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Sedimentary rock

Sedimentary rocks are formed from overburden pressure as sediment is deposited from air, ice, or water flows carrying solid particles in suspension. As the velocity of the fluid decreases, particles being to fall out of suspension and are deposited. As sediment deposition continues, the overburden (or lithostatic[?]) pressure squeezes the sediments into layered solids in a process known as lithification and the original connate fluids are expelled.

Sedimentary rocks are composed largely of silica (i.e. quartz), with other common minerals including feldspars, amphiboles, clay minerals[?] and sometimes more exotic igneous minerals. Sedimentary rocks are classified as clastic, that is, they are composed of discrete clasts of material (rather than being composed of organic material as is the case for a limestone).

Carbonate minerals precipitating out of the ocean cover the ocean floor[?] with layers of calcite which can later form limestone.

Sedimentary rocks are economically important in that they can be used as construction material. In addition, sedimentary rocks often form porous and permeable reservoirs in sedimentary basins in which petroleum and other hydrocarbons can be found.

It is believed that the relatively low levels of carbon dioxide in the Earth's atmosphere, in comparison to that of Venus, is due to large amounts of carbon being trapped in limestone and dolomite sedimentary layers. The flux of carbon from eroded sediments to marine deposits is known as the carbon-cycle[?].

The shape of the particles in sedimentary rocks has an important effect on the ability of micro-organisms to colonize them. This interaction is studied in the science of geomicrobiology. One measure of the shape of these particles is the roundness factor, also known as the Krumbein number after the geologist W. C. Krumbein.

See also: sandstone, list of minerals



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