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subsidence

subsidence, lowering of a portion of the earth's crust. The subsidence of land areas over time has resulted in submergence by shallow seas (see oceans). Land subsidence can occur naturally or through human activity. Natural subsidence may occur when limestone, which is easily carved by underground water, collapses, leaving sink holes on the surface, such as in Florida. Earthquakes can also cause subsidence of the land because of the movement of faults. Permafrost, or the permanently frozen ground in tundra regions, can subside during local warming trends, a phenomena called thermokarst. Oceanic crust produced at spreading ridges (see seafloor spreading) subsides after cooling, as do calderas, the craterlike features at a volcano's peak. An atoll is a coral reef that forms a ring with no apparent central peak and may form when volcanic islands subside—an explanation first proposed by Charles Darwin. Human activity has contributed greatly to subsidence over the last few centuries. For example, withdrawal of oil from the field at Long Beach, California, beginning in 1936 resulted in subsidence at rates ranging from 0.5 to 2.0 ft (0.15–0.61 m) per yr in the center of the field. By 1962 the center of the oil field had subsided slightly over 27 ft (8.5 m), caused by the removal of fluid from the pore spaces in the underground rock, allowing the grains to compact. Similarly, withdrawal of groundwater through well pumping has resulted in subsidence in such cities as Mexico City, Houston, Tex., and Venice, Italy. Subsidence is also caused by the collapse of underground salt, ore, and coal mines.

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subsidence

subsidence
1. A progressive depression of the Earth's crust, which allows sediment to accumulate and be preserved. Subsidence is caused by mantle convection and by sediment loading. The subsidence rate will control the proportion of deposited sediment which will be preserved in the subsiding area. Subsidence rates in sedimentary basins typically vary from 0.3 to 2.5 mm per year.

2. Sinking or settling of the ground surface due to natural or anthropogenic causes. Surface material with no free side is displaced vertically downwards with little or no horizontal movement.

3. Local sinking, due to underground mine workings.

4. Downward movement of air, characteristically gentle (1–10 cm/s) and often in large anticyclones, which is related to the divergence in lower layers near the ground surface. Subsidence results from radiation cooling or from convergence of air horizontally in the upper troposphere. It typically brings settled weather, with evaporation of cloud drops by adiabatic warming in the subsiding air mass, causing cloudless skies above the friction layer near the surface, so that in winter fog and low cloud may prevail when moisture is sufficient.

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subsidence

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Subsidence

Subsidence

Atmospheric subsidence

Geologic subsidence

Human causes of subsidence

Effects of subsidence

Resources

The term subsidence is used in both atmospheric and geological sciences. Atmospheric subsidence refers to the sinking of air that is denser (heavier) than the air below. As it subsides, increasing air pressure compresses the air parcel, causing it to warm. Geologic subsidence is a form of mass wasting that refers to the sinking of geologic materials (rocks or sediments) as underlying materials are removed or change their position.

Atmospheric subsidence

Atmospheric subsidence occurs when the normal upward flow of air in the atmosphere, known as atmospheric convection, is disturbed. To understand what happens during convection, imagine a parcel of air located immediately above the ground during sunrise. As solar energy warms the earth, heat is transferred to that parcel of air. Warm air is less dense than cold air, so the heated parcel has a tendency to rise upward, or convect, into the atmosphere. As the parcel rises, it expands, causing cooling. Cooling causes water vapor (gas) in the air to change state to a liquid; water droplets form, producing clouds. An air parcel will continue to rise upward until its density is equal to the surrounding atmosphere, that is, until it is stable.

Convection, which creates a large area of low pressure and converging winds at Earths surface, known as a cyclone, is not always present. Subsidence, or sinking of air, may happen instead, forming an area of high pressure, an anticyclone. Large scale subsidence occurs when air several thousands of feet overhead is denser than the surrounding air. This denser air is produced when winds aloft converge or air aloft is very cold, or warm, but unusually dry. The dense air sinks due to the pull of gravity, compressing the air, creating high pressure at the surface and diverging winds just above the surface. Warming of the air as it subsides increases evaporation, causing clear skies. That is why high pressure systems are usually associated with fair weather.

