165-1 Determining the timing of magmatic unrest, ascent, and deposition in explosive silicic eruptions

Session: GSA Mineralogy, Geochemistry, Petrology, and Volcanology Division Awards Session

Presenting Author:

Author:

Abstract:

The timescales of magmatic unrest, ascent, and deposition in explosive silicic eruptions are obtained with varying degrees of success, where some timescales remain out of reach. Here I present several notable examples and highlight areas that require further scrutiny. In these systems, quartz has primarily been used to extract timescales of pre-eruptive unrest. With renewed debate surrounding the diffusion coefficient of Ti-in-quartz, our ability to constrain the timing of pre-eruptive disturbances is strongly challenged. By compiling diffusion modeling results for multiple explosive silicic eruptions (>70 wt.% SiO₂; n=9) we find that all minerals (qtz: n=235, san: n=124, opx: n=54, cpx: n=33) consistently return decadal (qtz; cpx) to centurial (san; opx) timescales. This observation suggests that most, if not all, rhyolitic systems have a similar tipping point, although authors attribute these results to different processes. Further experimental testing could provide clarity. After magma ascent begins, water measurements in quartz-hosted melt inclusions have revealed that diffusive water loss can occur on the order of days, suggesting slow initial ascent from the storage region in some systems. The application of the melt embayment (open melt inclusion) to 14 separate rhyolitic eruptions suggests that final magma ascent rate (pre-fragmentation) does not vary based on erupted volume or VEI, unlike what is observed in less evolved systems. The development of new numerical approaches coupled with experimental validation has provided clarity surrounding the fidelity of embayments as recorders of ascent under varying conditions. Finally, detailed studies of field deposits continue to add nuance to earlier models of caldera formation. In Yellowstone, frequent evidence is observed for brecciation or deformation of earlier cooled ignimbrite flows that are later overlain by an indistinguishably aged but visually distinct eruption unit. These timescales are the most challenging to access, with most techniques unable to resolve the likely years to decades hiatus recorded by the physical characteristics of the deposit. This combined work highlights the range and complexity of timescales recorded in silicic systems, while also revealing remarkable consistencies in how these systems work. Moving forward, we are challenged to confirm that the recovered timescales are not limited by the tools applied and are in fact truly meaningful.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4301

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