Karst and Sinkholes

Karst topography forms in areas where the underlying bedrock is composed of material that can be slowly dissolved by water. Examples of this type of sedimentary rock include carbonate rocks such as limestone, halite, gypsum, dolomite, and anhydrite. Carbonate rocks may develop karst and other dissolution features due to the effects of circulating groundwater that has been made slightly acidic through the presence of dissolved carbon dioxide (which creates carbonic acid that reacts with the rock, dissolving it). Sinkholes and caverns can form, creating potential hazards (i.e., the land surface could subside or collapse into the underground openings). This may principally occur in areas where cavities filled with water are emptied through groundwater withdrawal or other natural processes, resulting in the cavities being filled with air and reducing support for the overlying rock. Karst is not overly prevalent in the Northwest Central, but it is found in abundance throughout the Black Hills of South Dakota, and is scattered throughout several other states (Figure 10.19).

Figure 10.19: Areas of karst in the continental US associated with carbonate and evaporate rocks. NOTE: Clicking on this image will bring up a high-resolution version that may take a few minutes to load. This version will expand to the size of your browser window, so enlarge the window for maximum visibility.

The Black Hills are surrounded and underlain by thick layers of Mississippian to Jurassic anhydrite and gypsum, which contain abundant karst features due to dissolution from groundwater and rain. Sinkholes are commonplace, ranging in size from small holes of a few meters (feet) across to large pits as wide as 140 meters (460 feet). The presence of other easily dissolved carbonate layers, laid down in Paleozoic and Mesozoic inland seas, has led to a variety of caves and small sinkholes found throughout the Northwest Central US. For example, the Little Belt Mountains in central Montana are underlain by a thick layer of limestone (the Madison Limestone) laid down in the Mississippian (Figure 10.20).

Figure 10.20: The Belt Meteor Crater southeast of Great Falls, Montana, is a sinkhole in the Madison Limestone measuring 10 meters (35 feet) deep and 30 meters (100 feet) across.

In Idaho, volcanic pseudokarst dominates the Snake River Plain. This type of topography is not technically karst—instead of forming through the dissolution on carbonate bedrock, these fissures, sinkholes, and caves were created by the extrusion of liquid lava. While sinkholes in volcanic pseudokarst are rare, they tend to be related to the collapse of old lava tubes.

Because karst terrain is very porous and fractures easily, groundwater pollution can be a serious problem. Contaminants that might otherwise be filtered through the sedimentary rock are quickly transported into aquifers by runoff. The hazards of pollution are increased by rampant industrial, agricultural, and residential development over karst features.