The alteration of rocks or crystallization of minerals by gases or supercritical fluids (generically termed magmatic fluids) derived from solidifying magma. At surface conditions, magmatic fluids sampled from active volcanoes contain steam with lesser amounts of carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen chloride, and hydrogen fluoride, and trace amounts of many other volatile constituents. Magmatic fluids may contain relatively high concentrations of light and heavy elements, particularly metals, that do not crystallize readily in common rockforming silicates constituting most of the solidifying magma; thus, valuable rare minerals and ores are sometimes deposited in rocks subjected to pneumatolysis. Magmatic fluids are acidic and may react extensively with rocks in the volcanic edifice or with wall rocks surrounding intrusions. Penetration of magmatic fluids into adjacent rocks is greatly aided by faults, fractures, and cracks developed during intrusion and eruption or created by earlier geologic events.