Describe and explain the changes organic material undergo in coalification

Organic material accumulates in an area of fast plant growth and anaerobic conditons (which stops the vegetation decaying), such as a tropical delta, the organic material is buried which leads to increases in temperature (due to the geothermal gradient) and pressure which expels volatiles from the organic material, forming peat. As the peat is buried further the temperature and pressure continue to increase, and the % of volatiles decreases, and so the rank of the coal increases through lignite, then bituminous coal and then finally anthracite. Throughout this process, the coal colour gets darker, the coal becomes more reflective, has a higher carbon %, a lower volatiles % and a higher enthalpy change of combustion.