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Sinkholes

Sinkholes


Sinkholes are one of the main landforms in karst topography , so named for the region in Yugoslavia where solution features such as caves, caverns, disappearing streams and hummocky terrain predominate. Karst features occur primarily in limestone but may also occur in dolomite, chert, or even gypsum (Alabaster Caverns in Oklahoma).

As the name implies, sinkholes are depressions formed by solution enlargement or the subsidence of a cavern roof. Subsidence may occur slowly, as the cavern roof is gradually weakened by solution, or rapidly as the roof collapses. Several of the latter occurrences have gained widespread coverage because of the size and amount of property damage involved.

An often-described sinkhole formed during May 1981 in Winter Park, Florida, swallowing a three-bedroom house, half a swimming pool, and six Porsches in a dealer's lot. The massive "December Giant" occurred near Montevallo, Alabama, and measured 400 ft (122 m) wide by 50 ft (15 m) deep. A nearby resident reported hearing a roaring noise and breaking timbers, as well as feeling earth tremors under his house.

Cenotes is the Spanish name for sinkholes. One sacred cenote at the Mayan city of Chichen Itza in Yucatan, Mexico, was known as "the Well of Sacrifice." Archaeologists postulate that, to appease the gods during a drought , human sacrifices were cast into the water 80 ft (24 m) below, followed by a showering of precious possessions from onlookers. Since most of the gold and silver objects from the New World were melted down, these sacrifices are now highly prized artifacts from pre-Columbian civilizations.

Although occurring naturally, sinkhole formation can be intensified by human activity. These sinkholes offer an easy pathway for injection of contaminated runoff and sewage from septic systems into the groundwater . Because karst landscapes have extensive underground channels, the polluted water often travels considerable distances with little filtration or chemical modification from the relatively inert limestone. Therefore, the most serious hazard posed by sinkholes is the access they provide for turbid, polluted surface waters. This allows bacteria to thrive, so testing of spring water that emerges within or below karst regions is vital.

Sinkholes also pose special problems for construction of highways, reservoirs, and other massive objects. Fluctuating water levels weaken the overlying rock when the water table is high, but remove support when water levels are low. Thornbury (1954) described the problems resulting from efforts by Bloomington, Indiana, to build a water supply reservoir on top of karst topography. Much valuable water escaped through channels in the limestone beneath the dam. This structure eventually was abandoned and a new reservoir constructed in a region composed of relatively impervious siltstone below the limestone.

Anthropogenic (human-caused) sinkholes form as a result of mine subsidence, often catastrophically. Collapsing mine tunnels within the 1,000-mi (1,600-km) labyrinth in the historic Tri-State Mines of Kansas, Missouri, and Oklahoma have created scenarios very similar to the Winter Park, Florida, example. Subsidence of the strata overlying underground coal mines is another rich source of these anthropogenic sinkholes.

[Nathan H. Meleen ]


RESOURCES

BOOKS

Coates, D. R., ed. Environmental Geomorphology. Binghampton, NY: State University of New York, 1971.

Keller, E. A. Environmental Geology. 4th ed. Columbus, OH: Charles E. Merrill Publishing Co., 1985.

Thornbury, W. D. Principles of Geomorphology. New York: John Wiley & Sons, 1954.

PERIODICALS

"Into the Well of Sacrifice," National Geographic Magazine (October 1961): 540561.

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