Sedimentary rocks are one of the three types of rocks that form the earth’s surface. The other two are igneous and metamorphic rocks. Sedimentary rocks are formed by the accumulation of small rocks particles in the earth surface through erosion, weathering, transportation of the sediments and precipitation of minerals. Because weathering and erosion are unavoidable in a planet that comprises 70% water, these rocks are usually formed in surfaces that are covered with water. Sand is an example of a sedimentary rock. Because of the different processes involved in the formation of sedimentary rocks, there are three major classifications of these types of rocks: clastic, biochemical and chemical.
Clastic sedimentary rocks
Clastic sedimentary rocks are products of weathering and erosion. These are composed of broken fragments from other rocks. The fragments or sediments are carried by water, blown by air or rolled down slopes by gravity and are deposited in an area. Geologists classify clastic sedimentary rocks further into categories base on the appearance, composition and size of the sediments. Examples are sand, silt and clay.
Biochemical sedimentary rocks
Biochemical sedimentary rocks are formed when biological elements take part in the chemical change in the rocks. An example is limestone which is made from the calcium-rich skeletons of mollusks and corals. The decomposition of marine life forms results to a sediment deposit in the ocean floor that is mixed with existing sedimentary rocks. Coal is an example of sedimentary rocks. Coal is formed from an accumulation of plant matter that decomposed in the sea bed. Through time, layer upon layer of carbon-rich dense material called peat is formed. Pressure, heat and time form these large coal beds that mined for fuel today.
Chemical sedimentary rocks
Chemical sedimentary rocks result from changes because of chemical processes. These are usually formed in bodies of water with high concentration of mineral crystals. It can happen when water evaporated leaving behind sediments. A rock salt is an example of such process. Rock salt’s mineral name is halite. Another example is a dolomite. Dolomites are formed by crystallization of limestone because of extreme temperature.
Different conditions create different types of sedimentary rocks. Geologists study them to know how they are formed and to understand the changes that happened in the earth’s surface. This includes the formation of fossils in the sedimentary rocks.
Sources:
http://en.wikipedia.org/wiki/Sedimentary_rock
http://www.windows2universe.org/earth/geology/sed_intro.html
http://earthsci.org/education/teacher/basicgeol/sed/sed.html
