Last Updated 28 Jan 2021

Sedimentary Rocks

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Plate tectonics are responsible for many of Earth’s natural occurrences. From volcanoes to earthquakes plate tectonics play are large role. Another natural occurrence that is not is discussed as much as volcanoes and earthquakes is the formation of sedimentary rock. Plate tectonics also plays a major role in the formation of this type of rock. Plate tectonics is a term that is used to describe how the different plates that make up the Earth move and the reactions that occur as they are moving. Before explaining how plate tectonics are related the concept of sediments must be understood.

Sediments are pieces of solid material are deposited on Earth’s surface by way of wind, water, ice, gravity or chemical precipitation. This occurs through weather and erosion (McClearly, 2005). As convergent plates move towards each other, one plate is subducted under the other plate and as this is occurring the plates are moving accumulated sediment and rocks with them. Divergent boundaries also have an effect on sedimentary rock. In divergent movement the plates are moving away from each other and the young sedimentary rock is pushed to the open boundary (Ryan, 2008).

Sedimentary rock actually originates from the disposition of the sediments after they are moved by the movement of plates. The climate controls how sediments are moved and how the rock weather. After weathering and erosion has occurred the sediments stop moving and settle. Where they settle is known as their disposition. The origination of sedimentary work is a combination of natural processes that result in the disposition of sediment. The way the sediment is actually formed is a process known as Lithification.

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Lithification is physical and chemical processes which loose sediment is transformed into sedimentary rock. Lithification begins with compaction. The weight of sediments on top of other sediments forces the sediment grains closer together, which results in physical changes. The next process that occurs is through a chemical process known as cementation. Cementation occurs when mineral growth, in combination with high temperatures, cement sediment grains together into solid rock (McClearly, 2008). After sedimentary rock is formed then it is organized on the Earth’s surface.

The pattern of organization is known as bedding. Bedding is the layered arrangement of strata in a group of sediment or sedimentary rock. Over time strata’s that are adjacent to each other may look different because the mineral composition of the sediments in a specific location may changes or the way the sediments are transported or deposited may be different. The bedding surface, which is the boundary between adjacent strata, is the indication that rock was once sediment. Once sediments have been formed they are classified into three different categories.

The first is clastic sedimentary rock. These rocks are formed from the sediment and debris from weathering and erosion. There are four different types of clastic, conglomerate (gravel), sandstone (sand),siltstone (silt), and mudstone (mud) or shale. These four categories are characterized by the size of their particles. The next category of sedimentary rock is chemical sedimentary rock. These rocks are formed by precipitation of minerals dissolved in lakes, rivers or sea water. This precipitation can occur is two ways.

The first is plants and animals living in water can alter the chemical balance of the water which has an effect on the sediments. The second way is the evaporation of fresh water because of warm dry climates, which leads to increased salinity, the salts become solid because they are not dissolving. The last category of sedimentary rock is biogenic sedimentary rock. These rocks are formed from sediments that are primarily composed of plant and animal remains. This can include the hard shells of animals as well as the bones and teeth.

From plants this can include fragments of plant matter such as roots, wood and leaves (Murck, Skinner, Mackenzie, 2008). Since there are three categories of sedimentary rock differentiating in the three is important. One way to tell the difference in the rock is the physical characteristics or physical location. Clastic rock is in the form of gravel, sand, slit or mud, which are all easily recognizable. Chemical sedimentary rock will be located in areas with water such as seas and lakes, since the chemical process which creates these rocks are done inside of water.

Biogenic sedimentary rock is easily recognizable because these rocks will have pieces of animal or plant matter in them. Fossils can be carved out of these types of sedimentary rock (McClearly, 2005). Sedimentary rock is one type of rock that is located all over the world. From mountains to oceans sedimentary rocks can be found. With the combination of plate tectonics, weather and erosion sedimentary rocks are formed. Clastic, chemical and biogenic rocks are all very different but together they make up the category of sedimentary rocks.

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