The coal formation process takes millions of years. The coal in use today started to form over 300 million years ago as living trees, ferns and other types of plant material. Coal is a nonrenewable resource because the time it requires for formation far exceeds the rate at which man uses it.
The coal-forming process, known as coalification, involves physical and chemical changes of peat. Peat includes plant parts, decaying plants and the products of the decay process. In order to form peat, the rate of plant growth must exceed the rate at which bacteria can break down the materials, as occurs in the water of bogs and swamps, where bacterial action slows due to a lack of oxygen.
Coalification requires the burial of thick layers of peat. The pressure of the layers deposited above the peat begins to compress the plant materials and squeezes the water from them. Pressure, heat and time cause many changes in the materials, as is evidenced by the formation of methane gas. The process increases the carbon percentage of the coal. The quality of coal depends on when miners remove it from the ground during the coalification process. The longer the coal remains buried, the greater the carbon content. With enough time and pressure, the materials transform to graphite, a mineral containing only carbon.