How do Sedimentary Rocks Form

If you are interested in geology, you may want to find out how do sedimentary rocks form. Whole of earth is covered with different kinds of rocks and minerals and even the floor of the oceans is full of rocks. The three main types of rocks are igneous, sedimentary and met amorphic. Much of the earth’s crust is covered with sedimentary rocks. How do sedimentary rocks form is a question that many people ask as they see these interesting rocks in different parts of the world. This article takes a look at the different ways in which these rocks get formed.

How do sedimentary rocks form – Process

Sedimentary rocks are formed from the sediments of pre-existing rocks

As the name implies, sedimentary rocks are the result of deposition of sediment that includes dead plant material, minerals and organic material. The compression of this sediment over a period that stretches to thousands of years, results in sedimentary rocks. Coal is a mineral rock that is a great example of sedimentary rocks. Other main types of these rocks are sandstone, limestone, flint, chalk, and mudstone.

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Most of the rocks found over the surface of the earth are sedimentary

Sedimentary rock can be formed from a scratch by the process of sedimentation or it can be formed from preexisting igneous, metamorphic or even other sedimentary rocks. The exposure of igneous and metamorphic rocks to the atmosphere and the elements results in sedimentary rocks. Weathering and erosion of these rocks start and large pieces get broken into smaller pieces. These small pieces get transported to other areas far away from their original place with the help of wind and water and even gravity. Thus, these pieces gather at the bottom of a water body like a lake or a river or over the surface of the earth from where these pieces cannot move further. Final accumulation takes place when the agents of nature lose steam and they cannot force these particles any further. It is not that only very small pieces of these rocks get transported as pieces as small as 0.002mm. Even pieces as large as big boulders can accumulate to lead to the creation of a sedimentary rock.

Formation of sedimentary rocks consists of four different processes

The four basic processes that are responsible for the formation of sedimentary rocks are as follows.

• Weathering- erosion of particles from rocks.
• Transportation- movement of particles because of gravity, water and wind.
• Deposition- accumulation of this sediment.
• Compaction and cementation- lithification.

How do sedimentary rocks form

Compaction takes place because of pressure from the top

When enough particles have accumulated at a place, their weight from the top starts to exert great pressure to initiate a process called lithification . This is a process of compaction of the particles by which they get converted into a sedimentary rock. It is this process that drives out all the moisture and cements the empty spaces in between the particles. In fact, the empty spaces get filled by the very minerals that are left behind by water that is driven out from these particles. The bodies of dead animals get converted into rocks. When the sediment contains more of dead plants, sedimentary rock formed is mostly coal. 


Image Courtesy:

  1. Inside Lower Antelope Canyon by Moondigger – Own Work (CC BY-SA 2.5)

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