Lexikon

sedimentary rock is the type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment was formed by weathering and erosion in a source area, and then transported to the place of deposition by water, wind, mass movement or glaciers. Though sedimentary rocks form just a small part of the Earth's crust, they cover the largest part of the Earth's surface. Sedimentary rocks are deposited in strata that form a structure called bedding. Sedimentary rocks contain important information about the history of Earth. They contain fossils, the preserved remains of ancient plants and animals. The composition of sediments provides us with clues as to the original rock. Differences between successive layers indicate changes to the environment which have occurred over time. Sedimentary rocks can contain fossils because, unlike most igneous and metamorphic rocks, they form at temperatures and pressures that do not destroy fossil remains. (http://en.wikipedia.org/wiki). Sedimentary rocks are classified into three groups. These groups are: clastic, chemical precipitate and biochemical or biogenic. Clastic sedimentary rocks are composed of discrete fragments or clasts of materials derived from other minerals. They are composed largely of quartz with other common minerals including feldspar, amphiboles, clay minerals, and sometimes more exotic igneous and metamorphic minerals. The classification of clastic sedimentary rocks is complex because there are many variables involved. Particle size (both the average size and range of sizes of the particles), composition of the particles, the cement, and the matrix (the name given to the smaller particles present in the spaces between larger grains) must all be taken into consideration. (http://en.wikipedia.org/wiki). Chemical sedimentary rocks form when minerals in solution become oversaturated and precipitate. In marine environments, this is a method for the formation of limestone. Another common environment in which chemical sedimentary rocks form is a body of water that is evaporating. Evaporation decreases the amount of water without decreasing the amount of dissolved material. Therefore, the dissolved material can become oversaturated and precipitate. Sedimentary rocks from this process can include the evaporite minerals halite (rock salt), barite and gypsum (http://en.wikipedia.org/wiki). Organic sedimentary rocks contain materials generated by living organisms, and include carbonate minerals created by organisms, such as corals, mollusks, and foraminifera, which cover the ocean floor with layers of calcite which can later form limestone. Other examples include coal and oil shale (derived from the remains of tropical plants and subjected to heat) (http://en.wikipedia.org/wiki).