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Sedimentary Rocks and their formation

Sedimentary Rocks

The upper 16 km thick earth’s crust contains only 5% of the sedimentary rocks, while the percentage of the sedimentary rock in the exposed rocks is about 75%. These rocks are found chiefly as an extensive cover over the continental crust. In the subject of rock formation, sedimentary rocks and their formation is a very important and interesting topic. Sedimentary rocks are formed by the consolidation, compaction, and cementation of sediments deposited under the water. Sedimentary rocks also include the rocks formed by the accumulation of chemically precipitated or organically derived material. Sedimentary rocks occur in layers and frequently contain fossils.

Formation of the Sedimentary Rocks

The formation of the sedimentary rocks takes place in three stages: (i) weathering and erosion of the pre-existing rocks, (ii) sedimentation, and (iii) lithification and diagenesis.

Weathering and Erosion

Weathering and erosion are collectively known as denudation. Weathering is the process of breaking and disintegration of particles/sediments from the pre-existing rocks, while erosion is a process by which the disintegrated sediments are separated and transported from the parent rocks to another place. The weathered material separated from the rocks is known as sediments. After separation of the sediments from their parent rocks, they are usually transported and deposited in the area of accumulation by the action of water or less frequently by glacial or wind action. During transportation, the sediments are roughly sorted and deposited according to size. Bigger rocks fragments such as gravel, settle first, and the fine grains or sediments or sand are next in order, while clays are deposited in the last. The minerals, which are dissolved by the water, travel in solution.

See also: Difference between weathering and erosion

Sedimentation:

Sedimentation is the process of accumulation and settling down of sediments at a site of deposition. The material carried in the solution precipitates and accumulates. Sedimentation is the intermediate stage in the formation of sedimentary rocks. Sedimentation is the first step toward the formation of sedimentary rocks. It takes place when the suspended particles settle out of the fluid which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration, or electromagnetism. Settling is the falling of suspended particles through the liquid, whereas sedimentation is the final result of the settling process. Sedimentation may pertain to objects of various sizes, ranging from large rocks in flowing water, to suspensions of dust and pollen particles, to cellular suspensions, to solutions of single molecules such as proteins and peptides. Even small molecules supply a sufficiently strong force to produce significant sedimentation. Source: Wikipedia

See also: Transportation and deposition by sea

Lithification and Diagenesis:

Lithification is a process by which soft and loose sediments are converted into hard and firm rocks. This process is also known as “Consolidation”. During this process many physical and chemical changes take place within the sediments. Such changes are called the “diagenetic changes” and the process is described as “diagenesis”. The diagenesis includes the following processes;

Sedimentary rock formation
Sedimentary rock formation
  1. Compaction: Compaction occurs when the weight of overlying layers compresses the sediments below. As the grains of sediments are pressed closer and closer together, there is a considerable reduction of pore spaces and volume. Fine-grained sediments, such as clays are consolidated more effectively by this process.
  2. Cementation: Cementation means the bonding and chemical integration of the particles with one another. When water circulates through the pores of coarse-grained sediments, the dissolved mineral matter is precipitated between the frains, which causes cementation. The most common cementing materials are silica, calcium carbonate (calcium mineral), iron oxides, and clay minerals. The identification of cementing material is a relatively simple matter. Calcite cement will effervesce with dilute hydrochloric acid, while iron-oxide gives the rock a characteristic of red, orange, or yellow colour. Silica, the hardest of the cement, produces the hardest sedimentary rocks.
  3. Recrystallization: Recrystallization further consolidates the sedimentary rocks. Although most sedimentary rocks are lithified by compaction, cementation, or a combination of both, some are consolidated chiefly by the recrystallization of their constituents. Chemically formed rocks, such as limestone, dolomites, salt, and gypsum are examples of the rocks consolidated by recrystallization.

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