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Aaron Marshall Dissertation Defense

July 1 @ 9:00 am - 11:00 am MDT

Aaron Marshall

The Curacautín eruption of Llaima volcano, Chile

Abstract: Mafic magmas are the most common magmas erupted on Earth and the Solar System. The low viscosity of mafic magmas results in eruptions that are primarily effusive to mildly explosive. Rarely, mafic magmas erupt as more violent, explosive events, and the causes of this transition in eruptive style is hotly debated. In this dissertation, I investigated the conditions in the conduit and shallow subsurface that generated the unusually explosive mafic, Curacautín eruption of Llaima volcano, Chile. The Curacautín ignimbrite (Ci) is a basaltic andesite ignimbrite consisting of four flow units of variable thicknesses. New 14C dates for five Ci exposures returned ages of ~12.6 ka suggesting the Ci was generated in a single eruptive event. Using new methods for volume estimation, I calculated a volume of 3.5–4.5 km3 DRE for the Ci. Pyroclast textures, including moderate vesicularities and high microlite number densities suggest rapid magma ascent rates prior to eruption. I calculated timescales of crystallization for Ci plagioclase microlites of <10 s to ~5 hrs using crystal size distribution (CSD) theory. To further test the rapid ascent hypothesis, I modeled plagioclase nucleation and growth rates of 6.1105 cm-3 hr-1 and 27.4 µm hr-1 for the Ci. I used these rates to conduct Monte Carlo simulations for Ci plagioclase CSDs and calculated ascent rates from <1–6 m s-1, further supporting the rapid ascent hypothesis. I was unable to produce the smallest size populations of plagioclase microlites. Finally, I observe textures consistent with the autobrecciation and welding of protopyroclasts prior to eruption. I call this new process fusing and suggest it records conduit conditions not previously considered in mafic eruptions. Size-restricted broken plagioclase crystals record fragmentation and secondary, post-fragmentation crystallization, which may be the cause of our inability to produce the smallest size population of plagioclase within the model. These observations have important implications for how we interpret the deposits of explosive eruptions. This research supports other work that suggests rapid magma ascent is the primary driver for highly explosive mafic eruptions in the absence of external water. Because people are living on or near volcanoes that erupt mafic magmas in greater numbers, it is paramount that we understand what causes these systems to transition in eruption style.

Advisor: Brittany Brand

Co-Advisors: Dorsey Wanless, Mark Schmitz, Michael Manga, Benjamin Andrews