The subsiding air may settle onto a cooler air layer, creating what is known as a temperature inversion, or subsidence inversion. In a temperature inversion, a warm air layer, several hundred or thousand feet above the surface, is trapped between cooler layers above and below. This inversion resists convection of surface air, since the surface air is stable, and causes air pollution to be trapped at the surface. Subsidence inversions commonly occur at high latitudes during the winter and over the eastern United States during the late summer months. During an inversion, an urban areas air pollution may become a serious health hazard.

Geologic subsidence

Geological subsidence involves the settling or sinking of a body of rock or sediment. Subsidence is a type of mass wasting, or mass movement-transport of large volumes of earth material primarily by gravity. Subsidence may occur as the result of either natural or human-caused events.

Earthquakes are commonly associated with subsidence. When two blocks of Earths crust slide against each other, causing an earthquake, ground movement may occur, raising or lowering the ground surface. During Alaskas 1964 Good Friday earthquake, an area of at least 70,000 sq mi (180,000 sq km), much of it coastline, subsided 3 ft (1 m) or moresome areas now flood at high tide.

Another way that earthquakes can cause subsidence is by rapidly decreasing the load-bearing capacity, or strength, of loose earth materials, or sediments, due to liquefaction. Liquefaction occurs when vibrations from an earthquake, or other disturbance, cause water-saturated sediments to temporarily lose their grain-to-grain contact, which is what gives them their load-bearing capacity. For just an instant, the weight of the overlying materials is supported only by the water between the grains. An instant later, when the grains begin to settle, the weight of the overlying sediment (or buildings) causes the grains to be forced closer together and the land to subside.

During the 1989 earthquake along the San Andreas Fault in California, some of the most serious damage occurred in San Franciscos Marina District. Buildings were constructed on old bay deposits, unconsolidated water-saturated sediments, which when shaken by the earthquake temporarily lost their strength due to liquefaction. In the Marina district, sediment that had previously supported large buildings quickly turned into a mud-like material that could no longer support a buildings weight.

Another example of natural subsidence can be found in regions where caves are common. Caves form when underground water dissolves limestone and carries it away. The resulting void spaces grow larger and larger over time until they become the features we call caves.

If limestone dissolves for a long enough time, the hole (cave) that forms becomes too large to support the weight of its walls. In such a case, the ceiling of the cave will subside, either slowly or in a catastrophic collapse, forming a large depression at the surface, known as a sinkhole. If conditions are right, the sinkhole may eventually fill with water, forming a lake. A landscape containing many dry or water-filled sinkholes formed by limestone dissolution is called karst topography.

Yet another cause of subsidence is volcanic eruption. Whether molten rock, or magma, is lost suddenly and dramatically, as in the Mt. St. Helens eruption of May 18, 1980, or the slow flow of lava (magma flowing on the surface), as happens in the Hawaiian Islands, land subsidence is likely to follow. The material ejected from the earths interior leaves an empty space that must be filled in one way or another. In many cases, a large section of the overlying crust along with the geologic and human features attached to itcollapses into Earths interior, forming what is called a caldera.

Subsidence may also result from the accumulation of large volumes of sediment at Earths surface in what is known as a sediment basin. An obvious setting in which this occurs is at river deltas. Each day, the Mississippi River deposits up to 1.8 million metric tons of sediment at its mouth near New Orleans. The weight of this sediment contributes to a gradual subsidence of the land on which New Orleans resides. Basins between mountains also can subside due to the weight of accumulating sediments.

Human causes of subsidence

Many forms of human activities can result in subsidence. One of the most widespread of these problems involves the removal of groundwater for agricultural, municipal, and other purposes. In large parts of the United States, for example, farmers and ranchers depend heavily on water removed from underground aquifers to irrigate their crops and water their livestock. Such activities have now been going on with increasing intensity for at least a century.

This practice will not lead to subsidence as long as enough rainfall filters downward to replace, or recharge, the groundwater removed by humans. However, when the rate of removal exceeds the rate of recharge, significant decreases in the volume of the aquifer begin to occur. The pore spaces between the grains of the aquifer, previously occupied by water, are emptied. The grains then begin to compact more tightly, and they collapse. Eventually, the aquifer begins to subside.

