Vent from which volcanic vapours issue.
Fumaroles, like geysers, are caused by hot spring s, which disperse groundwater from the upper parts of the Earth's crust after it has been heated by magma . As magma begins to solidify, the gases in it, mostly water vapour, become concentrated by the pressure of the remaining liquid. When the pressure becomes high enough, the liquid is forced into cracks in the surrounding rock. A fumarole forms if a crack extends to the surface.