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FEATURE ARTICLE

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

Joseph Martinez, Kenneth Johnson, James Neal

Figure 15. Cargill sinkClick to Enlarge Image

Evaporite rocks underlie about 35 to 40 percent of the United States and are found in 32 of the 48 contiguous United States, as well as in Canada and Mexico, and on other continents. Evaporite deposits form when various salts precipitate from evaporating water, mainly seawater. The principal evaporite rocks include gypsum (or anhydrite, the 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 solubility enable subsurface dissolution channels and sinkholes to form in a matter of only days, weeks or years, and catastrophic collapse can result. Unlike sinkholes in carbonate rocks, evaporite sinkholes may result from either natural causes or from human activities.


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