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A rheid // is a non-molten solid that deforms by viscous flow at least a thousand times faster than it would deform elastically under the same applied stress. The term, which was coined by S. Warren Carey in 1953, has the same Greek root as rheology, the science of viscoelasticity and nonlinear flow.
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 propagation of shear waves), it must be behaving as a rheid. Granite has a measured viscosity at standard temperature and pressure of about 4.5×1019 Pa·s  so it should be considered 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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