02-12-2018, 05:11 PM
What Causes Most Below-Sea-Level Depressions?
Most major depressions are associated with tectonic plate boundaries. They form when converging plates deform or when spreading centers open. A few are volcanic in origin. Most of these depressions are in the northern hemisphere for a good reason: this is where most of the land is and where most of the plate boundaries occur. Many are found in the vicinity of where the African, Arabian and Eurasian plates meet. Most are found in hot desert regions of the planet where high evaporation rates prevent them from filling with water. A few are found in temperature climes.
Many well-populated places in the world are below sea level. About a third of the Netherlands including Schiphol Airport is below sea level. So too are the Jordan River and parts of many coastal cities including New Orleans and Bangkok. Other regions below sea level are the Enriquillo-Cul de Sac Depression (Hispaniola), Chott Melrhir Depression (Algeria), Sebkhet Shunayn and Sabkhat Ghuzayyil (Libya), Shatt al Gharsah (Tunisia), Lammefjord (Denmark), Sebkha Tah (Morocco), Hachiro-gata (Japan), Sariqarnish Kuli (Uzbekistan), The Fens (UK) and Lake Eyre, Australia. Of the two hundred and fifty five recognized countries or protectorates in the world, 33 have land below sea level. Most of these are only a few meters down.
Some below-sea-level depressions are dry, most others have salt lakes. The lakes are undrained, which means water flows in but does not flow out. Since water cannot flow up hill, the lakes cannot empty into the sea like most do and so they accumulate salt as their waters evaporate. Where does the salt originate? Some of it comes from ancient sea floor salt deposits that have been uplifted then weather and release the salt which reaches the depressions via rivers. Many other rocks weather to yield sodium and chlorine ions that are carried to the lake by rivers. The Sea of Galilee is below sea Level but is a freshwater lake because it drains into the Dead Sea through the Jordan River.
Owing to variations in rainfall, evaporation rates, and human water use, most of the lake levels fluctuate from year to year. Some have come and gone in historic times. The present Salton Sea was produced by a canal construction accident in 1905, though the lake has come and gone naturally many times in the last thousand years.
The lowest exposed land on Earth is at the Dead Sea shore, at -413 meters. The deepest lake bottom in a below-sea-level depression also belongs to the Dead Sea at -750 meters. Lake Baikal in Russia is at an elevation of 447 meters, but with a maximum depth of 1637 meters, its bottom lies at -1190 meters.
The Bentley Subglacial Trench in Antarctica reaches 2555 meters below sea level. It is the lowest point on Earth that is not covered by ocean, although it is covered by ice. The trench is huge, roughly the size of Mexico. The deepest point on the ocean floors is -10,916 meters in the Mariana Trench. Trenches are the upper most parts of subduction zones and there are dozens of trench locations that are more than 5000 meters below sea level.
Strictly speaking, most of the surface of the Earth is below sea level because oceans cover 71 % of the planet. But sea level varies and during the Great Ice Age 18,000 years ago, sea level was 130 meters lower. Today, large areas are under water that during the Great Ice Age were well above sea level. It is also likely that long ago and far into the future, plate tectonics will produce higher mountains and deeper trenches and depressions than we have today.
Most major depressions are associated with tectonic plate boundaries. They form when converging plates deform or when spreading centers open. A few are volcanic in origin. Most of these depressions are in the northern hemisphere for a good reason: this is where most of the land is and where most of the plate boundaries occur. Many are found in the vicinity of where the African, Arabian and Eurasian plates meet. Most are found in hot desert regions of the planet where high evaporation rates prevent them from filling with water. A few are found in temperature climes.
Many well-populated places in the world are below sea level. About a third of the Netherlands including Schiphol Airport is below sea level. So too are the Jordan River and parts of many coastal cities including New Orleans and Bangkok. Other regions below sea level are the Enriquillo-Cul de Sac Depression (Hispaniola), Chott Melrhir Depression (Algeria), Sebkhet Shunayn and Sabkhat Ghuzayyil (Libya), Shatt al Gharsah (Tunisia), Lammefjord (Denmark), Sebkha Tah (Morocco), Hachiro-gata (Japan), Sariqarnish Kuli (Uzbekistan), The Fens (UK) and Lake Eyre, Australia. Of the two hundred and fifty five recognized countries or protectorates in the world, 33 have land below sea level. Most of these are only a few meters down.
Some below-sea-level depressions are dry, most others have salt lakes. The lakes are undrained, which means water flows in but does not flow out. Since water cannot flow up hill, the lakes cannot empty into the sea like most do and so they accumulate salt as their waters evaporate. Where does the salt originate? Some of it comes from ancient sea floor salt deposits that have been uplifted then weather and release the salt which reaches the depressions via rivers. Many other rocks weather to yield sodium and chlorine ions that are carried to the lake by rivers. The Sea of Galilee is below sea Level but is a freshwater lake because it drains into the Dead Sea through the Jordan River.
Owing to variations in rainfall, evaporation rates, and human water use, most of the lake levels fluctuate from year to year. Some have come and gone in historic times. The present Salton Sea was produced by a canal construction accident in 1905, though the lake has come and gone naturally many times in the last thousand years.
The lowest exposed land on Earth is at the Dead Sea shore, at -413 meters. The deepest lake bottom in a below-sea-level depression also belongs to the Dead Sea at -750 meters. Lake Baikal in Russia is at an elevation of 447 meters, but with a maximum depth of 1637 meters, its bottom lies at -1190 meters.
The Bentley Subglacial Trench in Antarctica reaches 2555 meters below sea level. It is the lowest point on Earth that is not covered by ocean, although it is covered by ice. The trench is huge, roughly the size of Mexico. The deepest point on the ocean floors is -10,916 meters in the Mariana Trench. Trenches are the upper most parts of subduction zones and there are dozens of trench locations that are more than 5000 meters below sea level.
Strictly speaking, most of the surface of the Earth is below sea level because oceans cover 71 % of the planet. But sea level varies and during the Great Ice Age 18,000 years ago, sea level was 130 meters lower. Today, large areas are under water that during the Great Ice Age were well above sea level. It is also likely that long ago and far into the future, plate tectonics will produce higher mountains and deeper trenches and depressions than we have today.
