Vent processes were examined at Karymsky volcano, Kamchatka, by measuring SO2 emissions using a correlation spectrometer (COSPEC). Continuous high-sensitivity COSPEC measurements and physical observations were collected on 11 and 12 September 1999, when Karymsky was producing small ashy eruptions every 5–20 min. Each eruptive event began with an explosion and audible rumbling (lasting ∼30 s) followed in some cases by audible chugging (lasting to 2 min). Gas plumes accompanied each event, and almost without exception the plumes dissipated and became invisible despite significant SO2 emissions. Variations in SO2 output show distinctive patterns that correlate with eruption activity. Maximum SO2 output occurred following each explosion and then declined rapidly to low, but detectable levels. In contrast, a second type of event, often associated with audible chugging, was characterized by high SO2 output long after the initial ash blast. Variations in degassing at Karymsky can be explained by secondary boiling, gas-pressure accumulation, and vent resealing. We developed a new application of the COSPEC technique to study the dynamic vent processes of erupting volcanoes. This application provides insights into the processes that occur at the otherwise inaccessible vents of erupting volcanoes particularly when a volcano changes from passive degassing and small explosions to degassing patterns that may precede a larger and more dangerous eruption.

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