May 15, 2020 Mary Gates Hall

boruco’s magmatic system before the event. Phase equilib- SESSION O-1I ria modelling suggests re-equilibration of orthopyroxenes ap- proximately in the upper 7 km during magma ascent. The LITHOSPHERETO BIOSPHERE: compositional and textural variability indicate magma mix- ing, which is a common process leading to explosive activity VOLCANOES,GLACIERS,CLIMATE in other TMVB volcanoes such as Popocatepetl´ and Volcan´ CHANGE, AND INSECTS de Colima. Session Moderator: Kirsten Nicolaysen, Geology 11:00 AM to 12:30 PM * Note: Titles in order of presentation.

Lavas, domes, and beetles: Probing the magmatic processes leading to the 1870-5 CE eruption of Ceboruco Volcano, Mexico. Gustavo Bejar Lopez, Senior, Geology, Whitman College Mentor: Kirsten Nicolaysen, Geology, Whitman College Mentor: Nick Varley, Universidad de Colima

As one of the westernmost stratovolcanoes in the Trans- Mexican Volcanic Belt (TMVB), Ceboruco exhibits a wide range of compositions correlating to its variable eruption style. With around 55,000 people residing within 10 km to the vent, Ceboruco represents an important hazard. Magma- tism in the Ceboruco region is the consequence of both com- pressional and extensional tectonic regimes: the small Rivera plate subducts under North America, and regional crustal ex- tension promotes the formation of three intersecting grabens in western Mexico, one of which hosts Ceboruco. In this tec- tonic setting, compositions are variable, and types of eruption fluctuate between moderately-effusive ones, like Ceboruco’s 1870-5 CE event, to large Plinian activity, like its caldera- forming eruption 1,060 ± 55 yr BP. Historically documented, the 1870-5 CE eruption led to the formation of a 7.5 km long lava flow, a small plug dome, and a larger dome complex (El Escarabajo). All these lavas exhibit the same trachydacitic composition (˜68 wt.% SiO2, ˜9 wt.% Na2O+K2O), signal- ing that they were derived from the same magma batch. As the eruption sequence and the subsurface pre-eruptive condi- tions remain unexplored, I probe those processes using bulk and mineral compositional analyses, microtextural observa- tions, and phase equilibrium modelling. Higher crystallinity and higher Ba abundance in the dome complex suggest its derivation from the denser, deeper portions of the magma body hinting that El Escarabajo erupted late in the 5-year pe- riod. Reverse zoning in plagioclases (An67 − 72) and Ca- depleted rims in clinopyroxenes indicate perturbation in Ce-

Undergraduate Research Program 1 www.uw.edu/undergradresearch