What is the importance of sedimentary rocks
Information sheet sedimentary rocks
Formation of the sediments
Sedimentary rocks or sedimentary rocks are rocks formed by deposits, which are often formed from fragments of older rocks. The rocks on earth have been exposed to weathering for millions of years. The action of gravity, water, ice and wind creates solid and dissolved minerals and decomposition products. The solid and dissolved components are removed by the exogenous and endogenous forces, transported and then deposited and sedimented in deeper places such as valleys. These processes produce unconsolidated sediments. The newly created deposits consist of unchanged, changed and new minerals. Solidification (diagenesis) turns the sediments into sedimentary rocks.
Formation of sedimentary rocks
Various processes can lead to the conversion of sediments to sedimentary rocks. One of the most important is compression. When numerous layers are stacked on top of one another, the pressure on the layers below increases, so that air and water are forced out of the spaces between the individual pores. The particles regulate and rotate in such a way that they interlock.
Another important process is puttying or cementing. In the process, dissolved minerals, such as calcite, are deposited in the water that flows through the sediment. Over time, the loose sediment turns into a solid mass.
All sediments are initially deposited in loose storage (loose sediments), but can be solidified to sedimentites through the above-mentioned processes of cementing or cementing and the load through diagenesis. With the help of these processes, the contact areas between the mineral grains and the strength of the rock are increased.
Both loose sediments and sedimentary rocks can weather again, be removed and transported. A sediment can run through the cycle of weathering, transport and deposition several times.
Classification of the sedimentary rocks
The sedimentary rocks can be divided into clastic, chemical and biogenic sediments.
- clastic sedimentary rocks
Sedimentary rocks composed of parts of older rocks and solidified by diagenesis are called rubble rocks or clastites.
If these consist mainly of rounded components such as gravel or rubble, one speaks of conglomerates. Sedimetites composed of angular debris fragments, however, are referred to as breccias (also known as breccias).
The clastic sediments consist of mechanically transported and unchanged rock material. The grain sizes are different and range from sorted (uniform) as in drifting sands to unsorted (mixed) as in moraines.
The loose sediments are called according to the grain size: clay (<0.002 mm), silt (0.002 - 0.063 mm), sand (0.063 - 2.0 mm), gravel (2.0 - 63 mm) and stone / coarse rubble (> 63 mm) . The solidified clastic sediments are called according to their composition: mudstone (slate clay, with flat discharge), siltstone and sandstone.
Special types of clastic sedimentary rocks are the arkose and the greywacke. The arkosis has a high proportion of chemically unweathered feldspar fragments. In addition to the quartz grains, the greywacke contains 50% other mineral grains.
- The chemical and biogenic sediments arise from precipitation from solutions or from biological processes. Limes are formed, for example, through chemical precipitation, while other sediments, in turn, are formed from organic material such as fossil remains of plants and animals.
Limestone consists mainly of calcium carbonate. It arises either through precipitation from a solution or through deposits of calcareous skeleton and shell residues. Marl, a mixture of lime and 35 - 65% clay, can be divided into marl, marl lime, clay marl and marl clay (the clay content increases). Dolomite is a calcium magnesium carbonate and is similar to lime. It was subsequently transformed into dolomite by adding a magnesium-containing solution to the lime. Sedimentary rocks also include coal seams and layers carrying crude oil and natural gas. Anhydrite, rock salt and potash salt belong to the evaporites, salts that are formed by the evaporation of solutions. These salts can be found in salt domes.
The sediments can also be identified by the means of transport (aeolian: transported by the wind, glacial: transported by the ice) and the place of deposition (terrestrial: on the mainland, fluvial: in the river, limnic: in the lake and marine: deposited in the sea) .
More than two thirds of the earth's surface is covered by sedimentary rocks. They give insights into the previous environmental conditions and the changing climates. With their help, statements can also be made about the history of changes in nature.
Source: Geography Information Center
Author: Julia Ludwig
Source date: 2008
Processing date: 06/08/2012
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