How Are Sedimentary Rocks Formed?

By Carly Dodd on August 20 2020 in Geography

Sedimentary rock cliffs. Image credit: Leene/Shutterstock.com
  • The main contributors to sedimentary rock formation are erosion, precipitation, or natural weathering; as well as lithification and dissolution.
  • Some of the more common types of sedimentary rock include sandstone, shale, limestone and coal.
  • There are two types of sedimentary rocks, referred to as either detritus or chemical.

Sedimentary rocks are, as the name suggests, formed from the buildup of sediment. This means they form over time on the surface of the Earth, unlike other types of rock, such as igneous or metamorphic, which are created deep within the Earth under great pressure or heat.

Sedimentary rocks are mainly caused by gradual but constant natural changes in the environment. The main contributors to sedimentary rock formation are erosion, precipitation, or natural weathering; as well as lithification and dissolution. These environmental phenomena slowly eat away at dirt or rock surfaces, or wash sediments together which eventually build up into rock formations. Some of the more common types of sedimentary rock include sandstone, shale, limestone and coal. In all cases, debris, organic material, or minerals slowly form into what are typically soft, dry and porous rock types.

Types Of Sedimentary Rocks

There are two types of sedimentary rocks, referred to as either detritus or chemical.

Detritus Rocks

Detritus sedimentary rocks are formed when rock fragments, debris or sediments accumulate over time and can be either organic or inorganic in makeup. These detrital rocks come together under great pressure, usually over many years. The debris, or matter that form the rocks, can be either organic or inorganic.

A chunk of coal. Image credit: Siberia Photo and video/Shutterstock.com

Organic matter would consist of dead and decaying vegetation or animal matter. Once under great pressure, this material then compresses gradually until it becomes rock. The primary example of this process is coal, which is formed under great pressure over long periods of time.

Sandstone rock in the Lower Antelope Canyon in the Navajo Reservation near Page, Arizona USA. Image credit: Vichie81/Shutterstock.com

Inorganic detritus rocks, conversely, are made up of non living things. These rocks, classified as ‘clastic’, form when other rock particles or minerals accumulate and are compacted over the duration of many years. Sandstone is the most well known example of this process, as it is rock formed from the compression of sand.

Chemical Rocks

Limestone in a quarry. Image credit: Parmna/Shutterstock.com

Chemical rocks are formed from the accumulation of certain chemicals – usually calcium – in a given place over time. One of the prime examples of this is limestone, which forms where calcium carbonate precipitates and collects on the bottom of the sea. Similarly, many caves are populated by chemical rock such as stalagmites and stalactites, which are made from calcium and carbonate deposits.

How Are Sedimentary Rocks Formed?

Shale, a type of sedimentary rock in Thailand. Image credit: Pongsakorn Matapan/Shutterstock.com

There are a number of different ways these types of sedimentary rocks can be formed. From natural causes to chemical interactions, rocks are broken down or built up in various ways. Over time and with continual pressure, sedimentary rocks transform from debris, into solid rocks or rock formations. Below are the different ways in which sedimentary rocks can be formed.

Erosion

One of the major factors which lead to the formation of sedimentary rock is erosion. Wind, water, and rain, will break down earth cutting through rock and soil. This constant friction of the elements against the Earth's surface erode rock and soil, turning it into small sedimentary particles or debris. In this way, large boulders, hill sides, and rock formations are worn down creating small dust like debris, such as sand or mud.

Weathering

Various weather patterns or meteorological phenomena will naturally break down rock by a process called weathering. Wind and rain can slowly wear away at large rock formations, boulders, river beds and mountains to create minute rock debris such as sand. Examples of this can be seen in places like large canyons and waterfalls. Canyons are formed from the constant flow of water over the same area, slowly carving a path into the ground and producing sedimentary rocks along the river bed. Similarly, rain water will slowly break down rock, leaving behind minerals and rock based particles. This debris, sand and mud is what forms sedimentary rocks.

A steep river bank with pronounced layers of sedimentary rocks. Image credit: Belozorova Elena/Shutterstock.com

Dissolution

Dissolution is another process which breaks down hard, larger rock formations into sediment. This type of break down happens due to chemical weathering, or acidic rain. Climate changes and greenhouse gases are the main causes of acid rain, which eats away at stone and rock.

Lithifiction

Lithification is one way in which new sedimentary rocks can be formed. Sediments can consist of debris, minerals and other small rock particles. These built up gradually, and when compressed, form new rocks. Lithification specifically is when mud, clay, sand or other sediments are pressed under the weight of water. These sediments would be found on the bottom of the ocean or lakes, and are compressed over a great span of time. This constant pressure from overlying sediments and water compacts the sediments until they form into a solid rock mass.

Precipitation

While the term precipitation is used most in relation to rain, in a chemical sense, it has another meaning. When a liquid precipitates, it separates liquid - in this case water- from any precipitates, i.e. insoluble solids found within the solution. When rain precipitates, and evaporates into the atmosphere, various chemicals and minerals transported by rainwater are left behind. This process forms another type of sedimentary rock, which is often found in dried up bodies of water or river beds.

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