HR: 17:15h
AN: V24B-06    [Abstracts]
TI: The Dynamics of a Strombolian Bubble Burst Derived From Doppler Radar.
AU: * Gerst, A
EM: Alexander.Gerst@zmaw.de
AF: Institute of Geophysics, University of Hamburg, Bundesstrasse 55, Hamburg, 20146, Germany
AU: Hort, M
EM: Matthias.Hort@zmaw.de
AF: Institute of Geophysics, University of Hamburg, Bundesstrasse 55, Hamburg, 20146, Germany
AU: Johnson, J B
EM: jeff.johnson@unh.edu
AF: Department of Earth and Environmental Science, New Mexico Tech, Soccorro, NM 87801, United States
AU: Kyle, P R
EM: kyle@nmt.edu
AF: Department of Earth and Environmental Science, New Mexico Tech, Soccorro, NM 87801, United States
AB: The exact mechanism of strombolian explosions is unknown to the present day, and almost no information has previously been gained on pressures and energies involved during an explosion. Erebus is one of the few volcanic open vent systems that allows a direct observation of these processes, as it contains a convecting phonolite lava lake, which is presumably connected to a magma chamber at depth. It is the source of frequent violent strombolian explosions, caused by large gas bubbles bursting at the lake surface, which had a diameter of ~40 m in 2005/06. We use data from a deployment of three continuous wave Doppler radar instruments at Mt. Erebus volcano, Antarctica, to derive information on the dynamics of Strombolian eruptions, including gas pressures, volumes, energies, directivity, and the physical structure of the volcano. The radar system consisted of three 24GHz Doppler radar instruments, and was deployed on the crater rim from November 2005 to January 2006. 50 large explosions were recorded from three different angles. The use of 24GHz (K-Band) microwaves allowed for a continuous and undisturbed observation of the source of these strombolian explosions, i.e. observing the top part of a magma column, independent of meteorological or visibility conditions, and unaltered by the atmosphere. We develop a preliminary simple model describing the bubble expansion from the time when the bubble reaches the surface of the lake until the time of burst. The expansion velocity of the expanding magma shell was measured with a sampling rate of 15 Hz. This enables us to retrieve information about the internal mechanism of the bubble burst, including estimates of the absolute gas pressure and the volume of gas. Data for a typical explosion suggest that the overpressure in the bubble just before the burst is in the range of 3 to 8 atmospheres, with a volume of 1000 to 10,000 m³. An energy budget for the same explosion will be shown, allowing an estimate of the relative and absolute partitioning of different energy types evolving with time. The estimated total power output exceeds 10¹⁰ W for a short time, with a total energy output of roughly 10¹⁰ J. Additionally, the relatively high sampling rate enables us to calculate the expected infrasound signal created by a bubble burst. This allows for a direct comparison to real infrasound data recorded on the crater rim, verifying the structure of our preliminary model. A combined processing of data from all three radars enabled us to calculate time series of 3D directivity vectors for 10 explosions, which describes the direction of preferred expansion of the bubble during an explosion. Such directivity information allows a comparison to dipole infrasound radiation patterns recorded during the explosions, therefore verifying interpretations deducted from infrasound recordings. Video observations of explosions confirm the directivity measurements. We conclude that at Erebus, the directivity of explosions is mainly controlled by random processes, suggesting a highly symmetrical uppermost conduit system with a vertical axis of symmetry.
UR: http://www.geophysics.zmaw.de/
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8419 Volcano monitoring (7280)
DE: 8428 Explosive volcanism
DE: 8494 Instruments and techniques
DE: 9310 Antarctica (4207)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting