Making coal is a three phase process:

Phase 1. Aerobic decay In the first few inches of peat, aerobic (oxygen needing) bacterial decay reduces the volume by as much as 50%. Because the water is stagnant and the peat is almost impermeable, the bacteria soon use up all the available oxygen and die, ending the first stage of decay.

Phase 2. Anerobic decay A second type of bacteria exists in the swamp that requires no oxygen. These anerobic bacteria continue the decay process reducing the volume still further. Anerobic decay produces more acids and when the acidity gets too high, it kills off the remaining bacteria ending all decay.

At this point the peat has changed into a black, cheesy, gel-like material gytta The amount of gytta continues to accumulate as the bacteria work on the new peat above. It is gytta that will eventually be changed into coal. A 30 ft thick coal seam have had at least 30 feet of gytta.

Phase 3. Bituminization After the bacterial decay stages, the peat (gel) must be buried under thousands of feet of sediment that provides an insulating blanket trapping the natural heat rising to the surface. Once the temperature reaches 100o C, (2120F) the bituminization process begins. Chemical reactions drive off water, oxygen and hydrogen which raises the percentage of carbon. When the carbon content reaches 85%, it has become subbituminous coal. At 90 % it is bituminous coal and 99% is anthracite.