DESCRIPTION:
Volcanic Gases and Emissions,
Fumaroles and Solfataras

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Gas and Ash Plume, Mount St. Helens, Washington.
-- USGS Photo by Lyn Topinka, May 18, 1982

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Fumarole sampling, Mount Baker, Washington.
-- USGS Photo by Thomas J. Casadevall

All magmas contain dissolved gases that are released during eruptions as well as between eruptive episodes. Gas emission often precedes eruptions, and gases can issue from fumaroles for hundreds or thousands of years after an eruption has ended.

Volcanic gases usually consist predominantly of steam, followed in abundance by carbon dioxide and compounds of sulfur and chlorine. Minor amounts of carbon monoxide, fluorine and boron compounds, ammonia, and several other compounds are found in some volcanic gases.

Distribution of volcanic gases is mostly controlled by the wind; gases may be concentrated near a vent but become diluted rapidly downwind. Even very dilute gases can have a noticeable odor and can harm plants and some animals tens of kilometers downwind from a vent.

Close to a vent, volcanic gases can endanger life and health as well as property. Acids and ammonia and other compounds present in volcanic gases can damage eyes and respiratory systems of people and animals, and accumulation in closed depressions of gases heavier than air, like carbon dioxide, can suffocate people or animals that enter such basins. Other harmful effects of volcanic gases on plants and animals, and corrosion of metals and other property, can be severe near and downwind form especially active vents.

Volcanoes emit gases during eruptions. Even when a volcano is not erupting, cracks in the ground allow gases to reach the surface through small openings called fumaroles. Ninety percent of all gas emitted by volcanoes is water vapor (steam), most of which is heated ground water (underground water from rain fall and streams). Other common volcanic gases are carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen, and fluorine. Sulfur dioxide gas can react with water droplets in the atmosphere to create acid rain, which causes corrosion and harms vegetation. Carbon dioxide is heavier than air and can be trapped in low areas in concentrations that are deadly to people and animals. Fluorine, which in high concentrations is toxic, can be adsorbed onto volcanic ash particles that later fall to the ground. The fluorine on the particles can poison livestock grazing on ash-coated grass and also contaminate domestic water supplies.

Cataclysmic eruptions, such as the June 15, 1991, eruption of Mount Pinatubo (Philippines), inject huge amounts of sulfur dioxide gas into the stratosphere, where it combines with water to form an aerosol (mist) of sulfuric acid. By reflecting solar radiation, such aerosols can lower the Earth's average surface temperature for extended periods of time by several degrees Fahrenheit. These sulfuric acid aerosols also contribute to the destruction of the ozone layer by altering chlorine and nitrogen compounds in the upper atmosphere.

All magmas contain dissolved gases that are released both during and between eruptive episodes. Volcanic gases generally consist predominantly of steam (H2O), followed in abundance by carbon dioxide and compounds of sulfur and chlorine ... Minor amounts of carbon monoxide, fluorine and boron compounds, ammonia, and several other compounds are found in some volcanic gases. ...

Volcanic gases: The most common gases associated with active volcanoes are water vapor, carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen, helium, carbon monoxide, and hydrochloric acid. Lesser amounts of hydrofluoric acid, nitrogen, argon, and other compounds are commonly associated with active volcanoes as well. Volcanic gases rarely reach populated areas in lethal concentrations, although sulfur dioxide can react with the atmosphere downwind and fall as acid rain to cause corrosion and a host of other effects. People with respiratory or heart diseases are especially susceptible to volcanic gases (fume). Carbon dioxide is heavier than air and tends to collect in depressions, where it can occur in lethal concentrations and cause suffocation. On occasion, toxic concentrations of fluorine from hydrofluoric acid have been absorbed onto ash and ingested by livestock or leached into domestic water supplies.

