What is a fumarole in geology ?

Fumarole : definition

Fumaroles are gaseous emanations from cracks or cavities (vents), located on the sides of active volcanoes or grouped in volcanic fields. These gases are emitted at high temperature (from less than 100°C to more than 1000°C). Their composition depends on the temperature : dominated by water vapor with CO2 at low temperature (between 100°C and 300°C), these emanations are enriched in H2S, SO2, HCl, F... with temperature increase.

"Low" temperature fumaroles, also known as solfatares, deposit native sulphur, nitrates, borates and ammonium sulphates at vents, by reaction with air, higher temperature fumaroles can also deposit hematite and magnetite. We speak of fumarole deposits.

"Mofettes" are "cold" fumaroles, less than 100°C, usually linked to carbon dioxide thermal springs.