Coal formation, also known as coalification, is a complex process that occurs over millions of years. It involves the accumulation and transformation of plant matter in specific geological conditions. Here is a simplified breakdown of the process:

1. Accumulation of Plant Matter:

   It begins with the accumulation of plant matter in swampy environments. These environments are ideal because the waterlogged conditions inhibit the complete decomposition of organic material. Plants such as trees, ferns, and mosses die and accumulate in these areas.

2. Peat Formation:

   The accumulated plant matter forms peat. Peat is a soft, spongy, brownish material with a high moisture content. It's the first stage in the transformation of plant matter into coal. Anaerobic bacteria (bacteria that live without oxygen) partially decompose the plant matter.

3. Increased Pressure and Temperature:

   Over time, the peat is buried under layers of sediment. The pressure from these overlying layers compacts the peat. Increased pressure and geothermal heat cause further chemical and physical changes. Water and other volatile substances are squeezed out.

4. Transformation into Coal:

   As pressure and temperature continue to increase, the peat is gradually transformed into different ranks of coal. The rank of coal depends on the degree of transformation. The main ranks, from lowest to highest, are:

   • Lignite (high moisture content, lowest carbon content)
   • Sub-bituminous coal
   • Bituminous coal
   • Anthracite (lowest moisture content, highest carbon content)

   Anthracite represents the highest rank of coal and has the highest carbon content and energy density.