Geologist measure time with the geologic time scale. The earth is over 4 billion years old. The majority of earths history is represented by the Precambrian which consist of unfossiliferous rocks that were deposited before there was abundant life on earth. The remainder of geologic time is divided into the Paleozoic, Mesozoic and Cenozoic Eras which represent ancient life, middle life and recent life, respectively. The last three eras are further divided into the Periods of the Geologic Time Scale.
The age distribution of coal indicates that the majority of the coal deposits are Pennsylvanian (P) in age and there are no coal deposits that are older than the Devonian (D). Land plants had not fully evolved prior to Devonian therefore coal deposits which are the remains of plants could not have formed. The Pennsylvanian Period which corresponds to the majority of coal deposits including the Appalachian Basin was a time when plant life flourished in North America. It is also a time in geologic history when major continental glaciers occurred in the southern continents. The coal deposits in the western states, such as Wyoming, are younger than those in the Appalachian Basin and are mostly Jurassic (J), Cretaceous (C) and Tertiary (T) in age.
Burial of peat by overlying sediments results in an increase in the temperature and pressure. One change that happens is compaction. It is estimated that it takes 20 feet of peat to form a one foot coal bed. In addition to compaction there is a loss of moisture and volatiles. Much of the water that is lost was trapped in pore spaces and is expelled during compaction. Some of the water, plus the volatiles (gases) are released due to chemical changes in the peat.
In comparing the peat to coal note that the peat has a very open structure with high porosity compared to the dense coal. Also peat is often brown whereas coal is black.