Mechanical properties of salt are important to the geologist because they determine the behavior of salt in large quantities. The ability of salt to flow or creep slowly under sufficient differential pressure is of particular importance in explaining the formation and shape of salt structures. The time-dependent strain or creep of salt and other non-metallic substances is not well understood, but available data show that, under suitable conditions of pressure, temperature, and environment, salt and other nonmetallic materials show a behavior similar to that of metals. Data concerning creep of salt obtained in the laboratory and in situ have been studied. From these data it is possible to derive, for the given conditions of stress, temperature, and environment, an equivalent viscosity for the salt. The values obtained show a variation of many orders of magnitude.
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“Diapir” and “diapirism” come from the Greek diapeirein, which means “to pierce.” Diapirism sensu lato is a process by which earth materials from deeper levels have pierced, or appear to have pierced, shallower materials; it is divided into magmatic intrusion and diapirism sensu stricto on the basis of the temperature at which piercement occurs. Diapirs s.s. are composed of evaporites, argillaceous sediments, coal, peat, ice, serpentine, or other earth materials which have the critical characteristics of low equivalent viscosity and low density. These materials range in age from Precambrian to Recent. Diapirs are found in all parts of the world except the shield areas. They have many forms, ranging from smoothly rounded pillows to complexly injected laminae, are either connected with or disconnected from the “mother” bed, and are present either at the surface, where they form distinctive features, or at considerable depth. Diapirs have well-developed internal structures indicative of an origin by flow. Strata around a diapir may be strongly affected structurally and/or stratigraphically by the diapir, or they may be unaffected. Field and model studies indicate that diapirs have developed as a result of horizontal compression, gravitational instability, or both. Diapiric structures of various types contain large quantities of oil and gas, sulfur, salt, and potash and are important for underground storage and nuclear testing.