Andesite magma genesis, conduit dynamics and variable decompression from shallow reservoirs drive contrasting PDC events at Volcán de Colima, Mexico

Abstract

Volcán de Colima, one of the Earth's most active and hazardous andesitic stratovolcanoes, experienced its last major explosive eruption on July 2015 with two distinct events. The July-10 event comprised the collapse of a large summit dome, forming block-and-ash flow deposits of dense dome lava clasts. The July-11 event produced pyroclastic density currents (PDCs) that flowed to maximum 10.5 km-distance from the crater, burned the vegetation, and formed valley and overbank deposits of up to 30 vol% vesicular scoria clasts. Whereas the July-10 event fits well within the last 20 years' typical activity of Volcán de Colima, the July-11 PDCs were unprecedented. We present a close examination of the July-11 deposits via combined field, microtextural, and chemical analyses, including electron microscopy and X-ray microtomography. Results show that andesite magma (58–59 wt% SiO2) at 1021 °C and having 2 wt% H2O degassed and crystallized (102–105 mm−3 vesicles and 102–103 mm−3 microlite number densities) while ascending to the surface from ∼2 km-depth reservoirs, driven by decompression rates of 0.4–1.7 MPa s−1. These variable rates reflected heterogenous andesite magma rheology produced by different stages of melt genesis, mobility, stalling, crystallization and vesiculation. Prior to experiencing fragmentation, the andesite magma mingled with rhyolite melts produced from partial melting of silicic mush stored at depths from ∼2 to 5 km. Magmas fragmented at maximum strain rates of 10−3 s−1, powering the July-11 energetic (106–107 kg s−1) and pulsating PDCs that released 102–103 m3 s−1 on surface. The rapid <20 h transition from the July-10 to the July-11 events suggests only hours timescales from dome collapse to magma decompression and explosive eruption.