A dramatic example of subsidence as a result of groundwater removal has taken place in a region southeast of Phoenix, Arizona. There a section of land covering 120 sq mi (310 sq km) has sunk more than 7 ft (2 m). This phenomenon has occurred at many locations above the Ogallala Aquifer, which lies beneath the High Plains region, stretching from Kansas and Nebraska to Wyoming and from Texas and New Mexico to Colorado.

The removal of fossil fuels is also a major human cause of subsidence. A traditional method of removing coal, for example, is known as room-and-pillar because vertical columns of the coal (the pillar) are left in position while the coal around it is removed. When such mines are abandoned, however, the pillars of coal left behind are often not strong enough to hold up the overlying ground. When the pillars break, the ceilings of the mined room collapse and the overlying ground does so also. With more than 90,000 abandoned mines in the United States, this source of subsidence is likely to be a problem into the foreseeable future.

The pumping of oil and natural gas from underground sources can have similar effects. Similar to removal of water from an aquifer, when these materials are removed from the reservoir, the reservoirs grains compact and the reservoir occupies a smaller volume than it did before the oil or gas was removed. As a result, overlying ground subsides as the reservoir slowly collapses.

One of the most famous of these instances occurred as far back as the late 1920s in Southern California. Oil removed from the Wilmington and Signal Hill oil fields caused unstable ground to subside by as much as 29 ft (9 m). Since this region lies along the coastline of the Pacific Ocean, drastic efforts were required to prevent ocean water from flowing into lands that were now lower in some places than sea level. By 1968, subsidence in the area had been stopped, but only after huge quantities of seawater had been injected into the empty oil wells in order to prop open the pores in the reservoir. This success in restoring stability to the area came at a high price, however, as docks, highways, sewer systems, and other municipal structures had, by that time, been completely rebuilt.

Effects of subsidence

Whether caused by natural or human activities, subsidence often has a number of serious consequences for human societies. Probably the most dramatic, of course, is the disappearance of whole sections of land, as occurred in Alaskas Good Friday earthquake. Today, the sudden appearance of sinkholes in Florida is no longer unusual news. In many cases, these sinkholes appear because the removal of ground-water has left limestone caves that are unable to support the land overlying them.

Even relatively modest subsidence can also damage a variety of human structures. Buildings are weakened and collapse, railway lines and roads are twisted and broken, and underground sewer, power, and water lines are torn apart. Due to its ability to destroy property on a large scale, subsidence is a very expensive type of mass wasting that also poses some risk to human lives.

See also Atmospheric circulation; Volcano.

KEY TERMS

Aquifer A formation of soil or rock that holds water underground.

Karst topography A region in which numerous caves, sinkholes, and other formations resulting from the dissolving of underground limestone rock are apparent.

Liquefaction (of rocks) The process by which changes in pressure cause a rocky material that was originally strong and stable to change into a liquid-like material.

Temperature inversion A situation in which a layer of cool air is trapped beneath a layer of warmer, less dense air.

Resources

BOOKS

Erickson, Jon, and Alexander E. Gates. Quakes, Eruptions, and Other Geologic Cataclysms. 2nd ed. New York: Facts on File, 2001.

David E. Newton

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Subsidence

Subsidence

To subside is to sink or fall. Subsidence is commonly associated with the lowering of the earth's surface due to actions that have occurred below the surface. Sometimes this is a natural phenomenon, such as the solubilizing and removal of minerals by water. When the underground support system is removed in the process, the surface of the land sinks to a new level. This often leads to a special topographic form known as Karst topography . Human activities that lead to the extraction of ores, minerals, and fossil fuels often lead to a weakened mineral structural support and subsidence of the surface of the earth. In the arid portions of the earth, extraction of water from sub-surface aquifers has led to subsidence of the earth's surface as well.

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Subsidence

Subsidence

The term subsidence is used in both atmospheric and geological sciences. Atmospheric subsidence refers to the sinking of air that is denser (heavier) than the air below. As it subsides, increasing air pressure compresses the air parcel, causing it to warm. Geologic subsidence is a form of mass wasting that refers to the sinking of geologic materials (rocks or sediments) as underlying materials are removed or change their position.


