This image of Australia's Great Barrier Reef shows organic sedimentary rocks in production!
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Courtesy of Abi Howe, American Geological Institute
Organic Sedimentary Rocks
The sediment in an organic sedimentary rock is made of fossils!
The mineral parts of animals, such as bones and shells, are much more likely to be preserved than the soft tissues, which typically decay. The bones, shells, and other skeletal parts can become cemented together over time to make sedimentary rock. Usually the bones are made of calcite, or a similar mineral called aragonite, and the resulting organic rock is called limestone. Sometimes the skeletal material is made of silica, such as with diatom algae and some other types of plankton, and the resulting organic rock is called chert.
Plants compressed together over millions of years make an organic sedimentary rock called coal. Coal doesn’t look like it is made of sediment. In fact, it is often difficult to see the plant fossils within it because they have become so compacted over time, some of the more volatile materials are no longer present, and all that is left is the organic carbon.
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The
Fall 2010 issue of The Earth Scientist, focuses on rocks and minerals, including articles on minerals and mining, the use of minerals in society, and rare earth minerals, and includes 3 posters!
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