One of the types of rocks that can be found on the Earth are sedimentary rocks. There are two other kinds which are igneous and metamorphic rock. Sedimentary rocks cover about 75 to 85 % of the land on Earth. When pieces of rock or parts of once- living organisms or minerals from precipitation deposit and solidify on Earth’s surface they for sedimentary rocks. (Murck, et al. , 2008) In addition, the way the plate tectonics move, can significantly affect sedimentation and the formation of sedimentary rocks. There are three main types of sedimentary rocks which are classified by the source of the sediments.
They are clastic, biogenic, and chemical sedimentary rocks. Each category of sedimentary rock has distinct characteristics and depositional environments. Once the sediments are deposited a process called “lithification” or rock forming begins. For the newly deposited, loose sediment to be lithified and be turned into rock, the individual particles must be bound together in a cohesive unit. Once there is a layer of sediment buried either by accumulation of more sediment or by the tectonic processes it is under high pressure, which leads to compaction.
Compaction is usually the first step in the lithification process. There are several ways that this can happen. The other two ways that this can happen is through cementation or recrystallization. Cementation is the process in which substances dissolve in pore water precipitates out and forms a matrix in which grains of sediment are joined together. Recrystallization is the process where the formation of new crystalline mineral grains from old ones. (Murck, et al. , 2008) Types of Sedimentary Rocks
When clastic sediment is lithifies it will produce a rock that’s properties will reflect the type of sediment that it originated from. There are four basic classes of clastic sediment which is conglomerate, sandstone, siltstone and mudstone or otherwise known as shale. Conglomerate is a clastic sedimentary rocks that has large fragments in a finer grains matrix. Sandstone is a medium grained clast and is usually dominated by quartz grains. (Murck, et al. , 2008) Biogenic sedimentary rocks are those that are formed by the accumulation of living things which become compacted and lithified.
Ancient plant deposits become coal, and deposits of animal shells or skeletons like corals, mollusks, and foraminifera found on oceanic beds, cover the ocean floors with layers of calcite which can later form limestone. The two main minerals in carbonate rocks such as limestone are calcite and dolomite. Chemical sedimentary rocks are form when minerals in solution become over saturated and precipitate. In marine environments, this is a method for the formation of limestone. Another common environment in which chemical sedimentary rocks form is a body of water that is evaporating.
Evaporation decreases the amount of water so the dissolved material can become over saturated and precipitate. These rocks include carbonate rocks such as limestone (both a biogenic and chemical sedimentary rock) and evaporate such as halite and gypsum. Sedimentary rocks are also a result of plate tectonics which not only affect sedimentation and the formation of sedimentary rocks, also provide an explanation how rocks move through the rock cycle to be igneous, sedimentary, and metamorphic rocks. The Earth is made of two layers: the lithosphere and the asthenosphere.
The lithosphere consists of the mantle and the crust where we live. The lithosphere is divided into tectonic plates which lie on the asthenosphere. The boundaries of these plates move in three ways: Convergent boundaries where they move towards each other; divergent boundaries where they move away from each other; transform boundaries where they move against each other. This movement causes earthquakes, volcanoes, mountain formation and ocean spreading. At convergent plate boundaries, plates are move towards each other and one plate gets subducted beneath the other.
As the plate that is being subducted moves beneath the other plate, it is taking with it the accumulated sediment and rocks. The subducted rocks and sediments are heated to the point that they melt to become magma, and eventually, igneous rock as seen in volcanic activity. An additional way that sedimentation and sedimentary rocks are affected by plate tectonics is when mountain are formed, a depression called foreland basin is formed which receives sediments that is eroded off from the mountain belt and gets filled with sedimentary layers.
At divergent boundaries such as ocean spreading ridges, sedimentation is also affected. In theory, sediments are laid down in horizontal layers and they form one on top of the other this means that a layer of sedimentary rock is of the same age. At divergent plate boundaries, because the plates are moving away from each other, new plate material is coming up at the boundary. Therefore, the youngest sediments would be found closest to the plate boundary. Reference Barbara W. Murk, Brian J. Skinner, Dana MacKenzie ,2008 Visualizing Geology