Sinkholes in Evaporite Rocks
Surface subsidence can develop within a matter of days when highly soluble rocks dissolve because of either natural or human causes
Sinkholes characterize karst topography the world over. A mainly natural phenomenon, their development in carbonate rocks such as limestones of Florida and the Yucatan Peninsula of Mexico is well known (see "Karst Lands," American Scientist, September–October 1995). However, the fact that sinkholes also form in or above highly soluble evaporite rocks, such as gypsum and salt, is far from common knowledge. And these sinks can be at least as dramatic and troublesome as those in carbonates.
Evaporite deposits form when various salts precipitate from evaporating water, mainly seawater. The principal evaporite rocks include gypsum (or anhydrite, its anhydrous form) and salt (halite), although potash (sylvite) and other rarer salts also are locally important. Evaporites have the highest solubility of common rocks; water that is unsaturated with respect to gypsum (CaSO4 · 2H2O) or salt (NaCl) rapidly dissolves them and carries them off in solution. Indeed, gypsum and salt are, respectively, about 150 and 7,500 times more soluble than limestone. Such high solubilities enable subsurface dissolution channels and sinkholes to form in a matter of only days, weeks or years, and catastrophic collapse can result. Evaporite rocks underlie about 35–40 percent of the United States and are found in 32 of the 48 contiguous states. They are also found in Canada and Mexico, and are widespread on other continents.
Unlike carbonate sinkholes, which result mostly from natural processes, those in or above evaporite rocks can form as the result of either natural processes or human activities. A number of striking natural sinks exist in gypsum beds in Texas, New Mexico, Oklahoma and Nova Scotia. They also show up in gypsum diapirs—domes or arches of rock ruptured by the more plastic gypsum pushing upward from below. Excellent examples of these can be found in the Magdalen Islands in the Gulf of St. Lawrence, Canada, and in the Sierra del Fraile, in northeastern Mexico. Natural karst features, including major sinkholes and subsidence troughs, also occur over bedded salt in Texas, New Mexico and Arizona. The mining of rock salt, or the drilling of boreholes into or through rock salt, have accidentally created a number of large, man-made sinkholes in Louisiana, Texas, Kansas, Michigan and New York. Regardless of their origin, sinkholes in evaporites pose significant hazards and are costly to society. Only through a better understanding of the processes by which sinkholes form and a recognition of the regions in which they are likely to form can we minimize such risks.