As time passes, plant debris accumulates on the bottom of the swamp (leaves, twigs, branches, roots etc.). This peat undergoes coalification in three main stages.

1. Aerobic decay In the first few centimeters, active aerobic bacterial decay goes on reducing the volume significantly, often 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.



2. Anaerobic decay A second type of bacteria, that require no oxygen exists in the swamp. These anaerobic bacteria continue the decay process reducing the volume still further. Decay produces acids and throughout stage one and two, the acidity is rising. When the pH get to about 4.0 it kills off the anaerobic bacteria. At this point the peat has changed into a black, cheesy, gel-like material Gytta The amount of inert gytta continues to accumulate as the bacteria work on the upper layers. It is this material that will eventually be changed into a coal seam. If the coal is 30 ft thick there must be at least 30 feet of gytta.



3. Bituminization While bacterial decay is the process in stages one and two, thermal processes take over for the final stage. This requires a burial of the peat (gytta) by at least two or three thousand feet of sediment depending on the geothermal gradient. This insulating blanket retains the natural heat that rises to the surface all over the earth. Once the temperature reaches 100º C the bituminization process begins. Chemical reactions drive off water, oxygen and hydrogen which raises the percentage of carbon. The degree to which the bituminization goes determines the coal Rank. When the C reaches 85% it has become subbituminous coal: at 90 % it is bituminous coal and 95% is anthracite. During bituminization, the distinguishable plant remains are lost (except microscopic ones) and the shiny black macerals are produced.