{"id":322,"date":"2018-08-24T15:09:40","date_gmt":"2018-08-24T19:09:40","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/9-3-organic-sedimentary-rocks-2\/"},"modified":"2023-07-04T12:53:13","modified_gmt":"2023-07-04T16:53:13","slug":"organic-sedimentary-rocks","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/organic-sedimentary-rocks\/","title":{"raw":"9.3 Organic Sedimentary Rocks","rendered":"9.3 Organic Sedimentary Rocks"},"content":{"raw":"Organic sedimentary rocks are those containing large quantities of organic molecules. Organic molecules contain carbon, but here we're referring specifically to molecules with carbon-hydrogen bonds, such as materials from the soft tissues of plants and animals. In other words, the carbon in calcite (CaCO3<\/sub>) wouldn\u2019t make calcite an organic mineral because it isn\u2019t bonded to hydrogen.\r\n\r\nAn important organic sedimentary rock is coal. Most coal forms in swampy land adjacent to rivers and within deltas, and where climates are humid and tropical to temperate. The vigorous growth of vegetation leads to an abundance of organic matter that accumulates within stagnant, acidic water. This limits decay and oxidation of the organic material. If this situation\u2014where the dead organic matter is submerged in oxygen-poor water\u2014is maintained for centuries to millennia, a thick layer of material can accumulate. Limited decay will transform this layer into peat <\/strong>(Figure 9.17a, Figure 9.18 upper left).\r\n\r\n[caption id=\"attachment_320\" align=\"aligncenter\" width=\"655\"]\"\" Figure 9.17<\/strong> Formation of coal. (a) Accumulation of organic matter within a swampy area forms a layer of peat; (b) The organic matter is buried under sediment and is compressed; (c) With greater burial, lignite coal forms; (d) At even greater depths, bituminous and eventually anthracite coal form. Source: Steven Earle (2015), CC BY 4.0.[\/caption]\r\n\r\nAt some point the swamp deposit is covered with more sediment \u2014 typically because a river changes its course or sea level rises (Figure 9.17b). As more sediments are added, the organic matter is compressed and heated as temperatures increase with depth. This has the effect of concentrating the carbon within the coal. The amount of heating will determine how far this process progresses.\r\n\r\nThe further the process does progress, the more the coal will go from having obvious pieces of plant material within it, to being a black, shiny mass.\u00a0 Low-grade lignite<\/strong> coal forms at depths between 100 m to 1,500 m and temperatures up to ~50\u00b0C (Figure 9.17c). This is still a relatively early stage in the coal formation process, so the lignite commonly displays plant fossils that have not yet been destroyed in the process of coalification (Figure 9.18 upper right).\r\n\r\nAt between 1,000 m to 5,000 m depth and temperatures up to 150\u00b0C m, bituminous<\/strong> coal<\/strong> forms (Figure 9.17d, 9.18 lower right). At depths beyond 5,000 m and temperatures over 150\u00b0C, anthracite<\/strong> coal forms (Figure 9.18 lower left). In fact, as temperatures rise, the lower-grade forms of coal are actually being transformed from sedimentary to metamorphic rocks.\r\n\r\n[caption id=\"attachment_321\" align=\"aligncenter\" width=\"596\"]\"\"<\/a> Figure 9.18<\/strong> The formation of coal begins when plant matter is prevented from decaying by accumulating in low-oxygen, acidic water. A layer of peat forms. Heating and compression of peat form lignite, bituminous coal, and finally anthracite, as pressure and temperature increases. Source: Karla Panchuk (2017), CC BY-NC-SA 4.0. Photos by R. Weller\/ Cochise College and U. S. Geological Survey. Click for more attributions and terms of use.[\/caption]\r\n\r\nThe transition from peat to anthracite results in a progressive increase in the carbon concentration, in hardness, and in the amount of energy available to be released upon combustion.\r\n
\r\n\r\n<\/a>Concept Check: Coal Formation<\/strong>\r\n\r\n
\r\n\r\nQuestion 1: Where does coal form? Fill in the blanks.<\/strong>\r\n\r\nCoal forms in \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 <\/span>\u00a0(hint:<\/strong> swampy or dry?) conditions where there is lots of accumulated \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0<\/span> matter, and \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 (<\/span>hint:<\/strong> a gas that many decomposer organisms need to live<\/span>)<\/span>-poor conditions limit decay.\r\n\r\nQuestion 2: How does coal form? Drag the words into the correct boxes.<\/strong>\r\n\r\nThe first material to form from the accumulating plant matter is \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0<\/span>. As sediments and then rocks accumulate above it, it's compressed and heated. If temperatures and pressures continue to increase over time, it can be transformed into \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0<\/span>, then \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0<\/span>, then \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0<\/span>.\r\n\r\nEarly in its formation, \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0<\/span> rock-forming conditions are present, but as the process progresses, conditions change to \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0<\/span>.\r\n\r\nFill-in-the-blank options:<\/strong>\r\n