Rheid

For places in Schleswig-Holstein in Germany, see Rheide (disambiguation).

In geology, a rheid (English pronunciation: /ˈriːɪd/) is a solid material that deforms by viscous flow. The term has the same Greek root as rheology, the science of viscoelasticity and nonlinear flow.

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 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.