Following 18 years of relative quiescence, Mount St. Helens volcano (MSH) became restless and began erupting again during September–December 2004. On 23 September, the U.S. Geological Survey's (USGS) David A. Johnston Cascades Volcano Observatory (CVO) and the Pacific Northwest Seismograph Network (PNSN) at the University of Washington detected the onset of a shallow earthquake swarm beneath the 1980–1986 lava dome. The Mount St. Helens Emergency Response Plan defines three alert levels that differ from normal background activity: Level 1, Notice of Volcanic Unrest (unusual activity detected); Level 2, Volcano Advisory (eruption likely but not imminent); and Level 3, Volcano Alert (eruption imminent or in progress).
Measurements of sulfur dioxide (S02) were made at the National Oceanic and Atmospheric Administration's Mauna Loa Observatory in Hawaii, during a 12‐month period beginning in December 1988. SO2 concentrations varied from background levels of less than 0.05 ppbv to a maximum of 50 ppbv, during episodes that lasted from 2 to 24 hours. Emissions from the Kilauea crater, approximately 35 km southeast of the observatory at an elevation of about 1000 m above sea level (asl), and the current eruption at Puu O′o 50 km east‐southeast, are the most likely sources for the higher concentrations. These episodes occurred 10–25 times each month, mostly during the day; peak concentrations were usually recorded at mid‐day. The SO2 concentrations can be grouped into three periods; low (June–September), high (October–January) and intermediate (February–May). A clear diurnal cycle of SO2 concentration exists throughout the year, although day‐night changes were greatest during October–January and were barely detectable during the June–September period. The highest SO2 concentrations were recorded when the predominant wind direction was northerly to northwesterly, even though the apparent sources are in the southeastern sector. Nighttime concentrations were usually at background levels; however, many exceptions were observed. A few cases of higher than background SO2 were observed when free tropospheric (FT) conditions were identified. The possibility that long‐range transport was the cause for elevated SO2 concentrations under FT conditions was examined using air mass back trajectories analyses. The highest nighttime SO2 concentrations, under FT conditions, were observed during periods with slow easterly trajectories, and the lowest concentrations were found during westerly flows. Twenty‐four nighttime free tropospheric events were recorded when the SO2 concentration exceeded 0.2 ppbv. During 18 of these episodes, unusually high CO2 concentrations were observed.
S ince 1980, scientists have monitored geologic unrest in Long Valley Caldera and at adjacent Mammoth Mountain, California. After a persistent swarm of earthquakes beneath Mammoth Mountain in 1989, geologists discovered that large volumes of carbon dioxide (CO 2) gas were seeping from beneath this volcano. This gas is killing trees on the mountain and also can be a danger to people. The U.S. Geological Survey (USGS) continues to study the CO 2 emissions to help protect the public from this invisible potential hazard. Invisible CO 2 Gas Killing Trees at Mammoth Mountain, California Areas of dead and dying trees Areas of dead and dying trees at Mammoth Mountain volcano total more than 100 acres. In 1990, the year after a persistent swarm of small earthquakes occurred beneath the volcano, U.S. Forest Service rangers first noticed areas of tree kill. When U.S. Geological Survey scientists investigated, they discovered that the trees are being killed by high concentrations of CO 2 gas in the soil. The seepage of CO 2 gas from below Mammoth Mountain and the continued occurrence of local earthquakes are signs of the ongoing geologic unrest in the area. The upper part of the 11,053-foot-high volcano (above 9,500 feet) is shown in darker shades of green.
In Roman mythology, Vulcan, the god of fire, was said to have made tools and weapons for the other gods in his workshop at Olympus. Throughout history, volcanoes have frequently been identified with Vulcan and other mythological figures. Scientists now know that the "smoke" from volcanoes, once attributed by poets to be from Vulcan's forge, is actually volcanic gas naturally released from both active and many inactive volcanoes. The molten rock, or magma, that lies beneath volcanoes and fuels eruptions, contains abundant gases that are released to the surface before, during, and after eruptions. These gases range from relatively benign low-temperature steam to thick hot clouds of choking sulfurous fume jetting from the earth. Water vapor is typically the most abundant volcanic gas, followed by carbon dioxide and sulfur dioxide. Other volcanic gases are hydrogen sulfide, hydrochloric acid, hydrogen, carbon monoxide, hydrofluoric acid, and other trace gases and volatile metals. The concentrations of these gas species can vary considerably from one volcano to the next.
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