Transition from Effusive to Explosive Activity during Lava Dome Eruption: The Example of the 2010 of Merapi Volcano (Java, Indonesia)

Abstract:

Understanding the transition between effusive and explosive activity in dome-forming volcanoes remains a challenging question for eruption forecasting and eruptive scenario definition. The explosive activity of 26 Oct. and 5 Nov. during the 2010 eruption of Merapi volcano offers the opportunity to explore this transition by quantifying the mechanisms that led to the dome explosion.

Forty-three pumice samples were analyzed by 1) scanning electron microscope for textural analysis and 2) elemental analyzer for water content. The SEM images were processed so as to determine the proportions of gas bubbles, microlites and glass in each sample. These data were combined with the glass water content to feed the simple physical model developed by Burgisser et al. [1,2] to calculate pre-explosive pressure, depth, and porosity level for each pyroclastic pumice sample.

Preliminary results indicate that the water content in the melt is high, reaching 7 wt.%. These water contents yield a wide range of pre-eruptive pressures. Samples from 26 Oct. originated at pressures from a few MPa to 280 MPa. These pressures correspond to depths ranging from a few hundred meters to more than 10 km. This suggests that large overpressures were associated with conduit evacuation that reached unexpected depths. Samples from the 5 Nov. event range from ~10 to ~100 MPa. This suggests that this event also evacuated a large part of the volcanic conduit. Pre-explosive porosities of both events are low (<10 vol. %) along the depth of the entire conduit, which suggests extensive permeable outgassing of the magma-filed conduit prior to each explosive evacuation.

Ongoing work includes analysis of melt CO₂ content due to preliminary evidence that it played an important role in the 2010 Merapi eruption. The modeled conduit properties serve as baseline data for conduit flow modeling and building plausible eruptive scenarios.

[1] Burgisser et al. (2010) J. Volcanol. Geotherm. Res. 194, 27-41. [2] Burgisser et al. (2011) J. Volcanol. Geotherm. Res. 199, 193-205.