From the types of sedimentary rocks found associated with coal, geologists have determined that coal originates in environments similar to today’s river deltas (Chapter 8). As illustrated in Figure 13.5, deltas are ideal because they contain expansive back swamps with lush vegetation. Here the low flow rates help produce stagnant water conditions with low levels of dissolved oxygen (O2), which greatly slows the decay of dead plant material and allows more organic matter to accumulate. Moreover, large deltas typically experience subsidence as the tremendous weight of sediment presses down on the crust. This progressive sinking allows for even greater amounts of plant matter to accumulate, which, in turn, compresses the underlying organic material into a more compact form called peat. At some point the river channels may migrate or sea level will rise, burying the peat with younger sediment. This burial seals the peat off from atmospheric oxygen, increasing the chance that it will be preserved. As the delta continues to receive new sediment and subside, layers of peat are buried even deeper and are exposed to progressively higher levels of heat and pressure. Eventually the peat may reach the depth where very low-grade metamorphism transforms it into the combustible sedimentary rock known as coal. During this transformation the original plant material undergoes physical and chemical changes that drive off water and other volatile compounds (easily turns to a gas), leaving behind solid coal that is more concentrated in carbon. Because this process is a function of temperature and pressure, the carbon concentration can increase over time should the coal become more deeply buried. As illustrated in Figure 13.6, coals are ranked based on the amount of carbon and volatiles thep contain Lignite is the lowest grade, with progressively higher ranks called coals are generally more compact, which is a reflection of being buried at greater depths and the volatile compounds being driven off.