A Geographic depression Is an area or zone that is characterized by having a lower height than the regions around it and in some cases because they are below sea level. It may be completely covered by water or on the contrary, it is a dry region (sometimes even arid).
Geographical depressions have very varied sizes. Throughout the world they can be found on a small scale, such as subsidence of only a few meters in diameter, or large depressions that reach continental scales.
Likewise, the causes and origins of geographical depressions are very diverse. In some cases, the movement of tectonic plates Causes depression. In others, the climate, the permeability of the land, human action, among other factors, are the cause of the abrupt decline of the land and subsequent depression.
Classes of geographical depressions
In geomorphology (a branch of geography and geology that has as its object the study of the forms of the terrestrial surface), a geographic depression is an area where the ground has suffered, for various reasons, a sudden descent and results in an area Located at a lower height than the surrounding region.
There are two kinds of geographic depressions: On the one hand we find relative geographic depression. This type of depression occurs when the surrounding terrain is higher than the area of the depression but is above sea level.
Examples of relative depressions are the Great Basin, located to the west of the United States and the Tarim Basin, in western China. Both geographic areas are considered among the most extensive relative depressions of planet earth.
On the other hand, we find absolute depression, which occurs when an area or zone has a lower height than the rest of the surrounding terrain and in turn is below sea level.
An example of this type of geographical depression is the Caspian Sea, the largest lake in the world, located between Europe and Asia.
This lake, at a height of -28 meters above sea level, constitutes one of the largest absolute geographic depressions of the planet with around 371,000 square kilometers in extension.
Causes of geographical depressions
There are various causes and factors involved in generating a geographical depression, whether relative or absolute.
Most large-scale geological depressions are related to tectonic plates and their movement, while other depressions are generated by the destabilization of the terrain due to erosion, volcanic activity, human action or climate, among others.
We can classify the causes of the generation of geographical depressions in several groups, among which we find:
- Geographical depressions related to soil erosion.
- Geographic depressions related to the collapse of the terrain.
- Geographical depressions related to an impact on the ground.
- Geographic depressions related to sediments in the terrain.
- Geographical depressions related to tectonic movements.
When it comes to a slow movement of land destabilization (in relation to the amount of land that sinks and the years that this movement takes) we speak of geographical depressions generated by subsidence that is, according to geology, the progressive collapse of the land.
Among the depressions related to subsidence are those that are generated by wind erosion, typical in arid ecosystems (usually dunes and dry soils). Also found are those depressions generated by erosion in glacial terrain and river valleys.
The endorheic basins are also part of the depressions formed by subsidence. An endorheic basin is an area where the water does not have a fluvial outlet to the ocean. This generates the accumulation of salts that end up destabilizing the terrain and derive in the generation of geographic depressions.
Sedimentation and human action can also generate destabilization on the ground and lead to the creation of a geographical depression. It is frequent to find geographic depressions near the places of extraction of petroleum or near to zones of mining.
In some cases, the depression is generated by collapse when the land yields by the accumulation of sediments, by the variation in the level of groundwater or in the so-called karstic zones.
When the collapse of rocks located on a hollow occurs a geographical depression called Dolina or Torca. The dolinas are frequent in the karstic zones and in the vast majority of cases they are filled with water.
Geographical depressions can also be generated by the movement of tectonic plates that collide with each other at the converging edge, by the impact of a meteorite on the ground, resulting in a crater or volcanic activity after an eruption that destabilizes the terrain.
Countries with absolute geographical depressions
Around the planet there are many geographic depressions, but only 33 countries have absolute geographical depressions, that is, land areas located below sea level.
The Netherlands is perhaps one of the most representative territories of this group. About two-thirds of the territory covered by the Netherlands is about 4 meters below sea level.
Thanks to a complex drainage system, the construction of dams and the continuous renovation and monitoring of highly populated geographic depressions, the Netherlands has managed to remain safe from floods.
In the United States, the area known as the Death Valley is considered a great geographic depression, as it is located at about -86 meters above sea level. It is called Valle de la Muerte because of the high temperatures that occur in this territory.
The lowest point of the earth lies in the so-called Dead Sea depression, with -413 meters above sea level. This geographical area contains the Dead Sea, a part of the Jordan River, the Sea of Galilee and various communities.
Other countries with absolute geographical depressions are Japan, Libya, Denmark, Spain, Algeria, Tunisia, Morocco, Australia, among others.
Bibliographic references
- David K. Lynch. Thule Scientific (2017) Land below sea level. Retrieved from geology.com.
- Depression (Geology) (2017) Retrieved from revolvy.com.
- Vanessa McKinney. ICE Case Studies (May 2007) Sea Level Rise and the Future of the Netherlands. Retrieved from american.edu.
- Hobart King. Geology.com (2017) Convergent Plate Boundaries. Retrieved from geology.com.
- Subsidence. (2017) Recovered from revolvy.com.
- Herrera, G.; Tomás, R.; López-Sánchez, J.M.; Delgado, J.; Mallorquí, J.; Duque, S.; Mulas, J. Advanced DInSAR analysis on mining areas: The Union case study (Murcia, SE Spain). Engineering Geology, 90, 148-159, 2007.
- Australian Broadcasting Corporation (February 8, 2017) Sinkholes explained: How are they caused and what are the warning signs? Recovered from abc.net.au.