The influence of crystals on the movement of bubbles through basaltic magmas is poorly understood. We study the interaction of bubbles with a suspension of crystals in a viscous fluid through analog experiments. In our experiments, an air bubble rises through a suspension of plastic beads in a viscous corn syrup - water mixture; we vary bubble volumes, crystal spacings and fluid viscosities. We observe the following change in interaction styles with increasing bubble volume: (1) bubble migration through the crystal network with little bubble deformation, (2) bubble movement through the crystal network with deformation (and sometimes bubble splitting), and (3) displacement of the liquid-crystal mixture by the rising bubble. Interactions change from type (1) to (2) when the bubble is approximately the same size as the crystals forming the network. Transition to type (3) behavior depends on both bubble volume and the thickness of the crystal-liquid layer. In all cases, bubble rise is impeded by the presence of crystals. Preliminary results suggest that impedance is most pronounced for bubbles slightly larger than the crystals (a condition that promotes the maximum bubble deformation). Additionally, very small bubbles may be trapped for long times in the crystal network, suggesting that a shallow reservoir of crystal-rich magma may actually trap rising bubbles from below. These observations provide an alternative interpretation to that of small undeformed bubbles representing late-stage bubble nucleation and large irregularly shaped bubbles forming by coalescence of smaller bubbles (e.g. Lautze and Houghton, 2006). Furthermore, we observe in our experiments that large bubbles can spread out and move laterally underneath a crystal layer. This is not usually considered in models of bubble migration and may explain focusing of gas escape from magma reservoirs and volcanic vents. We apply our experimental results to analysis of bubble populations at Stromboli volcano, Italy, where gases rising from a deep crystal-poor magma reservoir travel through, and entrain, shallow crystal-rich magma. Preliminary results from image analysis on SEM and optical microscope images suggest that the smallest bubbles are most abundant and that their sizes are within the modal size range of the crystals. This can mean that (i) smaller bubbles were initially more abundant or (ii) bigger bubbles have deformed and split into smaller bubbles, which would confirm our conclusion from the analog experiments that interactions change from type (1) to (2) when the bubbles reach the size of the crystals in the network. In addition, the abundance of crystal-size bubbles in these samples suggests relative accumulation, possibly through trapping or extreme impedance, of the bubbles within the crystal-rich layer. Combined with our observation (from the analog experiments) that most bubbles are indeed significantly slowed within the crystal layer, this could suggest that bubble number densities are not direct reflections of bubble nucleation rates.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 8499 General or miscellaneous