Sinkholes are considered a type of land subsidence, because they involve a vertical downward movement of the land surface. Sinkholes can be shallow or deep, small or large, but all are a result of the dissolving of the underlying limestone. Hydrologic conditions, including lack of rainfall, lowered water levels, or, conversely, excessive rainfall in a short period of time, can all contribute to sinkhole development.
Sinkholes are common in regions of karst, where mildly acidic groundwater has dissolved rock such as limestone, dolostone, marble, or gypsum. Areas in Florida, Pennsylvania, Missouri, Kentucky, Texas, Tennessee or anywhere in the Northern Plains are in karst regions and at risk from sinkholes. However, not all sinkholes are quite as dramatic as the ones in Florida and Pennsylvania. Some are hardly noticeable, causing the ground to sag slightly.
The occurrence of gradual land subsidence is seldom as obvious as is the case of catastrophic sinkholes or mine collapses. Where ground-water mining or drainage of organic soils are involved, the subsidence is typically gradual and widespread, and its discovery becomes an exercise in detection. Gazing out over the San Joaquin Valley, California today, one would be hard-pressed to recognize that fewer than 75 years ago the land surface was nearly 30 feet higher in some locations.
Soil subsidence is usually irreversible. The natural rate of accumulation of organic soil is on the order of a few inches per 100 years; the rate of loss of drained organic soil can be 100 times greater, up to a few inches per year in extreme cases. Thus, deposits that have accumulated over hundreds of years can disappear relatively quickly in response to human activity. In time, the remaining organic material becomes diluted through the incorporation of the organic layer into the mineral subsoil. This reduces the productivity of the soil.
- USGS – Water Science School – Sinkholes: http://water.usgs.gov/edu/sinkholes.html
- Colorado Geological Survey – Website: http://geosurvey.state.co.us/hazards/Pages/GroundSubsidence.aspx
- USGS – Land Subsidence in the United States: http://water.usgs.gov/ogw/pubs/fs00165/
- USDA – Soil Quality Degradation – Subsidence: http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1187276.pdf
- Image Source: http://www.usgs.gov/blogs/features/usgs_top_story/the-science-of-sinkholes/ [Accessed Jun 4, 2013]