What are Sedimentary Rocks?

Sedimentary rocks are formed by the process of sedimentation.

Sedimentary rocks are formed through subsequent cementation and deposition of sediment material within bodies of water and at the surface of the earth. The processes that cause organic or mineral particles to settle in place are collectively known as sedimentation. Sediments are the particles that form sedimentary rocks through accumulation. From the source area, sediments are formed by either erosion or weathering before being deposited elsewhere. The sediments are then transported to the place of deposition via agents of denudation which are mass movement, water, glaciers, ice, or wind. Another way that sedimentation might take place is when the shells of aquatic organism settle out of suspension or when minerals precipitate from water solutions. In science sedimentology is a discipline that involves the study of the origin and properties of sedimentary rocks; it is part of physical geography and geology. Sedimentology also overlaps with other earth sciences disciplines including geochemistry, pedology, structural geology, and geomorphology.


Sedimentary rocks are believed to cover about 73% of the current land on the surface of the Earth. However, their total contribution is approximately 8% of the crust’s total volume. Sedimentary rocks comprise of only a thin layer of the Earth’s crust which generally consists of metamorphic and igneous rocks; they are deposited as veneers of strata and form a structure known as bedding. It is important to study rock strata and sedimentary rocks since the information is essential for civil engineering. For instance, such information is used in constructing tunnels, roads, canals, and houses among other structures. Natural resources such as drinking water, coal, ores, or fossil fuels are produced from sedimentary rocks. The main source of understanding the history of the earth is the study of sedimentary rock strata’s sequence which includes the history of life, paleogeography, and paleoclimatology. Besides the Earth, there have also been sightings of sedimentary rocks on planets like Mars.

The Classification Of Sedimentary Rocks

Sedimentary rocks can be classified according to the processes of how they were formed and can be divided into four different groups, which include chemical sedimentary rocks, clastic sedimentary rocks, other sedimentary rocks, and biochemical or biogenic sedimentary rocks. Clastic sedimentary rocks are further subdivided depending on their dominant particle size and the composition of other particles of rocks originally cemented by silicate minerals. Clastic sedimentary rocks mainly consist of rock fragments, mica, quartz, clay minerals, and feldspar. Types of clastic sedimentary rocks include mudrocks, sandstones, conglomerates, and breccias. Biochemical sedimentary rocks form when materials that are dissolved in water or air is used by organisms to construct their tissue. Types of biochemical sedimentary rocks include coal, deposits of chert, and most types of limestone. Chemical sedimentary rocks are formed when the mineral constituents in solutions are inorganically precipitated after being supersaturated; examples include barite, halite, gypsum, and stylite. Other sedimentary rocks are the category of sedimentary rocks that are formed by uncommon processes such as volcanic breccias, impact breccias, and Pyroclastic flows, among others.

Properties Of Sedimentary Rocks

There are several factors that help classify sedimentary rocks, they include fossils, mineralogy, texture, color, and primary and secondary sedimentary structures. The texture of sedimentary rocks comprises of their orientation, size, and form. Despite texture being a small-scale property of a sedimentary rock, it can help determine other large-scale properties such as permeability, density, and porosity. Fossils are most commonly found in sedimentary rocks compared to igneous and metamorphic rocks. Unlike the other two types of rocks, sedimentary rocks are formed at pressures and temperatures that do not obliterate the remains of fossils. However, most of the times such fossils may not be seen by the human eye but are seen only under a microscope. In nature, dead organisms are often rapidly removed by erosion, bacteria, scavengers, or rotting. However, sedimentation is a major contributor to special circumstances where such natural process are not able to work, thus resulting to fossilization.

Color is a major property of sedimentary rocks and is usually identified by iron and its two main oxides which are iron (II) oxide and iron (III) oxide. For instance, iron (II) oxide or FeO forms only under circumstances that comprise of low oxygen also known as anoxic, thus giving the rock a greenish or grey color. On the other hand, iron (III) oxide or Fe2O3 which is usually found in the form of hematite, a mineral found in an environment that is richer in iron, contains more oxygen. Therefore, sedimentary rocks in such an environment are brownish or reddish in color. Minerology refers to the mineral structures found in rocks. A large number of sedimentary rocks comprise of either calcite or quartz. In comparison to metamorphic and igneous rocks, sedimentary rocks normally contain low levels of different significant minerals. Nonetheless, the origin of minerals in sedimentary rocks tends to be more complicated than in igneous rocks.The minerals in sedimentary rocks are either formed through diagenesis or precipitation during sedimentation.

Primary sedimentary structures is also another property used in identifying sedimentary rocks. Sedimentary structures comprise of large-scale features that are easier to study in the field unlike textures, they are used in indicating something about the sedimentary environment. For instance, sedimentary structures can help tell which side of a sedimentary rock initially faced up in an environment where tectonics have either overturned or tilted the sedimentary veneers. Secondary sedimentary structures are sedimentary structure that are only formed after deposition, these structures form within the sediment through biological, chemical, and physical processes. Such processes can indicate various circumstances after deposition and can even be used as geopetal indicator.

The Rate Of Sedimentation

Sediments are deposited at different rates mainly depending on the location they are found. For instance, deposition on a deep ocean floor only accumulates a few millimeters of sediment every year, whereas deposition on a channel found in a tidal flat can result in the accumulation of several meters of sediments in a day. However, there is a distinction between sedimentation that results from catastrophic processes and normal sedimentation. The former comprises of all types of sudden special processes including flooding, mass movements, or rock slides. Such processes can result in the sudden deposition of sediments in large amounts at once. While other environments with sedimentary rocks are dominated by ongoing or normal sedimentation, most sedimentary rocks were formed as a result of some catastrophic processes.

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