Rise and Burst of Overpressured Bubbles in a Particle-Rich Suspension: Consequences for Interpretation of Normal Activity at Stromboli Volcano, Italy.
Abstract
Normal activity at Stromboli is characterized by short, intermittent, and pulsatory eruptions that generate fountains of pyroclasts. These eruptions are generally explained as Taylor bubbles rising to the surface of a liquid-filled conduit. However, eruptions at Stromboli generate magma fragments that contain 45 - 55 vol% crystals on average, evidence of a crystal-rich layer near the surface. To explore how the presence of crystals may affect interpretation of Strombolian eruptions, we performed a series of analogue experiments where large bubbles of different volumes rise in a 25 mm diameter vertical tube. The tube is filled to 1.4 m with silicone oil surmounted by an oil-and-particle layer (the cap) of varying thickness (3 - 55 cm) and particle content (0 - 43 vol%). The particles are irregular and polydisperse, with average diameter 0.8 mm and a random close packing of 47 ± 2 vol%. The vertical pressure gradient in the tube is scaled by reducing the pressure at the liquid surface with a vacuum pump. At particle contents above 30 vol%, the cap became non-Newtonian and caused significant bubble deformations during rise in the cap. When bubbles reached the surface, they burst on the side (rather than the centre) of the tube, and generated a fountain of ejected clasts, reminiscent of those observed at Stromboli. We analysed pressure variations in acoustic signals emitted during burst for comparison with field observations. For a given bubble size and in the Newtonian regime (below 30 vol% particles), the peak excess pressure increases with particle fraction, consistent with increasing cap viscosity. However, in non-Newtonian caps, the peak excess pressures for a given bubble size were significantly lower and depended little on particle fraction. The total energy emitted was the same, but the signals lasted longer and were more ragged. Pulsatory signals occurred in experiments with the highest particle fractions in the cap, larger bubbles, and greater bubble expansions. We surmise that the gas is released gradually through a smaller aperture. In conclusion, the presence of a near-surface crystal-rich layer at Stromboli can significantly affect bubble migration and burst dynamics.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V23K0184O
- Keywords:
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- 0545 Modeling;
- COMPUTATIONAL GEOPHYSICSDE: 7280 Volcano seismology;
- SEISMOLOGYDE: 8428 Explosive volcanism;
- VOLCANOLOGY