is an opening in a planet's crust which emits steam and gases, but no liquid or solid material. The temperature of the gases leaving the vent ranges from about 100 to 1000 C. The steam forms when groundwater is superheated by hot rock, then flashes (boils due to depressurization) as it approaches the surface.

In addition to steam, gases released by fumaroles include carbon dioxide, sulfur oxides,