A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering.
Also known as nonmechanical. Sandstone and limestone are used for building stone. Phosphate-bearing sedimentary rocks are used for fertilizer. Quartz sand is used to make glass. Clastic sedimentary rocks may have particles ranging in size from microscopic clay to huge boulders. Their names are based on their clast or grain size. The smallest grains are called clay, then silt, then sand. Grains larger than 2 millimeters are called pebbles. Clastic sedimentary rocks are rocks composed predominantly of broken pieces or clasts of older weathered and eroded rocks.
Clastic sediments or sedimentary rocks are classified based on grain size, clast and cementing material matrix composition, and texture. Thus, a sandstone consisting of monocrystalline quartz that does not show undulatory extinction is mineralogically the most mature.
Although feldspars are the most common minerals in igneous and metamorphic rocks, feldspars are less stable than quartz at conditions near the Earth's surface.
Clasts are the fragments of rocks and minerals. Non-clastic rocks are created when water evaporates or from the remains of plants and animals. Limestone is a non-clastic sedimentary rock. During peak discharge [3] at this location, the water is high enough to flow over the embankment on the right, and it flows fast enough to move the boulders that cannot be moved during low flows. Clasts within streams are moved in several different ways, as illustrated in Figure 6.
Large bedload clasts are pushed by traction or bounced along the bottom saltation , while smaller clasts are suspended in the water and kept there by the turbulence of the flow. As the flow velocity changes, different-sized clasts may be either incorporated into the flow or deposited on the bottom.
At various places along a river, there are always some clasts being deposited, some staying where they are, and some being eroded and transported. This changes over time as the discharge of the river changes in response to changing weather conditions.
Other sediment transportation media, such as waves, ocean currents, and wind, operate under similar principles, with flow velocity as the key underlying factor that controls transportation and deposition. Clastic sediments are deposited in a wide range of environments, including glaciers, slope failures, rivers — both fast and slow, lakes, deltas, and ocean environments — both shallow and deep.
Depending on the grain size in particular, they may eventually form into rocks ranging from fine mudstone to coarse breccia and conglomerate. Lithification is the term used to describe a number of different processes that take place within a deposit of sediment to turn it into solid rock.
One of these processes is burial by other sediments, which leads to compaction of the material and removal of some of the intervening water and air. After this stage, the individual clasts are all touching one another.
Cementation is the process of crystallization of minerals within the pores between the small clasts, and also at the points of contact between the larger clasts sand size and larger. Depending on the pressure, temperature, and chemical conditions, these crystals might include calcite, hematite, quartz, clay minerals, or a range of other minerals. The characteristics and distinguishing features of clastic sedimentary rocks are summarized in Table 6.
If it is dominated by clay, it is called claystone. If it shows evidence of bedding or fine laminations, it is shale ; otherwise it is mudstone. Mudrocks form in very low energy environments, such as lakes, river backwaters, and the deep ocean. Accessed 12 Nov.
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