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Sinkholes

Sinkholes

Sinkholes are cavities that form when water erodes easily dissolved, or soluble, rock located beneath the ground surface. Water moves along joints, or fractures, enlarging them to form a channel that drains sediment and water into the subsurface. As the rock erodes, materials above subside into the openings. At the surface, sinkholes often appear as bowl-shaped depressions. If the drain becomes clogged with rock and soil , the sinkhole may fill with water. Many ponds and small lakes form via sinkholes.

Abundant sinkholes as well as caves, disappearing streams, and springs , characterize a type of landscape known as karst topography . Karst topography forms where ground-water erodes subsurface carbonate rock, such as limestone and dolomite , or evaporite rock, such as gypsum and halite (salt). Carbon dioxide (CO2), when combined with the water in air and soil, acidifies the water. The slight acidity intensifies the corrosive ability of the water percolating into the soil and moving through fractured rock.

Geologists classify sinkholes mainly by their means of development. Collapse sinkholes are often funnel shaped. They form when soil or rock material collapses into a cave . Collapse may be sudden and damage is often significant; cars and homes may be swallowed by sinkholes.

Solution sinkholes form in rock with multiple vertical joints. Water passing along these joints expands them, allowing cover material to move into the openings. Solution sinkholes usually form slowly and minor damage occurs, such as cracking of building foundations.

Alluvial sinkholes are previously exposed sinkholes that, over time, partly or completely filled with Earth material. They can be hard to recognize and some are relatively stable.

Rejuvenated sinkholes are alluvial sinkholes in which the cover material once again begins to subside, producing a growing depression.

Uvalas are large sinkholes formed by the joining of several smaller sinkholes. Cockpits are extremely large sinkholes

formed in thick limestone; some are more than a kilometer in diameter.

Sinkholes occur naturally, but are also induced by human activities. Pumping water from a well can trigger sinkhole collapse by lowering the water table and removing support for a cave's roof. Construction over sinkholes can also cause collapse. Sinkhole development may damage buildings, pipelines and roadways. Damage from the Winter Park sinkhole in Florida is estimated at greater than $2 million. Sinkholes may also serve as routes for the spread of contamination to groundwater when people use them as refuse dumps.

In areas where evaporite rock is common, human activities play an especially significant role in the formation of sinkholes. Evaporites dissolve in water much more easily than carbonate rocks. Salt mining and drilling into evaporite deposits allows water that is not already saturated with salt to easily dissolve the rock. These activities have caused the formation of several large sinkholes.

Sinkholes occur worldwide, and in the United States are common in southern Indiana, southwestern Illinois, Missouri, Kentucky, Tennessee, and Florida. In areas with known karst topography, subsurface drilling or geophysical remote sensing may be used to pinpoint the location of sinkholes.

See also Hydrogeology; Hydrologic cycle; Landscape evolution; Weathering and weathering series

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