A fumarole (Latin fumus, smoke) is an opening in Earth's (or any other astronomical body's) crust, often in the neighborhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide. The name solfatara, from the Italian solfo, sulfur (via the Sicilian dialect), is given to fumaroles that emit sulfurous gases.
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 flows. A fumarole field is an area of thermal springs and gas vents where magma or hot igneous rocks at shallow depth are releasing gasses or interacting with groundwater. From the perspective of groundwater, fumaroles could be described as a hot spring that boils off all its water before the water reaches the surface.
A good example of fumarole activity on Earth is the famous Valley of Ten Thousand Smokes, which was formed during the 1912 eruption of Novarupta in Alaska. Initially, there were thousands of fumaroles in the cooling ash from the eruption, but over time most of them have become extinct. Fumaroles 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. There are also an estimated four thousand fumaroles within the boundaries of Yellowstone National Park. Another example is an array of fumaroles in the Valley of Desolation in Morne Trois Pitons National Park in Dominica.
Sulpherous fumeroles. © James Shook under GNU.
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