GEOGRAPHIC DEPRESSION: TYPES, CAUSES AND EXAMPLES - GEOGRAPHY - 2024

A geographical depression is an area or a zone that is characterized by having a lower height than the regions around it and in some cases by being below sea level. It can be completely covered by water or on the contrary, it can be 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 sinkholes only a few meters in diameter, or large depressions that reach continental scales.

Example of geographic depression

Likewise, the causes and origins of geographic depressions are very diverse. In some cases, the movement of the tectonic plates causes the depression. In others, the climate, the permeability of the terrain, human actions, among other factors, are the causes of the abrupt descent of the terrain and the subsequent depression.

Types of geographic depressions

In geomorphology (branch of geography and geology whose object is to study the shapes of the earth's surface), a geographical depression is an area where the terrain has suffered, for various reasons, a sharp decline and results in an area located at a lower elevation than the surrounding region.

There are two kinds of geographic depressions: On the one hand we find the 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.

Some examples of relative depressions are the Great Basin, located in the western United States, and the Tarim Basin, in western China. Both geographical areas are considered among the largest relative depressions on planet earth.

On the other hand, we find the absolute depression, which occurs when an area or a 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 geographic 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 geographical depressions on the planet with around 371,000 square kilometers in size.

Causes of geographic depressions

There are various causes and factors that intervene in the generation of a geographic depression, whether relative or absolute.

Most large-scale geographic depressions are related to tectonic plates and their movement, while other depressions are generated by the destabilization of the terrain thanks to erosion, volcanic activity, human action or climate, among others.

We can classify the causes of the generation of geographic depressions into several groups, among which we find:

• Geographic depressions related to soil erosion.
• Geographical depressions related to the collapse of the terrain.
• Geographic depressions related to an impact on the ground.
• Geographical depressions related to sediments on the ground.
• 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 are talking about geographical depressions generated by subsidence which is, according to geology, the progressive sinking of the land.

Among the depressions related to subsidence are those that are generated by wind erosion, typical in arid ecosystems (generally dunes and dry soils). There are also those depressions generated by erosion in glacial lands and in 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 river outlet to the ocean. This generates the accumulation of salts that end up destabilizing the terrain and lead to the generation of geographic depressions.

Sedimentation and human actions can also generate destabilization on the ground and lead to the creation of a geographical depression. It is common to find geographic depressions near oil extraction sites or near mining areas.

In some cases, the depression is generated by collapse when the land gives way due to the accumulation of sediments, due to the variation in the level of groundwater or in the so-called karstic zones.

When the collapse of rocks located on a hole occurs, a geographical depression called Dolina or Torca is generated. The sinkholes are frequent in the karst areas and in the vast majority of cases they fill 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 by volcanic activity after an eruption that destabilizes the ground.

Examples of countries with absolute geographic depressions

There are a large number of geographical depressions around the planet, but only 33 countries have absolute geographical depressions, that is, areas of land located below sea level.

The Netherlands is perhaps one of the most representative territories of this group. About two-thirds of the territory comprised by the Netherlands is around 4 meters below sea level.

Thanks to a complex drainage system, the construction of dikes and the continuous renovation and surveillance of highly populated geographical depressions, the Netherlands has managed to stay safe from flooding.

In the United States, the area known as the Valley of Death is considered a great absolute geographic depression, since it is located around -86 meters above sea level. It is called Valley of Death because of the high temperatures that occur in this territory.

The lowest point on earth is 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 that have absolute geographic depressions are Japan, Libya, Denmark, Spain, Algeria, Tunisia, Morocco, Australia, among others.

Bibliographic references

1. David K. Lynch. Thule Scientific (2017) Land below sea level. Recovered from geology.com.
2. Depressión (Geology) (2017) Recovered from revolvy.com.
3. Vanessa McKinney. ICE Case Studies (May 2007) Sea Level Rise and the Future of the Netherlands. Recovered from american.edu.
4. Hobart King. Geology.com (2017) Convergent Plate Boundaries. Recovered from geology.com.
5. Subsidence. (2017) Recovered from revolvy.com.
6. Herrera, G.; Tomás, R.; López-Sánchez, JM; Delgado, J.; Mallorquí, J.; Duque, S.; Mulas, J. Advanced DInSAR analysis on mining areas: La Union case study (Murcia, SE Spain). Engineering Geology, 90, 148-159, 2007.
7. Australian Broadcasting Corporation (February 8, 2017) Sinkholes explained: How are they caused and what are the warning signs? Recovered from abc.net.au.