1.3 Coal-forming environments today
Coal formation begins with preservation of waterlogged plant remains to produce peat and then slow compression as the peat is buried. About 10 m of peat will compress down to form about 1 m of coal; clearly large amounts of plant debris must be available for preservation. Even so, for a significant thickness of peat to accumulate there must be a balance between the growth of plants and the decay of underlying dead material to form peat (a process known as humification).
Such a balance occurs in areas of poorly drained land known as mires (swamps).Whilst there are different types of mire, they all require the water table to be at or above the land surface for most, if not all of the year. This waterlogging of mire soils restricts the supply of oxygen. Such anoxic conditions prevent the complete decay of plant matter to carbon dioxide and water by aerobic bacteria. Instead, anaerobic bacteria convert some into methane, thereby reducing the hydrogen content of the decayed matter and increasing its carbon content, which is essential for the formation of coal. Rapid burial ensures that plant material decays and compacts fast enough to accommodate new plant growth in the mire above. The process of humification is fast in hot, humid tropical areas, but peat also accumulates in cooler, higher latitudes providing a humid climate is maintained. In regions such as Siberia and Canada, mosses rather than trees are the primary source of plant material.
Currently, 3% of the Earth's surface is covered by peat. However, not all of this is likely to form coal in the future.
Consider the extensive areas of peat which typify many upland districts of the UK and Ireland (where peat has been extracted for use as a fuel). Is this peat a good candidate for future coal formation?
Almost certainly not. Such peat was formed during a much wetter period of the geological past and in today's drier climate, large areas are above the water table undergoing oxidation. Therefore, this peat is likely to be eroded, rather than buried and turned to coal.
The next two sections look at different types of potentially coal-forming mires that occur today and are thought to have been significant in the geological record.