This work stresses the role of volcanic gases in global change and their use in assessment of volcano unrest. Sulfur dioxide (SO2) from explosive volcanism causes significant climatic and atmospheric impacts, but the source at the time of eruption for the surprisingly large amounts of released sulfur (S) remains very controversial. Volcanologists customarily assume the source is S dissolved in pre-eruption melt and thus use concentrations of S in melt inclusions to infer SO2 yields in explosive volcanism. Our SO2 source studies show that melt inclusion estimates of SO2 yields for recent eruptions at Mount St. Helens, Redoubt, and Pinatubo are >25 times lower than remote sensing-based SO2 yields and that commonly invoked sources of "excess sulfur" (nonerupted magma, commingled basalt, anhydrite breakdown) are not viable in these cases. Magmas in these examples were apparently vapor-saturated at depth (6-11 km) prior to eruption, and it is proposed that accumulations of S in the deep magmatic vapor provided the SO2 for these eruptions. Models of the properties of the magmatic vapor lead to the prediction that the magmas contained a large vapor fraction at depth--possibly up to 10 volume %. An important implication is that melt inclusion estimates of SO2 released in explosive eruptions of the (pre-remote sensing) past may be far too low and greatly underestimate impacts on climate and the chemistry of the atmosphere.
Project geologists are working on the development of new technology for measuring and monitoring volcanic gases. One such technique involves the use of a Fourier transform infrared (FTIR) spectrometer mounted in a small aircraft and used to measure gases in a volcanic plume. While instruments currently in use for measuring gases in volcanic plumes can measure only one gas, usually SO2, the FTIR system may be able to measure as many as nine volcanic gases at once. This will allow ratios of various gas species to be determined as well as emission rates for several different species to the atmosphere. This information may provide important clues useful in eruption prediction as well as estimates of the amounts of various gases released to the environment. The USGS Volcano Emissions and Global Change Project currently has a prototype FTIR under test.
In 1992, Volcano Emissions and Global Change Project began a program to systematically collect and analyze gases from potentially active volcanoes in the Pacific Northwestand Alaska. For each volcano studied, the USGS is (1) conducting reconnaissance studies of available fumaroles, and (2) collecting samples from selected fumaroles to determine the chemical and isotopic compositions of the gases. These data will help constrain the current status of degassing at these volcanoes and the compositions and origins of their volcanic gases. Frequent resampling of selected fumaroles may detect changes in the subvolcanic system, including the injection of new magma. Volcanoes studied to date include Mount St. Augustine, Mount Baker, Mount Hood, and volcanoes in Katmai National Park (Mount Griggs and Trident).
A few of the World's passively degassing volcanoes emit very high-temperature (>600 degrees Celsius) gases. Study of these hot volcanic gases is important because they constitute the bulk of the global emissions from passively degassing volcanoes, and, due to their probable magmatic source, these gases enable study of magmatic degassing. Ongoing work by Project scientists focuses on (1) collecting additional data on the bulk compositions of these magmatic gases, (2) determining the concentrations, origins, and emission rates of the 30-40 trace elements that are generally transported in these gases, and (3) reaction mechanisms in high-temperature gases. Two main goals of this work are to understand better the applications of major- and trace-gas sampling in detecting changes in the subvolcanic system and the impact of volcanic gas and metal emissions on the local environment.
Other topics currently under study include: research on volcanic SO2 scrubbing by water, development of improved methods for in-situ geochemical monitoring, studying the chemical structure of volcanic plumes, and measurements of air quality around volcanoes.
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05/24/04, Lyn Topinka