Textural Characteristics of Pumice from a Postglacial Plinian Eruption at Laguna del Maule, Southern Chile: Implications for Magma Ascent Dynamics

At the Laguna del Maule volcanic complex in Southern Chile, one of the most active volcanic systems in the world, geologists are trying to better understand its volcanic history to make predictions about future volcanic hazards. Eruptive compositions range from basalt to rhyolite and resultant eruption styles include both lava flows and widespread pumice and ash-producing explosive eruptions. In this paper, I quantify the textures and morphologies of over 150 pumice clasts from a pyroclastic flow deposit from a postglacial plinian rhyolite eruption at Laguna del Maule (the Rhyolite del Maule deposit). Analyses indicate that clasts are texturally heterogenous, with vesicularities ranging from 55 – 95%. Pyroclasts of extremely high vesicularities (e.g. reticulite, \u3e 94% vesicularity) and low bubble number densities (102 cm-2, 103 cm-2) were found. Pyroclasts of this textural type are typically produced from low explosivity, Hawaiian style eruptions, where bubble overpressures are low due to relatively slow ascent from shallow depth. In contrast, Plinian eruptions are high explosivity eruptions, where bubble overpressures are high, driving high eruption rates from great depth. Thus, we are presented with a paradox for a plinian rhyolite eruption that produced high vesicularity clasts with low bubble number densities. We conclude that the answer lies in the heterogeneity of the deposit, which contains a wide range in clast textures due to a variety of nucleation and growth events reflecting a variety of pre-eruptive storage depths and syn-eruptive ascent rates