sinkholes Sinkholes (French, dolines) are topographically closed depressions created by removal of materials downwards through their floors. Loss downwards differentiates them from other types of closed depressions, such as impact craters or deflation hollows that form when material is carried out over the enclosing rims. Four main processes create sinkholes: dissolution of soluble rocks such as salt, gypsum, limestone, and dolostone directly downwards from the surface; collapse of consolidated or partly consolidated rocks into underlying cavities formed by dissolution, compaction, piping, mining, etc.; piping of silts, sands, and gravels grain by grain into smaller cavities (analogous to emptying a sandhopper); and subsidence, the slower settling or sagging of strata into cavities without the large ruptures or total fragmentation that occur in collapses. The term ‘suffosion’ is also used for erosion by the slumping of unconsolidated sediments at the surface into solution cavity below.

The basic shapes of sinkholes are bowls, funnels, or cylindrical shafts, but all of these may be distorted by geological effects, such as elongation along a fracture or orientation down tilted strata. Dimensions range from less than a metre in diameter and depth for the smallest, fresh collapse or suffosion features, up to kilometres in length and hundreds of metres in depth for ancient dissolutional landforms. The largest dimensions and greatest variety of features are found on the soluble (karst) rocks, particularly limestones. Most large karst sinkholes are of composite origin, initiated by surficial dissolution but enhanced by periodic collapses of bedrock and the suffosion of any insoluble residuum. Blockage of their basal drains may create pond dolines. Lateral expansion of the rims often leads to amalgamation into compound sinkholes (uvalas, Slovene). Around the boundaries of karst outcrops, elongated stream sinks may form where streams flow in from other rocks. The outcrops themselves may come to be completely occupied by sinkholes, creating polygonal karst. Densities greater than 1000 sinkholes per square kilometre may be counted on very soluble rocks such as salt or gypsum. On thick limestones where the water-table is deep, large solutional sinkholes may be punched downwards to create ‘eggbox topography’ with sharp residual peaks scattered along the doline rims and a local relief of hundreds of metres. This is known as fengcong (‘peak forest cluster’) in southern China, where the greatest karstlands are found. They are the inspiration of much classical Chinese painting.

Sinkholes are a significant hazard. Thousands of collapse and suffosion features develop rapidly and quite naturally each year, destroying property. However, tens of thousands more are induced by human activity, chiefly mining, improper design of urban drainage, overpumping of aquifers that removes the buoyant support of water, or inadequate land filling that allows compaction cavities to form. Heavy trucks and buses, pitheads, high-rise buildings, and swimming pools have all foundered abruptly in recent decades, often with loss of life. Everywhere in the world sinkholes are looked upon as God-given sites for the convenient disposal of raw sewage and other refuse; as they are commonly the principal sources of recharge for aquifers as well, this practice ensures widespread water pollution, especially in the karst regions.

D. C. Ford

Bibliography

Ford, D. C. and and Williams, P. W. (1989) Karst geomorphology and hydrology. Chapman and Hall, London.
Waltham, A. C. (1989) Ground subsidence. Blackie, Glasgow.

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