Metamorphism is the process by which pre-existing rocks are altered by chemical reactions under changing temperature, pressure and shearing stress. The original mineralogy and fabric of both sedimentary and igneous rocks can be changed due to the effects of plate tectonic processes with the resultant increase in temperatures and pressures (dynamic metamorphism), the emplacement of younger hot igneous bodies (contact metamorphism) or the dispersion of relatively low-temperature fluids and gases, commonly mineralized, during the later stages of igneous activity (hydrothermal alteration).