Origin of Coal
Coal is formed from the compression and alteration of partly decomposed peat. Peat forms from the accumulation of partially decomposed plant materials under generally wet conditions having a restricted supply of oxygen. Such conditions exist in environments called mires which can be classified as limnic or paralic. Limnic coal deposits are inferred to have resulted from peat that formed in mires isolated from marine conditions in slowly subsiding basins. In contrast, paralic deposits resulted from peat mires which had a hydrological connection with the sea at the time of peat formation.
The composition of peat depends on the the type of plants that grew in the mire, nutrient availability, climatic conditions, level of the water table, and the pH and Eh conditions of the mire. Any organic part of the mire ecosystem may be preserved in some way in the peat, including large trees, herbaceous shrubs, grasses, aquatic plants, and the microorganisms that break down the organic material. Differences between coals of different geologic periods are, in part, due to differences in the dominant plant groups from those times.