Sedimentary Rocks - Essay Example

Through centuries, the soil on the Earth has undergone both chemical and physical transformations and rock is one of the resulting forms. Rocks, in general, are divided into three groups, namely, igneous rocks, metamorphic rocks and sedimentary rocks (Park, 2001, p.192). Scientists (Park, 2001, p.204) say that “the ultimate source of all rocks on the earth’s surface is magma, hence the igneous rocks (which are solidified magma) are described as primary rocks.” All other kinds of rocks are born out of igneous rocks by the process of weathering (Park, 2001, p.204). There are three ways in which sedimentary rocks are formed. “Cemented fragments of rocks worn from the land” get deposited at a surface and on the beds of water bodies to get hardened and become sedimentary rocks (Park, 2001, p.196). This formation is a process of slow settling. Secondly, the “organic materials and remains of once-living things” are deposited in a particular place, as sediment, to get hardened into rocks (Park, 2001, p.196). Thirdly, “chemicals deposited from water” precipitate to become yet another kind of sedimentary rock (Park, 2001, p.196). These three kinds of particles are either transported from a source area through soil erosion or are created in the same location itself. Thus sedimentary rocks thus can be categorised into three- clastic rocks, chemical precipitate rocks and biogenic rocks. Sedimentary rocks formed by deposits of weathered rock fragments, transported from elsewhere, are called clastic sedimentary rocks. These kinds of rocks are mostly composed of quartz. They may also have contents like feldspar, other minerals, clay minerals etc. Breccia and sandstone are two examples of clastic sedimentary rocks ( Park, 2001, p.197). The grain size of this category of rocks will be very small and ranges from 0.002 mm to 263 mm (Wild, 1993, p.39). Among them, Shales contain the finest grains. The next category is biogenic sedimentary rocks. This kind of rock is created by particles from living things. Corals, mollusks, carbonate minerals like limestone, coal etc. are included in this section. Botkin and Keller (1995, p.55) have observed that “life processes play an important role in the rock cycle through the incorporation of carbon into rocks; bio-sedimantary processes produce rocks such as limestone.” . Chemical sedimentary rocks are the third group in sedimentary rocks. This kind of rock is formed when minerals dissolved in a solution get over saturated. And they precipitate to form a rock structure (Botkin and Keller, 1995, p.64). Mostly such rocks are formed by evaporation of water from such a solution. Gypsum is an example for this kind of rock. The major source of the particles that form sedimentary rocks, is a process called weathering. When a rock weathers by breaking down into small particles without having any change in its chemical composition, the process is called mechanical weathering. Ice is the major catalyst in this process ( Park, 2001, p.196). When water sweeps into the creeks and fissures in the rocks and freezes, the volume of water is increased. This puts extra pressure on the rock from inside and the cracks widen. Along with this, when the rock is exposed to alternate heat and cold by day and night temperature differences or difference in temperatures in different seasons, the contraction and expansion that accompany such temperature changes break down it. (Chapman and Reiss, 1992, p.94) These are the major causes of mechanical weathering. There is another kind of weathering also. In that, the rock is broken down by chemical reactions. So, the minerals in the rock get separated and become small particles that can be easily transported by wind, water etc. This kind of weathering occurs mainly to igneous rocks in which the minerals usually exist in very unstable forms. This is because igneous rocks had been formed at very high temperatures. Water and acidic and alkaline solutions can bring about the breaking down of these rocks. The minerals in the igneous rocks thus turn into clay minerals or chemicals dissolved in solution. Even glaciers are found to carry out weathering and transportation of sediments in huge quantities. Cox and Moore (1999, p.27) also have noted down another kind of weathering that is catalyzed by ice. They say that “ a simple example is the invasion of vegetation following the retreat of glaciers in Alaska.” Cox and Moore (1992) goes on to describe this process by saying the followed: Warmer conditions cause the melting of ice, and the ice front gradually recedes, leaving bare rock surfaces and crushed rock fragments in sheltered pockets and crevices. Such primitive soils may berich in some of the elements needed for plant growth such as potassium and calcium.