How is a tectonic lake created

lake

The morphometric parameters of a lake can change rapidly due to tectonic and volcanic processes in connection with sedimentation and biological processes. Delta embankments, shifting the coast and silting up are the visible consequences. A working out of sea groups according to morphometric parameters must therefore be carried out very carefully and always in connection with the relative parameters. The grouping of the lakes according to their formation is to be carried out more clearly (Tab. 1.), whereby, however, multiple, but not uniformly determined morphometric parameters can be shown. Group formation is made more difficult if the formation of a lake can be traced back to combined processes, e.g. in the case of caldera lakes to volcanotectonic processes, in the case of poly lakes to the dissolving activity of the water and sedimentation. Endogenous, that is, lakes created by tectonics and volcanism, can be found all over the world. These include the mostly deep, elongated and geologically old grave lakes and the often circular but small crater lakes (maars). Exogenous lakes are created by wind, water and ice. They are therefore tied to certain climatic zones. These lakes make up the largest part in terms of numbers, with the lakes formed by ice movement and ice melting outweighing the majority. Worldwide is only 1.8% of the mainland, that is about 2.5 million km2, covered with lakes. In the European area of ​​the Pleistocene inland glaciation, the lakes take up 4%, in Sweden 8.6% and in Finland even 12% of the area. Lakes can be dammed up by ice and moraines from recent glaciers. When such lakes erupt, dangerous floods, combined with the transport of mud and debris (flash floods), can arise.

In the hollow forms abandoned by the ice and carved out in mountain regions or on the edges of mountains with transition to the sea, lakes form, often in steps one behind the other, or fjord lakes. Many lakes are located in the area of ​​the Pleistocene or later, large areas of glaciated areas. They are dammed up by terminal moraines or meltwater deposits (sander). After the ice has melted, lakes form in the area of ​​the ground moraine, including the many small so-called dead ice holes. Spatially located in the same area are the subglacial meltwater channels, some of which have a considerable length. Fjords have the same origins.

Hollow forms, deflation troughs in arid regions, which can form smaller lakes, go back to wind erosion. In many desert regions, however, these depressions are now dry or, in the case of rare precipitation events, only form short-term lakes. In mountain regions, rivers can form mostly smaller lakes due to erosion, especially in the case of strong regressive erosion, e.g. in whirlpool depressions. When constricting meander sections, lakes arise in the oxbow lakes. Due to sedimentation in the vicinity of the rivers, circulating lakes and dam bank lakes are formed, which are often found in the lowland areas of the lower latitudes. In the coastal area, bays are often separated by offsetting the coast. Relict lakes, also known as regression lakes, are created in the coastal area of ​​the oceans when the sea level drops or as a result of land uplift, whereby parts of the sea are isolated.

At the mouth of large rivers, sedimentation, sometimes combined with tidal movements, creates delta lakes. The dissolving power of water is the cause of underground cavities in the limestone, which after collapse form depressions, sinkholes and uvalas on the surface, which, if they are filled with water, are accordingly referred to as sinkholes or uvala lakes. The Polje lakes, which also occur in the karstified limestones, receive so much material from sediment-carrying flowing waters from parts of the non-karstified catchment area that the cavities are clogged. Polje lakes can occur episodically and periodically as a result of snowmelt and / or heavy precipitation events, depending on the spring and ponor activity in connection with the height of the groundwater level. All lakes created by dissolving the water are summarized under the term karst lakes. What they all have in common is that they have no runoff through surface water, rather the drainage takes place through ponors, also known as swallow holes. Since karst lakes often appear periodically, many of them are intermittent lakes.

The temperature and heat balance of a body of water control many physical, chemical and biological processes. According to Van't Hoff's rule, the reaction rate of many chemical and biochemical processes increases by a factor of two to three when the temperature rises by 10ºC, while the water's ability to dissolve gases decreases considerably. Figure 1 illustrates the importance of temperature and the energy exchange processes for physical-chemical-biological processes. The heat balance of a lake is determined by the energy flow on the lake surface, which is manifested in the radiation and heat exchange, the heat turnover in the lake, which is manifested by the energy flows between individual water body areas of the lake, and the advective energy flows that are carried out by the water transport . Massless and mass, advective energy flows must be calculated and treated separately. Lakes in mid-latitudes and in the subtropics can store large amounts of heat because radiation and air temperature have large amplitudes here. Tropical and sub-polar lakes store much less heat. Although the heat content of tropical lakes is very high, it is not released into the atmosphere due to the slight annual fluctuation in air temperature. The temperature conditions in a lake are due to the density anomaly of the water, which has its maximum density of 1.00000 g / cm3 reached at 3.98 ° C, very complex. It must be noted that the difference in density in 1 ° C temperature steps is much smaller with cool water than with warmer water. The density of the water also depends on its salinity. Salt-free water has a volume weight of 1.00000 at 4 ° C. If the water has a salt content of 4 g / l, which corresponds to 4 ‰, its volume weight increases to 1.00818.

The interplay of density, salinity and water temperature means that stratification can occur in a lake or that this stratification is absent at a uniform temperature of 4ºC of the body of the lake, which is referred to as homothermia. In the climatic areas with markedly thermal seasons, a lake adjusts itself to a double change between homothermia and stratification. Homothermia occurs in spring and autumn, and stratification in summer and winter. With homothermia, the entire body of water is in an unstable position. Wind activity then leads to a circulation of the lake water. This can extend to the bottom of the lake, which causes full circulation, or it can only cover parts of the water body, which is referred to as partial circulation. In the temperate climatic zones, there is pronounced summer stagnation in summer and, in a weak form, stratification with two temperature zones also in winter (Fig. 2). In summer there is usually homogeneous, warm water in the surface layer. This layer is called the epilimnion. Below is a relatively thin layer with a steep temperature gradient, which is known as a thermocline or metalimnion. There is no heat transport into the depths through the metalimnion. In the layer below the metal ion, the water with the greatest densities is at around 4 ° C. This layer is known as the hypolimnion. Lakes with two alternations of circulation and stagnation per year are referred to as dimictic lakes or holomictic lakes if at least one circulation covers the entire water mass. Meromictic lakes, on the other hand, are not mixed down to the bottom of the lake. Meromixis can be topographical (sheltered from the wind), morphological (small water surface in relation to the depth) or chemical (salty hypolimnion with formation of a chemical thermocline). The unmixed deep water of the meromictic lakes is called the monimolimnion.