Figure 2: Another collapsing sinkhole that swallowed a building and a truck. Photo Courtesy of Geotimes.

Figure 3 : Arial photo of several sinkholes located
Southwest of Winslow, AZ. Photo courtesy of
Louis J. Maher, Jr.

Figure 4: Process of developing a sinkhole. Diagram
courtsey of USGS.

Figure 5: Sinkhole that has been filled with water.
Photo courtesy of USGS.

Figure 6: The Wrath of the Sinkhole, destroying an
entire house. Photo courtesy of USGS.

Links:

http://www.dcr.state.va.us/dnh/lws.htm

www.caves.org/conservancy/ikc/images/sinkhole.pdf

http://www.dep.state.fl.us/geology/geologictopics/sinkhole.htm

The Bright Side of Sinkholes:

http://www.sinkmedia.com/sinkholes/

Sinkholes

Introduction

Karst topography is a type of landscape where areas rich in carbonate rocks are sculpted by water (Tarbuck and Lutgens, 2002). Associated with karst topography is the development of sinkholes, which are depressions created by the removal of soluable rock by groundwater (Tarbuck and Lutgens, 2002). Since karst topography occupies approximately 10% of the Earth's surface, understanding the process and side-effects are important (GCRIO, 2002). Sinkholes can range from a meter to over 100 meters wide (Murck & Skinner, 1999). Areas that display karst topography and have potential for sinkholes development are portions of Kentucky, Tennessee, southern New Mexico, southern Indiana, central and northern Florida, and the Appalachian Mountain Great Valley Limestone Belt (Schmidt, 2001).

Conditions for Formations

Chemical weathering occurs with minerals that are of the carbonate group, typically bedrock of limestone and dolostone. The reaction that the water has on the rock will proceed to dissolve it (Murck & Skinner, 1999). Less common, evaporite rocks are susceptible to the same process, except at a much faster pace. For example, gypsum is 150-7,500 times more soluble than limestone (Martinez et al, 1998). So in these areas the dissolution is very intense.

Geologic Processes

Since the bedrock in karst areas are typically limestone or dolostone, a reaction occurs with rainwater (which percolates down into the groundwater system). This reaction occurs because the water is mildly acidic (McKenzie et al, 2000). Carbonic acid, the most common acid in water, forms when water and carbon dioxide combine in the atmosphere (Carving Canada, 1995). Once the water reaches the bedrock, the chemical reaction begins. The acid then dissolves the carbonate minerals through this reaction:

CaCO3 + H2CO3 => Ca+2 + 2HCO3 -1

(Formula from McKenzie et al, 2000)

Products of the Geologic Process

This process can take several years to develop a porous medium that can transport groundwater (Carving Canada, 1995). Continuation of this will then produce large cavities or caves. From these large voids sinkholes are likely to form. Two different types of sinkholes develop from this process, subsidence sinkholes and collapsing sinkholes.
1. Subsidence Sinkholes: The development of fractures in the bedrock continues to grow from the chemical reaction with time. As the voids grow, groundwater flow increases, dissolving continues and the elevation slowly drops as the bedrock dissolves away (Tarbuck & Lutgens, 2002).
2. Collapsing Sinkholes: When void spaces become well developed, the arch becomes too large to support the overlying soils and an abrupt collapse can occur (White, Culver, et al, 1995). Factors usually include the water table dropping which results in soils becoming very saturated and dense (Hubbard, 1990). Eventually the cavern's roof cannot support the weight of the overlying material and the cavity collapses instantaneously (Hornsby, 2002).

Impact on Humans

The entire karst dissolving process can take several years to develop, however the actual release and formation of a collapsing sinkhole can happen in a matter of seconds. As seen in the figures, sinkholes can have damaging effects to large manmade objects. They are capable of destroying entire houses, swallowing portions of roads, or anything that sits above the unstable ceiling.

Literature Cited

Carving Canada, 1995, [Online]: http://www.cancaver.ca/docs/karst.htm (2/5/02).

Hornsby, A.G., Sinkholes. [Online]: http://www.aterquality.ifas.ufl.edu/PRIMER/sinks.html (2/6/02).

Hubbard Jr., D.A., 1990, Sinkholes. [Online]: http://www.mme.state.va.us/Dmr/PUB/Brochures/sink.html (2/6/02).

Martinez, J.D., Johnson, K.S. and J.T. Neal, 1998, Sinkholes in Evaporite Rocks. American Scientist, v.86 Jan/Feb: p.38-51.

McKenzie G.D., Storm, R.N, and J.R. Wilson, 2000, Groundwater Processes, Resources, and Risks, Laboratory Manual in Physical Geology 5th ed., Upper Saddle River, New Jersey, Prentice-Hall, Inc.

Murck, B.W. and B.J. Skinner, 1999, Geology Today: Understanding Our Planet, New York, New York, John Wiley & Sons, Inc.

Schmidt, W., 2001, Sinkholes in Florida. Geotimes, v.46 May: p.18

Skinner, B.J. and Porter, S.C., 1992, The Dynamic Earth: An Introduction to Physical Geology 2nd ed., New York, New York, John Wiley & Sons, Inc.

Tarbuck, E.J. and F.K. Lutgens, 2002, Earth: An Introduction to Physical Geology 7th ed., Upper Saddle River, New Jersey, Prentice-Hall, Inc.

The U.S. Global Change Research Information Office, 2002, Karst Activity, [Online]: http://www.gcrio.org/geo/karst.html (2/6/2002).

White, W.B., Culver, D.C. and J.S. Herman, 1995, Karst Lands, American Scientist, v.83 Sept/Oct: p.450-9.

Copyright ©-2001 Earlham College. Revised 25 January 2002. Send corrections or comments to parkero@earlham.edu