sedimentation

Property blown by severe sandstorms at the Dust Bowl in South Dakota , USA, 1936

Sedimentation or sedimentation (of lat . Sedimentum = sediment) is the deposition of particles from fluids or gases under the influence of the force of gravity or the centrifugal force . The layer of suspended matter that forms is called sediment , sediment or loose sediment . Unlike sedimentary rock is unconsolidated sediment , a loose rock .

Basics

During sedimentation, the deposited particles are layered according to their density and size due to their different sedimentation speeds (sinking speeds) . The particles with the greatest sedimentation speed are deposited first, i.e. they are at the bottom. Since the sedimentation speed is essentially determined by the density, different substances can be deposited separately in layers, which can also be used to separate the different substances of a mixture (see decanting ). If only one material is deposited or materials of similar density, large particles deposit more quickly and are on the bottom first, while small particles are on top. The greater the density, the faster the substance sinks to the ground. In the case of foamed material, for example eruptive pumice stones , inverse grading can occur, smaller particles then have a higher sedimentation speed and are stored below, while larger particles are stored above.

In the case of currents , sediments are mainly brought in by the erosion of the cross-section. Another effect is the sediment input . Sediments (and other solids) from the catchment area are brought in here.

Natural sediments

Inflow of the Rhone into Lake Geneva

Cross-section of a thin-layer sedimentation sequence

Natural sediments can be divided into three main groups according to their formation:

clastic sediments (particles transportedby water, wind, glaciers, debris flow,etc. and thus mechanically formed, e.g. sand, gravel bank , sandbar )

chemical sediments ( precipitated from aqueous solutions through chemical processes, e.g. carbonates )

biogenic sediments (deposits of organisms or from organism residues, e.g. coral reefs )

The storage location is another option. Here, fluvial , limnic , marine , aeolian , glacial and pyroclastic sediments can be distinguished from one another.

In the case of natural sedimentation, the suspended solids are usually brought in by erosion processes and, in this case, primarily by fluvial transport , whereby the parent rock is usually preceded by weathering . Depending on the distance to the ablation site and the flow velocity, the grain size distribution of the particles carried in the water shows significant differences. The rule here is that the grain size of the particles decreases with distance and with a decreasing flow velocity, since the largest or heaviest particles ( debris ) sediment first and the flow is often no longer able to stir them up from the water bed.

Especially in stagnant water , these suspended matter form sediment layers through gravitational deposition, some of which are used to determine the age ( stratigraphy ). This is mainly due to the fact that, in contrast to rivers, there is no longer a current and therefore very small particles can also be deposited. In addition, depending on the climate system, the sedimentation often shows a different pattern in the course of the year, since the finer particles settle in a frozen body of water in winter, for example. Thus, similar to the annual rings in trees, coarser and finer layers develop each year, which are called varves . These often include living beings or their traces, which can develop into fossils in the course of fossilization . The formation conditions ( paleoclimate ) of the individual layers are also often documented in these, which is why sediments represent important climate archives . Marine, shallow marine and sea deposits in particular have a high informative value , which is why they also represent the main goal of climatological research boreholes.

While advancing sedimentation increases the thickness of the sediments, the rising pressure in the deeper layers can trigger further geological processes. The diagenesis forms the sedimentary rocks from the loose sediments . Snow represents a special case , which can also be layered and compressed to ice under the influence of pressure . If this effect lasts for several years, it can lead to the formation of a glacier .

application

Sedimentation plant

The principle of sedimentation is used in areas of natural science, but also in everyday life:

The mechanical clarification of water in the settling basin of a sewage treatment plant is based on the principle of sedimentation.

In smaller decentralized oil mills , the principle is used to the in the pressing of vegetable oil produced trub oil to clean of sediment. The pressed oil is put into a container in which it can stay for a long time (up to several weeks) while the heavier sediments slowly sink to the ground due to the gravitational pull. After the sedimentation, the cleaned oil is slowly removed from the container so that the lowered particles remain in the sedimentation container. Newer vegetable oil sedimentation processes run continuously. The trub oil to be cleaned flows through a system of pipes through containers connected one after the other. The sediments are deposited in the individual containers so that a high degree of purification is achieved after leaving the last container.

In the mill laboratory a is sedimentation carried out in which the volume of the sediment of a flour-water suspension as a measure of the swellability of the protein in the flour applies. In soil science , sedimentation experiments are used to determine the grain size distribution of a soil .

Sedimentation analyzes are carried out in the pharmaceutical industry in order to check the quality of medicinal powders for the treatment of asthma, for example. The size of certain particles and their distribution in this powder can be determined by measuring the sedimentation velocities. Photosedimentation is an important example of such an analytical method . Sedimentation analysis is also often used to determine the grain size distribution of a soil .

In so-called split systems, the sedimentation effect is exploited, for example in biological preparation, using the density-dependent sedimentation speed to separate particles such as protein molecules or biological cells .

In the production, storage and processing of suspensions such as For example, paints , casting resins , concrete or some foods, sedimentation is often a disturbing effect.

literature

Maurice E. Tucker: Introduction to Sedimentary Petrology. Enke, Stuttgart 1985, ISBN 3-432-94781-X .
Andreas Schäfer: Clastic Sediments. Facies and sequence stratigraphy. Elsevier, Spektrum Akademischer Verlag, Munich 2004, ISBN 3-8274-1351-6 .

Web links

Wiktionary: Sedimentation - explanations of meanings, word origins, synonyms, translations

Mineral Atlas: Sinter (wiki)

Individual evidence

↑ The biogenic sediments are also classified as a subgroup of chemical sediments.
↑ Skoog, DA; Holler, FJ; Crouch, SR: Instrumental Analytics. , 6th edition, Springer-Verlag , Berlin / Heidelberg, 2013, ISBN 978-3-642-38169-0 , pp. 931-934.
↑ Chwan Bor Fuh: Split-flow Thin Fractionation. In: Analytical Chemistry. Vol. 72, No. 7, April 2000, ISSN 0003-2700 , pp. 266 A-271 A, doi: 10.1021 / ac0027688 .

[1] The biogenic sediments are also classified as a subgroup of chemical sediments.

[Skoog-2] Skoog, DA; Holler, FJ; Crouch, SR: Instrumental Analytics. , 6th edition, Springer-Verlag , Berlin / Heidelberg, 2013, ISBN 978-3-642-38169-0 , pp. 931-934.

[3] Chwan Bor Fuh: Split-flow Thin Fractionation. In: Analytical Chemistry. Vol. 72, No. 7, April 2000, ISSN 0003-2700 , pp. 266 A-271 A, doi: 10.1021 / ac0027688 .