Fumaroles, which emit mixtures of steam and other gases, are fed by conduits that pass through the water table before reaching the surface of the ground. Hydrogen sulfide (H2S), one of the typical gases issuing from fumaroles, readily oxidizes to sulphuric acid and native sulfur. This accounts for the intense chemical activity and brightly colored rocks in many thermal areas.

Fumarole - a vent that releases volcanic gases, including water vapor (steam).

Fumarolic activity - volcanic gas emissions, that may be accompanied by a change in the temperature of the gases of fluids emitted.

Fumaroles (also called solfataras), which emit mixtures of steam and other gases, are fed by conduits that pass through the water table before reaching the surface of the ground. Hydrogen sulfide (H2S), one of the typical gases issuing from fumaroles, readily oxidizes to sulphuric acid and native sulfur. This accounts for the intense chemical activity and brightly colored rocks in many thermal areas.

Volcanoes emit gases during eruptions. Even when a volcano is not erupting, cracks in the ground allow gases to vent to the surface through fumaroles.

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COSPEC setup on Sugar Bowl (northwest flank of Mount St. Helens) to measure SO2 emissions from top of dome.
-- USGS Photo by Lyn Topinka, October 21, 1983

Gas emissions are measured regularly in conjunction with seismicity and ground deformation to monitor eruptive activity. Mount St. Helens continuously emits volcanic gas from fumaroles on and around the dome. Most of the gas emitted by the volcano is water vapor, but emissions also include sulfur dioxide, carbon dioxide, hydrogen, and lesser amounts of helium, carbon monoxide, hydrogen sulfide, and hydrogen chloride.

Gas studies include (1) frequent airborne measurements of sulfur dioxide and, in 1980 and 1981, carbon dioxide in the plume and (2) less frequent sampling of gases from crater fumaroles. The emissions of sulfur dioxide are measured in the plume by a correlation spectrometer (COSPEC) designed originally for pollution studies. The instrument measures the amount of solar ultraviolet light absorbed by sulfur dioxide in the plume and compares it with an internal standard. Three to six traverses are made beneath the plume at right angles to the plume trajectory several times each week to calculate daily emission rates.

The emission rates of sulfur dioxide peaked during summer 1980 at about 1,500 tons per day, decreased rapidly in late 1980, and remained low at about 100 tons per day through 1983. Emission rates of carbon dioxide decreased rapidly in late 1980 until they were below the detection limit of 1,000 tons per day. These patterns correspond generally to a change in eruptive style from the explosive activity of 1980 to the now predominantly nonexplosive activity. The patterns suggest steady outgassing of a single batch of magma under the volcano to which no significant new magma has been added since mid-1980.

Increased rates of sulfur dioxide emissions measured before several nonexplosive eruptions are interpreted as the result of accelerated degassing of a small volume of magma as it moved toward the surface. During the nonexplosive eruptions, gas emissions remained elevated during the active extrusion of lava and generally dropped to preeruption levels once extrusion stopped. The occasional outbursts of gas and tephra are accompanied by brief, sudden increases in the emission rate of sulfur dioxide, water vapor, and probably other gases as well. It is not known whether this increase in gas is derived directly from magma within the dome or released during periodic, geyserlike flashing of a shallow hydrothermal system.

Mount St. Helens Volcanic Gas Emissions Menu

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Fumarole sampling, Mount Baker, Washington.
-- USGS Photo by Thomas J. Casadevall

Most hydrothermal activity at Mount Baker is concentrated within Sherman Crater, although a small area of fumaroles, known as the Dorr Fumarole Field, is present on the north flank of the volcano at an altitude of 2,300 to 3,500 meters.

Mount Baker Volcano Menu

Jökulhlaups (glacial-outburst floods) have been recorded from the Zigzag, Ladd, Coe, and White River Glaciers (at Mount Hood). ... The more frequent outbursts from White River Glacier may be due in part to an increase in size of the fumarole field at the head of the glacier at Crater Rock.

Mount Hood Volcano Menu

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