Depression (geology)

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A depression in the sense of geology is a tectonically caused depression on the mainland that is lower than sea ​​level .

Definitions and Conditions

The so-called Salton Trough in California is a geological depression. A tectonically active expansion basin that extends to approx. 84 m below sea level, with the Salton Sea , a 980 km 2 salt lake at its deepest point.

Depression is a special case of a depression , the bottom of which is always lower than the entire environment, but can also be higher than the level of the ocean. Geomorphological as well as other geographic depressions are to be differentiated from lowlands that are only partially surrounded by higher terrain, so that their bottom is predominantly above the discharge horizon. Only in climatic regions where the amount of precipitation is less than the evaporation can depression be superficially recognizable as naturally accessible land below sea level. At the deepest point they have a salt marsh or salt lake , the water level of which is often subject to strong seasonal fluctuations.

On the other hand, if the bottom of a lake is deeper than sea level, this is called cryptodepression , i.e. hidden depression. Depressions that can be walked on in humid climatic regions are only possible through human intervention. In terms of area, the most significant area is permanently drained marshland located just below mean sea level, and damming depressions such as the Dutch polders . The deepest depressions in humid regions are opencast mines that are either currently being exploited and drained, or have not yet been completely backfilled or watered after they were abandoned.

Major natural depression

The deepest depressions are the Jordan Valley with the Dead Sea (level currently −425 m, bottom down to −800 m) at the deepest point and the Sea of ​​Galilee (fresh water, level at −212 m), Lake Assal in Djibouti (level at at least - 155 m), the Central Asian Turfan Depression with Lake Aydingkol (mirror at −155 m), in Egypt the Qattara Depression (−133 m) and the Fayyum Basin with Lake Qarun (mirror −42 to −45 m), in the Afar triangle the Koba Depression (−115 m) and the Danakil Depression (−110 m), the Laguna del Carbón (mirror −105 m) in Argentina, the American Death Valley ( Badwater −85.5 m), the Caspian Depression with the Caspian Sea (mirror currently at −28 m, bottom down to −1,023 m), in Tunisia the Chott el Gharsa (−17 m) and in Australia the Eyre Lake , mirror −11 m (9,500 m) km²) to −17 m (empty).

Crypto depressions

A water-filled depression (lake), the bottom of which is below sea level, but which is so filled with water that the lake level is above sea level, is called cryptodepression (cf. Greek kryptos for "hidden"). In a humid climate, hollow forms are filled with lakes until they overflow. Therefore, apart from artificial drainage, there is only cryptodepression here. These emerged particularly often as a result of Ice Age glaciation, such as around the Baltic Sea and the peripheral lakes of the high mountains . In fjord lakes of this kind , glaciers have carved out valleys to below sea level. Terminal moraines or rock bars or hundreds of kilometers of land mass keep the lake level high and sea water away.

The crypto depressions in the Alpine region include Lake Garda (lowest point -281  m slm ), Lake Como ( -228  m slm ), Lake Maggiore ( -179  m above sea level ), Lake Iseo ( -66  m slm ) and the Lake Lugano ( -17  m above sea level ). Cryptodepressions in northern Germany are the bottoms of these lakes: Arendsee ( 27 m below sea level ), Plöner See , Hemmelsdorfer See ( 39.5 m below sea level ) in Ostholstein , Ratzeburger See , Schaalsee ( 36.5 m below sea level ), Schweriner See , Malchiner See , Kummerower See , Großer Stechlinsee and Tollensesee .

The deepest crypto depressions, however, often have a tectonic origin and are located near rift breaks : The deepest example worldwide is Lake Baikal (level 456 m above sea level, lowest point 1,186 m below sea level). Furthermore, Lake Tanganyika (level 782 m above sea level, lowest point 688 m below sea level) is of tectonic origin.

Damming depression

On shallow seashores and in some estuaries, land that is little below sea level is artificially kept free of sea water and used as cultivated land through the dike and constant drainage - this is called the damming depression. More than a quarter of the Netherlands is lower than mean sea ​​level . The deepest damming depression in Germany at 3.54  m below sea level is located in Neuendorf-Sachsenbande in the Steinburg district .

See also

Individual evidence

  1. ^ Hermann Wagner : Textbook of Geography. Second part physical geography. Hahnsche Buchhandlung, 1921 ( online ).
  2. Georg Schulz: Lexicon for determining the landforms in maps . Berliner Geographische Schriften 28th Institute for Geography at the Technical University of Berlin, Berlin 1989, ISBN 3-7983-1283-4 ( online ).
  3. Klaus-Peter Konerding, Andrea Lehr (ed.): Linguistic theory and lexicographical practice: Symposium lectures , Heidelberg 1996. Max Niemeyer Verlag, 1997, doi : 10.1075 / babel.44.4.17cla ( online ).
  4. Alexander Supan: Fundamentals of physical geography . Veit & Comp., 1884 ( online ).
  5. James M. Watts et al .: Thermal, mixing, and oxygen regimes of the Salton Sea, California, 1997-1999. In: Hydrobiologia 466, Springer, Dordrecht 2001, pp. 159-176 ( pdf ).
  6. ^ John Grotzinger, Thomas Jordan: Press / Siever General Geology. Springer, 2016, ISBN 978-3-662-48342-8 , doi : 10.1007 / 978-3-662-48342-8 ( online ).
  7. Rhodes W. Fairbridge: Crypto depression. In: Geomorphology. Encyclopedia of Earth Science. Springer, Berlin / Heidelberg 1968, pp. 231-233.