Bubble Coalescences Found in a Scoria from 2014-2015 Aso Eruption

Abstract:

November 2014, Aso Volcano resumed the eruption after approximately 20 years quiescence. The main activity was ash eruption with silent large plumes, but Strombolian eruption of spouting scoriae without ash also was observed. Most of scoriae are highly vesiculated, and have low density (Yokoo and Miyabuchi, 2015). In order to understand the evolution of bubble texture, I observed the scoriae in three methods, the microscope, CT scan, and SEM those can observe different scales of 10 mm, 1 mm, 10μm, respectively.

The microscope images show that larger bubbles (10mm) are surrounded by small bubbles (< 1mm), which shows elongated structure and suggesting that deformed by the large bubbles. CT images also show that larger bubbles (1mm) are surrounded by small bubbles (100μm). SEM images show that the bubble film thickness is approximately 1μm or less.

 According to the observation of pumices in other volcanoes, the typical bubble film thickness is estimated to be about 0.1-1μm (Nguyen, et al, 2013). In general, the volume fraction of bubbles and number density determine the thickness of the bubble films. For a smaller number density of bubbles, bubble size becomes larger in a same bubble volume fraction, so that the bubble film becomes thicker. In other words, the bubble films in the bubbly magma with a large number density reach the minimum thickness at a lower degree of bubble fraction. As a result, the bubble films rupture, or by coagulation, the larger bubble assimilates small bubbles. By the increase of the bubble size, thickness of the bubble film increases to be larger than the minimum thickness of 1μm. If such coalescences occur, the bubble size must have variety. From the varying bubble size and 1 1μm film thickness of scoriae, I infer that bubble coalescence occurred in the ascending bubbly magma in the conduit.