Thesis Defense: Sahand Hajimirza, Ph.D. Candidate
Department: Earth, Environmental and Planetary Sciences
Defense Date: Monday, April 13th, 2020
Time: 12:30 p.m.
Eruptive conditions in explosive rhyolitic eruptions: constraints from bubble number density
Rhyolitic eruptions are among the most explosive eruptions on earth with hazardous regional and global impacts. The behaviour of such eruptions is controlled by the subsurface processes during magma ascent which are inaccessible to direct observations, and thus needs to be inferred from eruption products. Common to all these eruptions are highly vesiculated pyroclasts with bubble number densities that exceeds 1 million bubbles in 1 mm3 of glass. These bubbles form predominantly during magma ascent through shallow volcanic conduits and are thought to record eruptive conditions. The object of this dissertation is to infer eruptive conditions, in particular magma ascent rate, in explosive rhyolitic eruptions from observed bubble number densities. This dissertation is organized in three chapters. First, a nucleation model is developed based on classical nucleation theory. Given that surface tension is the key in nucleation, it is shown that current surface tension measurements for bubble nuclei are not capable of describing nucleation rate in decompression experiments. Using the well-known Tolman correction, a surface tension model is defined that allows the simulation to reliably predict experimental bubble number densities. Second, the simulation predictions on nucleation kinetics which suggest that nucleation is a continuous process and potentially last for minutes is tested. Experiments with identical initial and final pressures that were quenched at different times reveal that nucleation continued in experiments for several minutes and thus validate the simulation predictions. Third, nucleation in several most explosive rhyolitic eruptions on record is simulated. Simulation results suggest that nucleation in these eruptions were heterogenous that is it was facilitated by presence of Fe-Ti oxide crystals in nano scale. Ascent rates of magma estimated by heterogeneous nucleation suggest that in rhyolitic explosive eruptions magma travels through volcanic conduits with a rate of, on average, 1-100 m/s, which is consistent with independent estimations.