A basin in the sense of geomorphology is a self-contained and often drain-free depression in the earth's crust lying above or below sea level ; it lies either on the surface of the earth ( land area ) or on the sea floor .
The extent of the subsidence can be very different. The large ocean basins measure 2,000 to 4,000 kilometers in diameter (6 of them in the Pacific ), while the 20 largest continental basins are 300 to 1,000 km in diameter. In terms of area, they are surpassed by several flat continent shields, but they have no hollow shape. In the case of smaller, strictly closed terrain forms , one speaks of depressions , 'basins' are also called basins , i.e. closed valleys that have a drain. Many large landscapes are referred to as "basins", although they are not necessarily actually subsidence or weak zones of the continental crust , in this case it is either large-scale areas without drainage or spaces separated by mountain ridges that are too large to be called basins to call.
Sedimentation takes place in almost all basin locations , which can be caused by water (bodies of water, ocean currents), wind, ice, landslides or chemical processes. The deposits on land are mainly sand , gravel , clay and loess , in the sea and lakes also mud , organic matter and lime .
Landscapes with very thick deposits are called sedimentary basins : They usually have a drain, but sediments are deposited in the same place over a long period of time. As a result, the lowest layers condense into rock, while the surface slowly becomes a plain (e.g. Pannonian Plain or Rhine Rift). Many of these basins are weak zones in the earth's crust, where geological fault lines or large-scale tectonics lead to permanent subsidence .
Basin on land
Geomorphological basins on land differ from so-called lowlands in that they are above-ground depressions without drainage. However, an outflow can be secondary (possibly repeated). This can sometimes make the demarcation of basin landscapes and lowlands difficult in parlance. The altitude of a basin can be around (sea level) in the vicinity of the river or the coast up to a pronounced high altitude in the interior of the continent. Individual bowl-like areas are even below sea level and are called depression . The deepest point of such depressions without a drain is often occupied by a salt lake .
In continental arid regions , the basins are mostly deserts , and drainage-free tub landscapes can become extensive salt pans . Basically, the formation of salt pans and (residual) lakes compared to the formation of runoff (e.g. through the formation of a breakthrough valley at the edge of the basin) depends on the ratio of water drainage (through evaporation and seepage) to water access. Rivers can end in basins or flow through them, surrounding mountains can locally increase the amount of precipitation or (often) shield the basin from precipitation. Sedimentation in the basin can promote the formation of an inland delta or swampy landscapes with anastomising rivers . The temperate zone has numerous loess basins in which the winds of the Ice Age deposited fine clay dust. Examples are the Paris Basin , the Hungarian Puszta , the Ukraine and large areas in northeast China .
Some elongated pools run along large mountain ranges , while inside there are often smaller pools, also in the form of broad valley floors or raised bogs . At the edge of mountains, ice age glaciers often carved out large tongue basins from which, for example, the lake landscapes in Bavaria or in the Salzkammergut have formed.
Basin locations can promote the formation of inversion weather patterns.
Examples of basins on land:
- Aral Sea - Asia; Uzbekistan, Kazakhstan
- Great Basin - North America; United States
- Okawango Basin - Africa; Botswana
- Tarim Basin with the Lop Nor - Asia basin ; China
Basin in the sea
Sedimentation plays a very different role in the sea. On average, the sea floor is covered with 1500 m to 2000 m thick sediments , which, however, are strongly consolidated. Above it is a layer of mud that varies in thickness and is on average 300 m thick . The geological underground is the oceanic crust , which is about 5000 m thick.
At the continental margins (see shelf ) the sedimentation is very different and the fineness depends on the predominant rocks in the catchment area of the big rivers . On the slopes of the mid-ocean thresholds , volcanic and basaltic sediments are added to the otherwise predominant heel rocks (sand, clay, limestone) .
Examples of pools in the sea:
The individual oceans have very many small and numerous large such basins; a list can be found here .
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