In geology, a rheid is a solid material that deforms by viscous flow. To be considered a rheid, deformation by flow should exceed elastic deformation by at least a factor of three.
Types of rheids
Almost any type of rock can behave as a rheid under appropriate conditions of temperature and pressure. For example, the Earth's mantle is believed to undergo convection over long time scales. Since the mantle is known to be solid (it supports the propogation of shear waves), it must be behaving as a rheid. Halite, the mineral form of salt, is a geological material that behaves as a rheid over relatively short time periods. As salt is buried by other types of sediments, it will often flow laterally towards regions of less confining stress. Through this mechanism, salt domes and other structures are formed. In some areas, such as the Gulf of Mexico, these structures often serve as traps for petroleum and natural gas.
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