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Sedimentary Rock

Sedimentary Rock

Sedimentary rock is one of the three main groups in the rock cycle and includes common building and flooring stones such as limestone, sandstone and travertine. It is generally softer and more porous than igneous and metamorphic rock which is what makes it a popular material in construction.

Sedimentary rock covers as much as 80% of the Earth's surface however only about 5% of the Earth's crust is made up of it. It is formed by the process of weathering, erosion, deposition and compaction of other rock and organic materials from the Earth's surface. The sediments which form sedimentary rock are generally broken down by the elements, wind and rain, before being washed down or transported into a low lying area such as the ocean floor. Here, an accumulation of these sediments builds up. The resulting pressure compacts or cements these materials together to form layers of new rock. This rock forming through pressure is known as lithification.

The sediments involved in this process can be classified into three main groups:

Clastic Sediments:

Fragments of rock which have been broken down through mechanical and chemical weathering. Ice is a common agent in mechanical weathering. Water makes its way into the cracks and fissures of rock at the Earth's surface, freezes, and expands under cold temperatures. This results in the breakdown of the rock and increased surface area which makes it vulnerable to chemical weathering from water. These rock pieces are broken down into various sizes before being deposited in new locations by wind, water and glaciers. The grain sizes of different sediments can vary with a scale ranging from 0.002mm for shale particles to sandstone at up to 2mm. Conglomerate and Breccia grains can be larger still at up to 2.7mm. Geologists identify clastic sediments by sieving and referring to the universally accepted Wentworth Scale.

Organic Sediments:

Also referred to as biochemical or biological sediments. These are derived from the death and deposition of organic life forms and plants. Huge quantities of shell, coral and other marine organisms and the minerals they contain are involved in this process. Calcite from shells and skeletons will form fossiliferous limestone. The chalky limestone which makes up the white cliffs of Dover originated from the shells of tiny animals called Forminfera. Decaying marsh plants and ferns which are compacted over millions of years become an organic sediment known as coal.

Chemical Sediments:

These are sediments which have derived from deposits of minerals which were once in solution (dissolved in water). These minerals can crystallise when the water evaporates which creates what are known as evaporites. A well known chemical sediment is rock salt which is a crystallized precipitate created by the evaporation of sea water. Another is gypsum (calcium sulfate) which is used to produce plaster of paris and portland cement. Flint rocks are formed from dissolved silica.

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