Coal Formation

Coal formation
Coal comes from the massive accumulation of dead land-based plant life, mainly trees. This organic matter was deposited in sedimentary basins on land (of continental origin), where the water was shallow.
These basins were either close to the sea, often in the form of large lagoons, or inland, in the form of lakes or marshes. As a result of climatic variations, for example an increasingly heavy annual rainfall, it is thought that enormous forests sank below water, and their debris accumulated in sedimentary basins where it was rapidly covered by large quantities of mud and sand.
This sudden and premature burial sheltered the debris from the air, thus preventing it from rotting quickly. In the maritime areas, a sudden subsiding of the basin resulted in an inflow of seawater, decimating the forest. After these catastrophic episodes, the forest grew again until a new disaster happened, and so on. This repetitive cycle of events explains why, in the substratum, layers of coal alternate with layers of clay or sandstone (compacted sand). Subsequently the sedimentary basin sinks gradually under the weight of the sediments and the layers of dead plant matter are slowly transformed under conditions of increasing temperature and pressure. The cellulose in the wood is first of all changed into humic acids (products of incomplete decomposition, which give the earth its brown colour), then into bitumens and finally into elementary carbon. The process is extremely long: the oldest coal has the highest carbon content (90 to 95%) The most favourable period for the creation of coal was the Carboniferous Period (carboniferous means “coal-bearing”), between 360 and 290 million years ago. However, smaller quantities of coal continued to be formed in certain regionsduring all the subsequent epochs: the Permian Period (290 to 250 million years ago), and the Secondary Period (Mesozoic Era, 250 to 65 million years ago). The vegetal masses buried in the Tertiary Period...