Heat is critical for the occurrence of salt intrusion. Increased temperature greatly reduces the ultimate strength of salt and eliminates work hardening. When salt is heated above 400°F (205°C), it becomes soft and plastic and flows indefinitely with a pressure gradient of about 33-100 kg/cm2 (460-1,400 psi). It is plastic during the entire process of intrusion, and even during extrusion at the surface. Thus, at the time of extrusion, salt can flow by simple gravity, like a “glacier,” as long as it remains hot.
When buried at a depth of more than 25,000 ft, sedimentary salt becomes mobile because of the high temperature and behaves hydrodynamically; it moves laterally to places of lower overburden pressure, where doming or piercement occurs. Once flow is initiated, it will continue until the supply of salt is depleted or cut off, either by the coming together of the overlying and underlying strata or because additional supplies of salt have not been heated to the temperature necessary to maintain plasticity. The energy impelling the lateral or radial flow to the place of piercement can be attributed only to an imbalance in geostatic load of the overburden, but after piercement occurs, the geostatic load differential and the ever-increasing effect of buoyancy cause the salt to rise rapidly through the overlying strata. Buoyancy becomes an effective force only when the height of the intrusion has increased greatly. Buoyancy is not a requirement for intrusion, but it has a modifying effect.
The emplacement of igneous masses such as volcanic plugs, granite batholiths, diamond pipes, carbonatites, and serpentine bodies (Gussow, 1962), and of such intrusive masses as mud volcanoes, shale diapirs, ice piercements or pingos (Gussow, 1954), and frost boils (Gussow, 1962), is similar to that of salt piercements. In all cases the prime motivating force for intrusion is the weight differential of the overburden, or geostatic load (Gussow, 1962).
The writer postulates that salt-dome intrusion is a thermally activated process and that the rate of intrusion is rather rapid—probably catastrophic on a geologic time scale. The movement which has been interpreted as salt-dome growth is actually a measure of rate of compaction of the adjacent sediments. The fundamental mechanics outlined for salt diapirism are applicable to igneous intrusion generally, and to other forms of diapirism.
Figures & Tables
“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.