Subsidence (geology)
Subsidenz ( German lowering ) describes the process of subsidence in geology . This can be locally limited, such as mountain subsidence , or over a large area, over tens of thousands to several million square kilometers, and occur over time spans of many millions of years. Areas of the earth's surface or upper crust affected by subsidence are generally referred to as subsidence zones .
Large-scale subsidence usually leads to the formation of a sedimentary basin in which sediments with a thickness of many hundreds to several thousand meters accumulate over geological time periods . In the North German Basin, for example, the sediment cover is up to 10 km thick in places.
causes
Tectonic Subsidence
Within the continental crust , the subsidence of a crustal area begins with a tectonic stretching of the crust. The expansion of the crust can be symmetrical or asymmetrical and can only include the continental upper crust or the entire lithosphere , as in the case of rift formation . The tectonic expansion of the crust is a relatively rapid process that spans a few million years.
Subsidence through surcharge
An area of the earth's crust can also be lowered by loading. One of the best-known examples is the Scandinavian shield , which was indented several hundred meters in the center by the inland ice of the last glacial period . Since then (about the last 10,000 years) the earth's crust has been lifting again after the discharge.
Another case of load subsidence occurs in connection with hotspot volcanism in oceanic regions: The mass of the basalt islands formed by volcanism weigh on the relatively thin oceanic lithosphere. This gives way under the weight and sinks deeper into the asthenosphere . A further lowering of the islands then takes place via the gradual cooling and increase in density of the rocks, so that the volcanic chains get below the sea surface with increasing distance from the hotspot, where they are formed into guyots , reinforced by erosion .
In sedimentary basins, the load of the accumulated sediments can increase the primary basin subsidence caused by other factors.
In cities, buildings also contribute to subsidence as a burden, which can be particularly noticeable in large cities in coastal regions.
Subsidence through abstraction of groundwater
In some mega-cities in the coastal regions, the abstraction of groundwater is one of the most important factors that cause subsidence. In Tokyo, groundwater abstraction was stopped at the beginning of the 1960s; ten years later the subsidy had come to a standstill.
Thermal Subsidence
The consequence of a tectonic expansion of the earth's crust is always an imbalance in temperature, which is caused by hot rock at a relatively shallow depth. A further subsidence must therefore take place through cooling. The cooling is a slow process that lasts a few hundred million years and is also effective in oceanic crust . This arises on the mid-ocean ridges from rising magma and moves away from the ridge with the drift of the tectonic plate . The cooling and the associated shrinking of the rock compacts the material and sinks deeper into the asthenosphere. Subsidence is proportional to the square root of the elapsed time or the age of the rock.
Subsidence through halokinesis
Halokinesis is the migration of rock salt in the deeper underground as a result of sedimentary load and inhomogeneities (“ paleorelief ”) in the basement below the salt deposits . The salt, which behaves plastically under high pressure in geological time periods, “flows” to an area of lowest pressure, rises from there in so-called diapirs towards the earth's surface and forms salt domes . In the areas from which the salt migrates, the mountains above sink due to the volume shrinkage in the subsurface , which blows through to the surface of the earth. The result is a so-called peripheral edge sink . The name refers to the fact that these subsidence zones are in the vicinity (“on the periphery ”) of salt domes.
Measurement of Subsidence
The aim of measuring subsidence is to determine the subsidence of a section of the earth's crust as a function of time in order to be able to determine a subsidence curve for this section. The subsidence curve determined can provide information on the development history of the investigated section of the earth's crust, including the possibility of the formation of mineral resources such as crude oil or natural gas .
Ideally, one can use a borehole with numerous, chronostratigraphically well documented sections to obtain a subsidence curve. In addition to the sediment thicknesses that were deposited in the time periods considered, information about the development of the porosity of the rocks and about the eustatic sea level fluctuations is required. These figures result in a correction of the lowering, because the load of the sediments and the sea water causes a part of the subsidence.
To determine the temperature history and the carbonization of the different areas of a sedimentary basin , subsidence analyzes, e.g. B. from seismic data . The purpose of this is to determine the suitability of a potential petroleum parent rock with regard to the possibility of petroleum formation and the likely degree of maturity of the petroleum via the former temperature conditions.
The current subsidence can be observed by optical methods, by GPS and by methods of remote sensing ( Lidar and InSAR ).
In several Asian coastal cities, a subsidence of the mainland is superimposed on a rise in (global) sea level, with the result that the relative sea level there rises much more than would be expected due to global warming alone .
Web links
Individual evidence
- ↑ Hans Murawski: Geological Dictionary . 8th edition. Enke, Stuttgart 1983, ISBN 3-432-84108-6 , pp. 280 .
- ↑ a b c d G. Erkens, T. Bucx, R. Dam, G. Lange, J. Lambert: Sinking coastal cities . In: Proceedings of the International Association of Hydrological Sciences . tape 372 , p. 189-198 , doi : 10.5194 / piahs-372-189-2015 .