fumarole

A fumarole is a vent in a volcanic or geothermal system through which steam and volcanic gases escape to the surface. The term comes from Latin fumus, meaning smoke, reflecting the characteristic plumes of vapor and gas often seen at these vents.

Fumaroles form when groundwater is heated by underlying magma or hot rock. The water turns to steam and carries dissolved gases to the surface. Gas compositions commonly include water vapor (the majority), carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen, and sometimes methane. The temperature and chemistry of the emission determine the vent’s appearance: high-temperature fumaroles produce extensive steam plumes, while cooler vents may deposit sulfur crystals or silica and leave mineral crusts around the vent opening.

Fumaroles are categorized by temperature and emission style. High-temperature steam fumaroles emit near-boiling gases, sometimes with little liquid water, and may create dense, opaque plumes. Low-temperature or diffuse vents release gases more slowly and can be less conspicuous. Deposits around fumaroles often include sulfur crystals, fumarolic dust, and silica sinter, formed as vapor condenses and minerals precipitate.

In geology and geothermal exploration, fumaroles signal active degassing from a subsurface system and can indicate magmatic or hydrothermal activity. They pose hazards to nearby people and ecosystems due to toxic or corrosive gases and unstable terrain. Monitoring fumarole activity helps scientists assess volcanic unrest, map hydrothermal reservoirs, and study degassing processes that shape volcanic landscapes.