(p.27) Hence, the roots of plants go in search of food into the crevices and they widen and even rock is broken into pieces. Park (2001) has described four major principles of weathering: All rocks are subjected to weathering, some rocks weather faster than others (because of their mineral composition, bonding and particle cementing), the same rock type weathers differently and at different speeds in different climate regimes and cracks and cavities in a rock enhance weathering processes by allowing them to occur deeper in the rock” (Park, 2001, p.205). Weathering by its transforming effect on different kinds of rocks, thus causes a ‘rock cycle’(p.204). Park (2001, p.204) has elaborated about this rock cycle in the statement below: Some igneous rocks are weathered and eventually buried and lithified into sedimentary (secondary) rocks. Other igneous rocks are transformed into metamorphic (tertiary) rocks by heat, pressure and chemical change. Sedimentary and metamorphic rocks at the edges of tectonic plates are subducted back into the mantle, where they are melted and can eventually be released as magma and form new igneous rocks. Sedimentary rocks can be transformed into metamorphic rocks by heat, pressure and chemical change. Metamorphic rocks can be weathered, buried and lithified into sedimentary rocks. (p.204). Actions of certain bacteria, fungi, plants etc. can also contribute to the process. Wild (1993, p.80) has noted that “ on dry rock surfaces, the algae are protected from desiccation by association with a fungus, the association being known as a lichen, which is an agent of rock weathering.” Increasing pressure and temperature on the lower layers of the sediment also induce certain chemical reactions. (Goudie, 1989, p.108). Wild (1993, p.40) has described one aspect of this phenomenon by saying, “ at a given temperature, the longer the weathering rock remains wet, the greater is the degree of chemical weathering.” To understand the grand scale in which weathering takes place, let us consider a similar phenomenon as described by Chapman and Reiss; (1992, p.13) “wind picks up some 1500 Tg (10 raised to 12 gm) of crustal material annually from the Sahara and the Arabian peninsula.” Chapman and Reiss (1992, p.12) have also described the way in which these particles exist in the atmosphere. These particles eventually get deposited by the force of gravity to become sedimentary particles. Chapman and Reiss (1992, p.12) have said that “there are significant quantities of aerosols in the atmosphere. These are suspended particles of sea-salt, mineral dust (particularly silicates), organic matter and smoke. These originate from different sources like “ dry surfaces.. coal fires.. biomass burning, volcanic dust” etc. (Chapman and Reiss, 1992, p.12). The rock particles, thus are transported by wind or water to lower altitudes. In this process also, weathering continues, thereby reducing the particle size more. A kind of sorting occurs by this natural process and size-wise, the particles are deposited in different places. Water-deposited sediments form flood plains, deltas, and the bottom of rivers, lakes and seas. Based on the location, two broad categories of sedimentary rocks are marine sedimentary rocks and terrestrial sedimentary rocks (Goudie, 1989, p.35). Sediment undergoes certain chemical and physical changes after being deposited. The loose material first turns into compact matter. The next step is lithification in which the consolidation is increased caused by the pressure build up. ( Park, 2001, p.196). When the minerals in the sediment, precipitate, the pore space in the rock is lessened and cementation process is started. (Goudie, 1989, p.38). When new rock layers form over the older ones, the particles in the lower layers mechanically rearrange themselves to become more hard (Goudie, 1989, p.38). Goudie (1989, p.38) also have described how the “ground water, seeping through the sediment, may deposit the minerals that it carries in solution, such as calcite, and may in due course unite the whole into a solid mass.” Sedimentation that occurs in the sea is of one nature near the coastline and of different nature in deep sea. ( Barrey and Chorley, 1992, p.359-360). Sediments having larger grains get deposited near the cost because of the higher wave action. Finer particles get deposited away from the coast. This sediment will undergo changes of a bio chemical kind. For example, carbonate rocks are seen far away from the coast but in shallow waters. This is created from the skeletons of organisms that live in such environment. ( Barrey and Chorley, 1992, p.359-360). Sudden currents that happen in the depths of the ocean also create sedimentary deposits in huge quantities. “Sedimentary rocks cover more than two-thirds of the earth’s surface area” (Goudie, 1989, p.39). They are the best archives of earth’s geographical and biological history as they store inside, fossils, and marks that indicate the differences in time in which they were formed. References Park, C. (2001). The Environment: Principles and Applications (2nd Edition). Routledge, London.  Barry, R.G. and Chorley, R J. (1992). Atmosphere, Weather and Climate. Methuen, London.  Botkin, D. and Keller, E. (1995). Environmental Science: Earth as a Living Planet. Wiley, Chichester.  Chapman, J.L. and Reiss, M.J. (1992). Ecology: Principles and Applications. Cambridge University Press, Cambridge.  Cox, B. and Moore, P.D. (1999). Biogeography. Blackwell, Oxford.  Goudie, A. (1989). The Nature of the Environment. Blackwell, Oxford.  Wild, A. (1993). Soils and the Environment. Cambridge University Press, Cambridge. Read More