Peat is soil-like, partially decayed plant material that accumulates in wetlands. Most people learn that coal is formed in swamps, but this is not completely accurate. The term “swamps” can be applied to many different types of wetlands, but coal only forms from peat-accumulating wetlands. Hence, peat deposits form in wetlands, but not all wetlands form peat. Peat-accumulating wetlands, also called peatlands or mires, include peat bogs, peat fens, and peat forests.

For peat to accumulate, the accumulation of plant debris in a mire must exceed the rate of bacterial decay of the plant debris. The process of partial decomposition of plant material in swampy, waterlogged environments is called peatification.

Peatification involves bacterial decay. The surface layer of most peats is dominated by aerobic bacterial decay (with oxygen) and detritus-eating organisms, so has high decay rates. If conditions in a wetland don’t allow for peat accumulation to exceed the rate of aerobic bacterial decay, or local rates of erosion, a peat will not accumulate or be preserved. In stagnant standing water, or in pore ater within the peat itself, oxygen can become depleted. This happens naturally in organic-rich waters in which oxygen is used up by the aerobic decay process. Under anaerobic (without oxygen) conditions, the bacterial decay rate is greatly reduced, and peat accumulates. Many peatlands at high latitudes (e.g., northern Canada and Siberia) have extensive peats greater than 15 feet thick. Some modern tropical peats (e.g. Sumatra) may exceed 50 feet in thickness. If these modern peats were buried to great depths, under the right geologic conditions, they could become coal.

Peat deposits are quite varied and contain pristine plant parts (roots, bark, spores, etc.), decayed plant parts, decay products, sediment, and charcoal from fires in the peat swamp. They also contain minerals (mineral matter) from sediment carried into the swamp by wind and water. The variable constituents in peat lead to a wide variety of organic and inorganic constituents in the resulting coals. The constituents within coal can vary both laterally and vertically in a coal seam, influencing the coal’s physical appearance and quality.

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