HR: 09:15h
AN: V11F-06    [Abstracts]
TI: From the Sound of Erta Ale Lava Lake (Ethiopia) to Eruption Dynamics Into a Magma Reservoir
AU: * Bouche, E
EM: bouche@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252, France
AU: Vergniolle, S
EM: vergniolle@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4 place Jussieu, Paris, 75252, France
AB: The basaltic volcano of Erta Ale, located on the East African Rift, has a permanent lava lake whose behaviour presents similarity with a shallow magma reservoir. In March 2003, continuous measurements of acoustic pressure, images from video, temperature, seismicity and wind velocity were perfomed to quantify degassing of the lava lake in order to understand the eruptive behaviour of this volcano. The videos show that two types of gas bubbles break at the lava lake surface. Modelling acoustic pressure gives bubble overpressure and size. Bubbles are either large (radius 2 m) and overpressurised (4.10⁴ Pa) or of intermediate size (radius 1 m) and weakly overpressurised (450 Pa). The large bubbles come from the conduit at the base of the lava lake whereas bubbles of intermediate size are produced by the destabilisation of a foam accumulated below the crust overlying the lava lake. Hence, their overpressure is related to capillary pressure of the rising small bubbles, suggesting that their diameter is 3.6 mm. The formation of bubbles of intermediate size is related to the local foam coalescence because of foam sluggish drainage. However, overpressure of intermediate size bubbles shows sudden peaks every eighteen hours, up to 6000 Pa. Each peak is related to a massive coalescence of a foam having reached its critical thickness. This involves a much larger number of bubbles than foam drainage, hence a much larger overpressure and energy. The rapid and massive coalescence leads to a sudden withdrawal of the foam. The disappearence of the foam suppress the buoyancy that sustained the cold and dense crust at the top of the lava lake, forcing the crust to sink. The average gas flux (6.10^-3 m³s^-1) is estimated over an eighteen-hour cycle from modelling the frequency of sound waves. Furthermore the diameter of the small bubbles deduced from the overpressure on synthetic waveforms can be combined with gas volume fraction observed on videos to estimate the gas flux between 3.5.10^-3 m³s^-1 and 7.10^-3 m³s^-1. The excellent agreement between these two independant methods reinforces the validity of our approach in the understanding of the physical processes at work in the lava lake.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8430 Volcanic gases
DE: 8445 Experimental volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting