Sedimentary Rocks Essay Example | Topics and Well Written Essays - 1250 words

Sedimentary Rocks - Essay Example The key characteristic feature of sedimentary rocks is that they are layered. Although there cover of the earth’s crust is extensive (about 75 per cent), sedimentary rocks only constitute about five per cent of the total volume of the crust. The earth’s crust mainly consists of the igneous and metamorphic rocks. Sedimentary rocks are vital sources of the natural resources such as coal and drinking water (Stille and Fichter, 2008). Formation of Sedimentary Rocks Sedimentary rocks are usually formed in areas where there is water or water had existed in one particular time. The dead materials of animals, plants, and pieces of rocks or minerals are then transported to these areas (water bodies) by wind, water, ice, or gravity where they are often dropped off and sink to the bottom. At the bottom of the water bodies, they little by little begin to form layers which build on top of one another. This puts pressure on the lower layers thus the weight of all of the layers pushes down the lower layers until they harden into rock.   This process does not occur in few years but takes millions of years to happen.   During this process, the water body dries up and the sedimentary layer that used to be under water becomes a surface layer (Boggs, 1995). Categories of Sedimentary Rocks According to Stille and Fichter (2008), sedimentary rocks are categorized into three main groups based on sediment type. The three categories include: Clastic Sedimentary Rock: they are formed from mechanical weathering debris. Examples include sandstone, conglomerate and breccias. Organic Sedimentary Rocks: they are formed from accumulation of dead plant and animal materials. Examples include coal and some limestones. Chemical Sedimentary Rocks: they are formed by precipitation of dissolved materials from solutions. Examples include some limestones and rock salt. Distinguishing Characteristics between the Three Categories of Sedimentary Rocks Clastic Sedimentary Rocks These are the most common type of sedimentary rocks and are comprised of granular materials which occur in sediments. Clastic rocks largely consist of surface minerals such as clay, feldspar and quartz which are formed as a result of physical breakdown of rocks and alteration of rocks chemically. Clastic sedimentary rocks are classified according to the size of the particle which is dominant in the rock. For example, Breccia is composed of large angular fragment, sandstone is composed of sand size weathering debris and conglomerate is made up of large rounded particles. Clastic sedimentary rocks have clastic texture; they are made up of clasts (Stille and Fichter, 2008). Organic Sedimentary Rocks Organic sedimentary rocks are composed of organic matter: animal and plant fragments. This type of rock is usually formed in two ways. One type (which includes limestone and chest) forms in the sea due accumulation of dead plankton shells whereas as the other one (coal) is formed due accumulation of dead plant on the earth surface (Boggs, 1995). Chemical Sedimentary Rocks Chemical sedimentary rocks are formed by inorganic precipitation of supersaturated mineral constituents. They form within the depositional basin from components of chemicals dissolved in water. Unlike clastic sedimentary rocks, the direct origin of chemicals is not east to esteblish. The chemicals could originate from magma or from an