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Karst is a special type of landscape that is shaped by the dissolution of soluble bedrock. Soluble bedrock includes carbonate rock (limestone and dolomites) and evaporite rock (that include gypsum, anhydrite, and halite (rock salt) minerals). The minerals in these types of rocks dissolve in water. Karst systems include subterranean drainage networks and cavern formation. Karst surface landforms include cave openings, closed drainage systems with emerging and disappearing creeks, and ground depressions.
Sinkholes are a form of ground subsidence where subsurface voids or caverns exist below the ground surface. The roofs of these voids weaken over time and cave in. If this cave-in can migrate to the surface, it creates a depression or ground opening called a sinkhole. The spontaneous opening and collapse of the ground surface are hazardous and potentially dangerous.
In Colorado, most of our sinkholes, also called dolines, are related to the dissolution of evaporite rocks. Evaporite karst hazards have been recognized in several areas of the state, including high growth areas of the Roaring Fork River and the Eagle River valleys. Both of these areas lie in regional collapse centers where subsidence of hundreds to thousands of vertical feet has occurred by the dissolution and deformation of evaporite rocks. The CGS has mapped those and other similar areas in Colorado that are underlain by evaporite rocks where sinkholes hazards occur, producing the Colorado Map of Potential Evaporite Dissolution and Evaporite Karst Subsidence Hazards report.
Some soils in arid areas of Colorado are dispersive and easily eroded. Underground pipes and fissure-type voids commonly form that also create sinkholes. The sinkhole landform from this type of soil erosion is commonly referred to as pseudo-karst. Examples of pseudo-karst can be seen here.
Human-caused subsidence commonly occurs from the collapse of underground mine workings or they may occur if water mains or culverts break so that soil can wash away below the ground surface.
More information about the different types of ground subsidence is in CGS Rocktalk Vol. 4, No. 4.