Atmospheric subsidence

Atmospheric subsidence occurs when the normal upward flow of air in the atmosphere, known as atmospheric convection , is disturbed. To understand what happens during convection, imagine a parcel of air located immediately above the ground during sunrise. As solar energy warms the earth , heat is transferred to that parcel of air. Warm air is less dense than cold air, so the heated parcel has a tendency to rise upward, or convect, into the atmosphere. As the parcel rises, it expands, causing cooling. Cooling causes water vapor (gas) in the air to change state to a liquid; water droplets form, producing clouds . An air parcel will continue to rise upward until its density is equal to the surrounding atmosphere, that is, until it is stable.

Convection, which creates a large area of low pressure and converging winds at the earth's surface, known as a cyclone, is not always present. Subsidence, or sinking of air, may happen instead, forming an area of high pressure, an anticyclone. Large scale subsidence occurs when air several thousands of feet overhead is denser than the surrounding air. This denser air is produced when winds aloft converge or air aloft is very cold, or warm, but unusually dry. The dense air sinks due to the pull of gravity, compressing the air, creating high pressure at the surface and diverging winds just above the surface. Warming of the air as it subsides increases evaporation , causing clear skies. That is why high pressure systems are usually associated with fair weather .

The subsiding air may settle onto a cooler air layer, creating what is known as a temperature inversion, or subsidence inversion. In a temperature inversion, a warm air layer, several hundred or thousand feet above the surface, is trapped between cooler layers above and below. This inversion resists convection of surface air, since the surface air is stable, and causes air pollution to be trapped at the surface. Subsidence inversions commonly occur at high latitudes during the winter and over the eastern United States during the late summer months. During an inversion, an urban area's air pollution may become a serious health hazard.


Geologic subsidence

Geological subsidence involves the settling or sinking of a body of rock or sediment. Subsidence is a type of mass wasting, or mass movement-transport of large volumes of earth material primarily by gravity. Subsidence may occur as the result of either natural or human-caused events.

Earthquakes are commonly associated with subsidence. When two blocks of the earth's crust slide against each other, causing an earthquake , ground movement may occur, raising or lowering the ground surface. During Alaska's 1964 Good Friday earthquake, an area of at least 70,000 sq mi (180,000 sq km), much of it coastline, subsided 3 ft (1 m) or more—some areas now flood at high tide.

Another way that earthquakes can cause subsidence is by rapidly decreasing the load-bearing capacity, or strength, of loose earth materials, or sediments, due to liquefaction. Liquefaction occurs when vibrations from an earthquake, or other disturbance, cause water-saturated sediments to temporarily lose their grain-to-grain contact, which is what gives them their load-bearing capacity. For just an instant, the weight of the overlying materials is supported only by the water between the grains. An instant later, when the grains begin to settle, the weight of the overlying sediment (or buildings) causes the grains to be forced closer together and the land to subside.

During the 1989 earthquake along the San Andreas Fault in California, some of the most serious damage occurred in San Francisco's Marina District. Buildings were constructed on old bay deposits, unconsolidated water-saturated sediments, which when shaken by the earthquake temporarily lost their strength due to liquefaction. In the Marina district, sediment that had previously supported large buildings quickly turned into a mud-like material that could no longer support a building's weight.

Another example of natural subsidence can be found in regions where caves are common. Caves form when underground water dissolves limestone and carries it away. The resulting void spaces grow larger and larger over time until they become the features we call caves.

If limestone dissolves for a long enough time, the hole (cave ) that forms becomes too large to support the weight of its walls. In such a case, the ceiling of the cave will subside, either slowly or in a catastrophic collapse, forming a large depression at the surface, known as a sinkhole. If conditions are right, the sinkhole may eventually fill with water, forming a lake . A landscape containing many dry or water-filled sinkholes formed by limestone dissolution is called karst topography .

Yet another cause of subsidence is volcanic eruption. Whether molten rock, or magma , is lost suddenly and dramatically, as in the Mt. St. Helens eruption of May 18, 1980, or the slow flow of lava (magma flowing on the surface), as happens in the Hawaiian Islands, land subsidence is likely to follow. The material ejected from the earth's interior leaves an empty space that must be filled in one way or another. In many cases, a large section of the overlying crust—along with the geologic and human features attached to it—collapses into the earth's interior, forming what is called a caldera.

