A sinkhole (images) is a natural hole that forms in the Earth's surface as a result of the chemical weathering of carbonate rocks like limestone, as well as salt beds or rocks that can be severely weathered as water runs through them. The type of landscape made up of these rocks is known as karst topography and is dominated by sinkholes, internal drainage and caves.
Sinkholes vary in size but can range anywhere from 3.3 to 980 feet (1 to 300 meters) in diameter and depth. They can also form gradually over time or suddenly without warning. Sinkholes can be found all over the world and recently large ones have opened in Guatemala, Florida and China.
Depending on location, sinkholes are sometimes also called sinks, shake holes, swallow holes, swallets, dolines or cenotes.
Natural Sinkhole Formation
The main causes of sinkholes are weathering and erosion. This happens through the gradual dissolve and removal of water absorbing rock like limestone as percolating water from the Earth's surface moves through it. As the rock is removed, caves and open spaces develop underground. Once these open spaces become too large to support the weight of the land above them, the surface soil collapses, creating a sinkhole.
Typically, naturally occurring sinkholes are most common in limestone rock and salt beds that are easily dissolved by moving water. Sinkholes are also not normally visible from the surface as the processes that cause them are underground, but sometimes however, extremely large sinkholes have been known to have streams or rivers flowing through them.
Human Induced Sinkholes
In addition to natural erosion processes on karst landscapes, sinkholes can also be caused by human activities and land-use practices. Groundwater pumping for example can weaken the structure of the Earth's surface above the aquifer where the water is being pumped and cause a sinkhole to develop.
Humans can also cause sinkholes to develop by changing water drainage patterns through diversion and industrial water storage ponds. In each of these instances, the weight of the Earth's surface is changed with the addition of the water. In some cases, the supporting material under the new storage pond for example, may collapse and create a sinkhole. Broken underground sewer and water pipes have also been known to cause sinkholes when the introduction of free flowing water into otherwise dry ground weakens soil stability.
An extreme example of a human induced sinkhole occurred in Guatemala in late May 2010 when a 60 foot (18 meter) wide and 300 foot (100 meter) deep hole opened in Guatemala City. It is believed that the sinkhole was caused after a sewer pipe burst after tropical storm Agatha caused a surge of water to enter the pipe. Once the sewer pipe burst, the free flowing water carved out an underground cavity that eventually could not support the weight of the surface soil, causing it to collapse and destroy a three-story building.
The Guatemala sinkhole was worsened because Guatemala City was built on land made up of hundreds of meters of volcanic material called pumice. The pumice in the region was easily eroded because it was recently deposited and loose - otherwise known as unconsolidated rock. When the pipe burst the excess water was easily able to erode away the pumice and weaken the structure of the ground. In this case, the sinkhole should actually be known as piping feature because it was not caused by entirely natural forces.
Geography of Sinkholes
As previously mentioned, naturally occurring sinkholes mainly form in karst landscapes but they can happen anywhere with soluble subsurface rock. In the United States, this is mainly in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania but about 35-40% of the land in the U.S. has rock beneath the surface that is easily soluble with water. The Department of Environmental Protection in Florida for example has a focus on sinkholes and how to educate its residents on what to do should one open up on their property.
Southern Italy has also experienced numerous sinkholes, as has China, Guatemala and Mexico. In Mexico, sinkholes are known as cenotes and they are mainly found on the Yucatan Peninsula. Overtime, some of these have filled with water and look like small lakes while others are large open depressions in the land.
It should also be noted that sinkholes do not occur exclusively on land. Underwater sinkholes are common around the world and formed when sea levels were lower under the same processes as those on land. When sea levels rose at the end of the last glaciation, the sinkholes became submerged. The Great Blue Hole (image) off the coast of Belize is an example of an underwater sinkhole.
Human Uses of Sinkholes
Despite their destructive nature in human-developed areas, sinkholes people have developed a number of uses for sinkholes. For example, for centuries these depressions have been used as disposal sites for waste. The Maya also used the cenotes on the Yucatan Peninsula as sacrificial sites and storage areas. In addition, tourism and cave diving is popular in many of the world's largest sinkholes.
Than, Ker. (3 June 2010). "Guatemala Sinkhole Created by Humans, Not Nature." National Geographic News. Retrieved from: http://news.nationalgeographic.com/news/2010/06/100603-science-guatemala-sinkhole-2010-humans-caused/
United States Geological Survey. (29 March 2010). Sinkholes, from USGS Water Science for Schools. Retrieved from: http://ga.water.usgs.gov/edu/earthgwsinkholes.html
Wikipedia. (26 July 2010). Sinkhole - Wikipedia, the Free Encyclopedia. Retrieved from: http://en.wikipedia.org/wiki/Sinkhole