Basaltic thermals and Subplinian plumes: Constraints from acoustic measurements at Shishaldin volcano, Alaska

Bulletin of Volcanology
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Abstract

The 1999 basaltic eruption of Shishaldin volcano (Alaska, USA) included both Strombolian and Subplinian activity, as well as a “pre-Subplinian” phase interpreted as the local coalescence within a long foam in the conduit. Although few visual observations were made of the eruption, a great deal of information regarding gas velocity, gas flux at the vent and plume height may be inferred by using acoustic recordings of the eruption. By relating acoustic power to gas velocity, a time series of gas velocity is calculated for the Subplinian and pre-Subplinian phases. These time series show trends in gas velocity that are interpreted as plumes or, for those signals lasting only a short time, thermals. The Subplinian phase is shown to be composed of a thermal followed by five plumes with a total expelled gas volume of 1.5×107m3.

The initiation of the Subplinian activity is probably related to the arrival of a large overpressurised bubble close to the top of the magma column. A gradual increase in low-frequency (0.01–0.5 Hz) signal prior to this “trigger bubble” may be due to the rise of the bubble in the conduit. This delay corresponds to a reservoir located at ≈3.9 km below the surface, in good agreement with studies on other volcanoes.

The presence of two thermal phases is also identified in the middle of the pre-Subplinian phase with a total gas release of 4.3×106mand 3.6×106m3. Gas velocity at the vent is found to be 82m.s1 and 90m.s1 for the Subplinian plumes and the pre-Subplinian thermals respectively.

The agreement is very good between estimates of the gas flux from modelling the plume height and those obtained from acoustic measurements, leading to a new method by which eruption physical parameters may be quantified. Furthermore, direct measurements of gas velocity can be used for better estimates of the SO2 flux released during the eruption.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Basaltic thermals and Subplinian plumes: Constraints from acoustic measurements at Shishaldin volcano, Alaska
Series title Bulletin of Volcanology
DOI 10.1007/s00445-005-0035-4
Volume 68
Issue 7
Year Published 2006
Language English
Publisher Springer International
Description 20 p.
First page 611
Last page 630
Country United States
State Alaska
Other Geospatial Shishaldin volcano
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