Eruption forensics: Deciphering the imprint left by the conduit-flow regime on individual crystals through multi-scale, multi-physics models

Abstract:

One of the fundamental problems in volcanology is to infer the complex magmatic processes occurring prior to eruption from limited data. Crystals trapped in magma quenched upon eruption record many aspects of the pre-eruptive condition in the volcanic conduit directly. However, we rarely leverage this crystal-scale data to test physical models of eruptive processes, partly because models typically operate at the scale of tens to hundred kilometers and hence do not entail testable model predictions at the scale of individual crystals.

In this talk, I discuss how crystal-scale models can help us decipher the subtle yet distinctive imprint of pre-eruptive conduit-flow conditions on individual crystals and, in some cases, derive specific, quantitative attributes of the conduit flow field from petrographic observations. To demonstrate the potential of our multi-scale approach, we study olivine aggregates contained in scoria samples erupted in 1959 at Kilauea Iki, Hawaii. A puzzling aspect of these aggregates is that crystals within the aggregate are separated by large misorientation angles that are seemingly unfavorable from a hydrodynamic perspective. While known for over 50 years, a physical explanation for why crystals aggregate in this way has remained elusive.

Here, we argue that the observed misorientation angles in the Kilauea Iki aggregates are the consequence of the crystals’ exposure to a traveling wave in the conduit prior to eruption. This wave arises naturally in bidirectional conduit flow at the interface between bubble-rich, buoyant magma ascending and degassed, heavy magma descending, but is absent in unidirectional flow. Apart from enabling us to test the compatibility of different conduit models with petrographic data, we find that the wave required for reproducing the observed misorientation angles propagate downwards into the conduit. We briefly discuss the potential implications of this finding for the dynamics of the eruption.

For the specific context of Kilauea volcano, our analysis suggests that monitoring misorientation angles of individual olivines in erupted aggregates over time might help us monitor and better understand the processes driving the waxing and waning of lava fountaining.