Fumaroles: The Steam Vents of Volcanoes, From Katmai to Yellowstone

An aerial view of a snowy mountainside with plumes of steam rising from it

Active fumaroles on Fourpeaked Mountain, Katmai National Park, 2006. U.S. Geological Survey.

A fumarole is a vent in the ground where volcanic gases and steam escape. Fumaroles are common on active volcanoes and are one of the surest signs that a volcano is alive, because they show that volcanic heat lies below. Between eruptions they are among a volcano's most changeable features: they hiss and growl, give off noxious gases and can be hazardous to approach.

A fumarole may be a hole, a small crack or a line along a fissure, and they often come in groups, most commonly in or near a volcano's vents and craters.

What comes out

The gases include water vapour, sulfur dioxide, carbon dioxide, hydrogen chloride and hydrogen sulfide. The water vapour, usually the main part, mostly comes from groundwater fed by rain and snow; the carbon dioxide, sulfur dioxide and other gases come from the volcano.

That makes fumaroles valuable to volcanologists, who monitor their temperature and the make-up of their gases. Increased activity often goes with unrest and can mean that magma is moving inside the volcano. Around the vents, sulfur, opal, gypsum, hematite and other minerals are deposited, and sulfuric acid slowly turns the volcanic rock to clay.

Katmai: the Valley of Ten Thousand Smokes

In 1912 Novarupta–Mount Katmai in Alaska produced the largest eruption of the 20th century, rated VEI 6, "colossal". It buried a valley in a vast ash-flow deposit, an ignimbrite up to 660 ft (200 m) thick, laid down at about 1380 °F (750 °C). When scientists first reached it in 1916, thousands of fumaroles were still steaming, and they named it the Valley of Ten Thousand Smokes. The park was established in 1918.

In 1917, pits dug just 3 feet into the surface came close to boiling, and the expedition's leader, Robert Griggs, estimated that more than 1,000 fumaroles sent steam and gas more than 500 ft into the air. By 1929 only a few hundred remained, and by the 1990s the last had gone out. Most of their steam had come from buried snowfields, rain and groundwater. Their openings, round and in lines, can still be seen as discoloured, altered rock on the valley floor.

A barren valley floor with oval patches of discoloured rock, with snowy mountains behind

An extinct fumarole in the Valley of Ten Thousand Smokes. Photograph by M. Fitz, National Park Service.

Most of Katmai's active volcanoes still have fumaroles, among them Trident, Martin, Mageik, Mount Griggs and Douglas.

Steam rising from vents in a volcanic summit crater

Vigorous fumaroles in the summit crater of Mount Martin, August 26, 2006. U.S. Geological Survey.

Other parks

Lassen Volcanic National Park, California. Fumaroles steam at Sulphur Works, Little Hot Springs Valley, Bumpass Hell and Devils Kitchen. They are cooler in spring, when plenty of shallow groundwater mixes in, and hotter and drier in late summer and in droughts. Big Boiler in Bumpass Hell, measured at up to 322 °F (161 °C), is one of the hottest fumaroles in the world.
Yellowstone National Park. About 2,000 fumaroles, at Norris Geyser Basin, Roaring Mountain, the Mud Volcano area and elsewhere. They are the park's hottest thermal features and chemically unlike its geysers and hot springs: fumaroles and mudpots are acidic, rich in sulfate, while geysers and hot springs carry chloride and silica and have a high pH.
Mount Rainier National Park, Washington. Steam from the summit fumaroles has melted a network of ice caves into the glaciers filling the craters.
Lake Clark National Park, Alaska. Redoubt and Iliamna volcanoes both have active fumaroles whose plumes can be seen from far away. Redoubt's fumaroles grew more active in the months before its 2009 eruption.
Hawaiʻi Volcanoes National Park. At the summit of Kīlauea, Steaming Bluff (Wahinekapu) gives off steam from groundwater heated by magma deep below, while the Sulphur Banks (Haʻakulamanu) also release sulfur dioxide and hydrogen sulfide.

A steaming vent in pale, altered ground

Big Boiler in Bumpass Hell, Lassen Volcanic National Park. National Park Service.

A white plume of steam rising from two holes in a glacier on a snowy volcano

Hot gas and steam escaping through holes in the Drift glacier on Mount Redoubt, January 31, 2009, weeks before its eruption. U.S. Geological Survey, Alaska Volcano Observatory.

Steam rising from the forested rim of a crater

Steaming Bluff (Wahinekapu), Hawaiʻi Volcanoes National Park. Photograph by J. Wei, National Park Service.

Fumaroles are also found at Aniakchak National Monument and Wrangell–St. Elias National Park in Alaska, and at Valles Caldera National Preserve in New Mexico.

Sources

National Park Service, "Fumaroles," in the series Volcanic Features; rewritten in hubnx's own words. Photographs from the National Park Service and the U.S. Geological Survey; a photograph credited to a Flickr user and a 1917 expedition photograph whose owner is not named left out.