Origin of Coal

Coal is found in deposits called seams that originated through the accumulation of vegetation that has undergone physical and chemical changes.

These changes include decaying of the vegetation, deposition and burying by sedimentation, compaction, and transformation of the plant remains into the organic rock found today. Coals differ throughout the world in the kinds of plant materials deposited (type of coal), in the degree of metamorphism or coalification (rank of coal), and in the range of impurities included (grade of coal).

There are two main theories for the accumulation of the vegetal matter that gives rise to coal seams [4]. The first theory, and the one most accepted as it explains the origin of most coals, is that the coal formed in situ (that is, where the vegetation grew and fell), and such a deposit is said to be autochthonous in origin. The beginning of most coal deposits started with thick peat bogs where the water was nearly stagnant and plant debris accumulated. Vegetation tended to grow for many generations, with plant material settling on the swamp bottom and converted into peat by microbiological action. After some time, the swamps became submerged and were covered by sedimentary deposits, and a new future coal seam was formed. When this cycle was repeated, over hundreds of thousands of years, additional coal seams were formed. These cycles of accumulation and deposition were followed by diagenetic (i.e., biological) and tectonic (i.e., geological) actions and, depending upon the extent of temperature, time, and forces exerted, formed the different ranks of coal observed today.

While the formation of most coals can be explained by the autochthonous process, some deposits are not easily explained by this model. Some coals appear to have been formed through the accumulation of vegetal matter that has been transported by water. According to this theory (i.e., allochthonous origin), the fragments of plants have been carried by streams and deposited on the bottom of the sea or in lakes where they build up strata, which later become compressed into solid rock.

Major coal deposits formed in every geological period since the Upper Carboniferous Period, 350 to 270 million years ago; the main coal-forming periods are shown in Figure 1-1 [5], which shows the relative ages of the world's major coal deposits. The considerable diversity of various coals is due to the differing climatic and botanical conditions that existed during the main coal-forming periods along with subsequent geophysical actions.

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