Fumaroles (Latin fumus, smoke) are vents from which volcanic gas escapes into the atmosphere. Fumaroles may occur along tiny cracks or long fissures, in chaotic clusters or fields, and on the surfaces of lava flows and thick deposits of pyroclastic (explosive ash) flows. They may persist for decades or centuries if they are above a persistent heat source or disappear within weeks to months if they occur atop a fresh volcanic deposit that quickly cools.
Fumaroles are common on Earth; for example, there are an estimated four thousand fumaroles within the boundaries of Yellowstone National Park. The Mars Exploration Rover Spirit has identified possible fumarolic deposits in Gusev crater.