the basis for the separation of the coal types. Each type has a range of carbon content as well as different average heat values associated with it. Each type is also slightly different because even within each type of coal, there can be further differences depending on the types of plants that formed the peat and also the exact conditions under which the peat was subjected.


The first type of coal is the youngest type, which is lignite. This coal has only been able to form a carbon content of about 25 to 35 percent. Its heat value range is the lowest of the four types but lignite makes up the largest portion of the coal reserves for the entire world. The normal black color that people associate with coal is not completely present in lignite as it still has a brownish coloring to it.


The next type of coal in the stages is the subbituminous coal. This type of coal has about 35 to 45 percent carbon content. Despite having a lower heat value than the other coming stages of coalification, subbituminous has a lower sulfur content and is therefore more desirable because it will burn cleaner.


After subbituminous is bituminous coal. Bituminous coal has higher carbon content at around 45 to 85 percent and one of the largest heat values of the four types. This is much more plentiful in the United States and is used to generate power and for coke which is used in the steel industry.


The final type of coal is really the one that people would be most familiar with, which is anthracite coal. Without becoming pure graphite (100 percent carbon) anthracite coal has between 85 and 98 percent carbon content and a high heat value. This type of coal is commonly used for home heating on the few homes that still use coal.