Sedimentary rock formation begins with igneous, metamorphic, or other sedimentary rocks. When these rocks are exposed at the earth’s surface they begin the long slow but relentless process of becoming sedimentary rock. Weathering
All rocks are subject to weathering. Weathering is anything that breaks the rocks into smaller pieces or sediments. This can happen by the forces of like wind, rain, and freezing water. Deposition
The sediments that form from these actions are often carried to other places by the wind, running water, and gravity. As these forces lose energy the sediments settle out of the air or water. As the settling takes place the rock fragments are graded by size. The larger heavier pieces settle out first. The smallest fragments travel farther and settle out last. This process of settling out is called deposition. Erosion
The combination of weathering and movement of the resulting sediments is called erosion. Lithification
Lithification is the changing of sediments into rock. There are two processes involved in this change. They are compaction and cementation. Compaction
Compaction occurs after the sediments have been deposited. The weight of the sediments squeezes the particles together. As more and more sediments are deposited the weight on the sediments below increases. Waterborne sediments become so tightly squeezed together that most of the water is pushed out. Cementation happens as dissolved minerals become deposited in the spaces between the sediments. These minerals act as glue or cement to bind the sediments together. The process of sedimentary rock formation takes millions of years to complete only to begin a new cycle of rock formation. Sedimentary rock can be formed in various ways, and is classified by the method of formation or content. Broadly, sedimentary rock is classified as being clastic, organic, or chemical. Following are the three types and how they are formed.
Clastic sedimentary rock:
The majority of sedimentary rock on Earth is formed from particles of pre-existing rocks. This route begins with the weathering and erosion of sedimentary, igneous, or metamorphic rocks. Through these processes, larger rock is broken up into smaller particles which are transported by moving water, ice, gravity or wind, and deposited at the bottom of a lake, a river delta, an ocean, or similar location where further movement is restricted or slowed down. The rock particles can range in size from boulders to clay particles less than .002 mm in diameter. If these rock particles are covered by additional particles, eventually the weight from above will start the process of lithification. Lithification is the compaction and cementing together of sediment particles which form rock. Compaction squeezes out the fluids and space that exist between the particles, and cementation results when minerals form from the migrating solution. Once cementation occurs, the new sedimentary rock has been formed. Rocks formed in this manner include sandstone, shale, mudstone, breccia, and conglomerate.
Organic sedimentary rock:
Chalk and other fossiliferous limestones are formed from the skeletons of marine organisms. Coal is formed from vegetation that originated and was deposited in swampy and marshy waterlogged soils which prevented their full decay after their death. As their remains piled up and were covered by more and more deposits, they were compacted and cemented in the same manner as clastic rock.
Chemical sedimentary rock:
Sedimentary rock can also form when minerals in a body of water have so saturated the water that they precipitate out, like a cloud that is so full of moisture that it pours out rain. Some limestones are formed in this manner as the mineral calcite precipitates out of a saturated solution and undergoes the process of compaction and cementation. The supersaturated solutions can also be caused by evaporation of a body of water high in mineral solution content. As the water...
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