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A relatively mobile mass that intrudes into preexisting rocks. Diapirs commonly intrude vertically through more dense rocks because of buoyancy forces associated with relatively low-density rock types, such as salt, shale and hot magma, which form diapirs. The process is known as diapirism. By pushing upward and piercing overlying rock layers, diapirs can form anticlines, salt domes and other structures capable of trapping hydrocarbons. Igneous intrusions are typically too hot to allow the preservation of preexisting hydrocarbons.