Sedimentary Rocks

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Types of Rocks

Our world is divided into land, air, and water. Now, if we focus on what actually constitutes land masses, we’ll come to see that the solid landmasses which make up the outer crust of the earth, which mostly consist of rocks. Essentially rocks are aggregate masses of natural minerals and can be divided into three categories, igneous rocks, metamorphic rocks, and sedimentary rocks. Igneous rocks refer to solidified magma from underneath the earth’s crust that is usually found on the ground surface of the earth. Metamorphic rocks are those rocks which are formed when existing rocks undergo a drastic change in their physical properties due to heat and pressure. Finally, sedimentary rocks are formed when sediments of earlier rocks keep depositing on top of each other on any given surface. 

What is Meant by Sedimentary Rocks?

The earth's crust is predominantly composed of igneous rocks and metamorphic rocks, which in turn are exposed to years of weathering, attrition, and erosion due to temperature fluctuations and other weather conditions. As a result, the dregs from these rocks are eroded away and deposited on the surface of the earth. After years of accumulation, these deposits become compact under environmental pressure. The rocks formed in this process are known as sedimentary rocks. Sedimentary rocks cover 73% of the earth’s land surfaces. However, they are the least available type of rock in the entirety of the earth’s crust, constituting only 8% of its total volume. 

The Common Types of Sedimentary Rocks 

Sedimentary rocks may be classified into further subgroups depending either on their process of formation or based on their mineral composition. Based on their origin, sedimentary rocks may be clastic, biochemical, and chemical rocks. 

Clastic Sedimentary Rocks: These are sedimentary rocks made out of pre-existing rocks. The pieces of these rocks are loosened by weathering and then transported to a basin where the sediments accumulate to form sedimentary rocks.

Biochemical Sedimentary Rocks: Biochemical sedimentary rocks include rocks that are formed with the help of organic processes as well. These include calcium carbonate shells by snails or clams, which can help in the formation of limestone. 

Chemical Sedimentary Rocks: When minerals in solutions become supersaturated and thus, become inorganically precipitate, they form chemical sedimentary rocks like halite, gypsum, etc.

How Are Sedimentary Rocks Formed?

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Since sedimentary rocks are formed from the sediments of the pre-existing rocks, they are named likewise. The process of formation of sedimentary rocks takes place through a geological process called lithification, which involves several steps. To begin with, erosion and weathering of existing rocks which essentially involves the physical disintegration of the rocks and then transportation of the debris away from the original rock. Sediments are also often formed through the process of chemical decomposition through either rainfall or dissolution. The next stage of the formation of sedimentary rocks involves the transportation of the detritus. Gravity plays an important role in facilitating transportation since most of the sediments are either carried by wind or by moving masses of water like rivers or glacial ice. 

Once transported, the sediments start depositing. The deposition of sediments can also take place due to precipitation when exposed to a suitable temperature and atmospheric pressure.  Organic reefs are an example of such sedimentary rocks. 

Once the sediments start to deposit, layers of such sediments start piling up on each other through a process called burial. Once the accumulation reaches a certain depth, these sediments start getting compacted and cemented into solid rocks through the process of lithification. This is how sedimentary rocks are formed. Some of the common examples of sedimentary rocks are chalk, clay, flint, and shale. 

What are the Characteristics of Sedimentary Rocks?

Now that we know how sedimentary rocks are formed let us list and describe three features of the sedimentary rocks. 

  • Sedimentary rocks have porosity varying between the range of 1% to 50%. This is because the process of formation of both organic or clastic sedimentary rocks, as well as chemical sedimentary rocks, take place over time, the particles are not always evenly packed. So, in more compact sedimentary rocks, the porosity is measured at 27% while in a more loosely packed rock, it can go over 47%. While sandstones can have a porosity of 5%, clays have porosity exceeding 50%. 

  • Although some sedimentary rocks are porous, they are largely impermeable. Only some varieties of sedimentary rocks are pervious if they have joints or cracks. 

  • The sphericity of the sedimentary rocks varies according to their course of the journey during the process of transportation. Further, the sedimentary rocks are from their original rock, the more rounded they are, and their sphericity increases. Parallelly, sediments that are carried through longer distances have more sorted debris rather than those that have travelled relatively shorter distances.

FAQ (Frequently Asked Questions)

1. What Are Some Of The Common Uses Of Sedimentary Rocks?

Some of the Uses are: 

  • Cement, a key building material, is made out of limestone, which is a sedimentary rock. 

  • Quartz is an important ingredient in the glass manufacturing industry and is a sedimentary rock. 

  • For the preparation of plaster, which is a commonly used binding material in construction, is made from sedimentary rock gypsum. 

  • Sedimentary rocks are important sources of a large variety of minerals. Sedimentary rocks are the largest source of salts on earth. 

  • Some sedimentary rocks which are rich in phosphate are used in the production of fertilizers. 

2. What Are The Other Types Of Sedimentary Rocks? Give 2 Examples Of Sedimentary Rocks.

Depending on their composition, sedimentary rocks may be further divided into siliciclastic (mostly containing silicate minerals) rocks, carbonate (mostly containing carbonate minerals) rocks, evaporite rocks (formed due to evaporite minerals like carbonates and chlorides), organic-rich rocks (contains more than 3% total organic carbon), siliceous rocks (mostly contains silica), iron-rich rocks (contain more than 15% iron) and phosphatic (containing mostly phosphate minerals) rocks. Examples of sedimentary rocks include sandstone and shale.

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