Subsidence may also result from the accumulation of large volumes of sediment at the earth's surface in what is known as a sediment basin . An obvious setting in which this occurs is at river deltas. Each day, the Mississippi River deposits up to 1.8 million metric tons of sediment at its mouth near New Orleans. The weight of this sediment contributes to a gradual subsidence of the land on which New Orleans resides. Basins between mountains also can subside due to the weight of accumulating sediments.


Human causes of subsidence

Many forms of human activities can result in subsidence. One of the most widespread of these problems involves the removal of groundwater for agricultural, municipal, and other purposes. In large parts of the United States, for example, farmers and ranchers depend heavily on water removed from underground aquifers to irrigate their crops and water their livestock . Such activities have now been going on with increasing intensity for at least a century.

This practice will not lead to subsidence as long as enough rainfall filters downward to replace, or recharge, the groundwater removed by humans. However, when the rate of removal exceeds the rate of recharge, significant decreases in the volume of the aquifer begin to occur. The pore spaces between the grains of the aquifer, previously occupied by water, are emptied. The grains then begin to compact more tightly, and they collapse. Eventually, the aquifer begins to subside.

A dramatic example of subsidence as a result of groundwater removal has taken place in a region southeast of Phoenix, Arizona. There a section of land covering 120 sq mi (310 sq km) has sunk more than 7 ft (2 m). This phenomenon has occurred at many locations above the Ogallala Aquifer, which lies beneath the High Plains region, stretching from Kansas and Nebraska to Wyoming and from Texas and New Mexico to Colorado.

The removal of fossil fuels is also a major human cause of subsidence. A traditional method of removing coal , for example, is known as room-and-pillar because vertical columns of the coal (the "pillar") are left in position while the coal around it is removed. When such mines are abandoned, however, the pillars of coal left behind are often not strong enough to hold up the overlying ground. When the pillars break, the ceilings of the mined room collapse and the overlying ground does so also. With more than 90,000 abandoned mines in the United States, this source of subsidence is likely to be a problem into the foreseeable future.

The pumping of oil and natural gas from underground sources can have similar effects. Similar to removal of water from an aquifer, when these materials are removed from the reservoir, the reservoir's grains compact and the reservoir occupies a smaller volume than it did before the oil or gas was removed. As a result, overlying ground subsides as the reservoir slowly collapses.

One of the most famous of these instances occurred as far back as the late 1920s in Southern California. Oil removed from the Wilmington and Signal Hill oil fields caused unstable ground to subside by as much as 29 ft (9 m). Since this region lies along the coastline of the Pacific Ocean, drastic efforts were required to prevent ocean water from flowing into lands that were now lower in some places than sea level . By 1968, subsidence in the area had been stopped, but only after huge quantities of seawater had been injected into the empty oil wells in order to prop open the pores in the reservoir. This success in restoring stability to the area came at a high price, however, as docks, highways, sewer systems, and other municipal structures had, by that time, been completely rebuilt.


Effects of subsidence

Whether caused by natural or human activities, subsidence often has a number of serious consequences for human societies. Probably the most dramatic, of course, is the disappearance of whole sections of land, as occurred in Alaska's Good Friday earthquake. Today, the sudden appearance of sinkholes in Florida is no longer unusual news. In many cases, these sinkholes appear because the removal of groundwater has left limestone caves that are unable to support the land overlying them.

Even relatively modest subsidence can also damage a variety of human structures. Buildings are weakened and collapse, railway lines and roads are twisted and broken, and underground sewer, power, and water lines are torn apart. Due to its ability to destroy property on a large scale, subsidence is a very expensive type of mass wasting that also poses some risk to human lives.

See also Atmospheric circulation; Volcano.

Resources

books

Erickson, Jon. Quakes, Eruptions, and Other Geologic Cataclysms. New York: Facts on File, 1994.


David E. Newton

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Aquifer

—A formation of soil or rock that holds water underground.

Karst topography

—A region in which numerous caves, sinkholes, and other formations resulting from the dissolving of underground limestone rock are apparent.

Liquefaction (of rocks)

—The process by which changes in pressure cause a rocky material that was originally strong and stable to change into a liquid-like material.

Temperature inversion

—A situation in which a layer of cool air is trapped beneath a layer of warmer, less dense air.

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