DESCRIPTION:
Mount St. Helens Volcanic Gases

MSH81_collecting_gas_samples_base_dome_09-24-81.jpg
USGS geologists collect gas samples around the dome. Samples were gathered from vents on the dome and crater floor, and were used to monitor changes in chemical composition. Additionally, sulfur dioxide gas was measured from a specially-equipped airplane before, during, and after eruptions to determine "emission rates" for the volcano. During eruptions, emission rates typically increased to 5 to 10 times their pre-eruptive value.
USGS Photograph taken on September 24, 1981, by Thomas J. Casadevall.
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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.

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