Pre-eruptive storage conditions and eruption dynamics of a small rhyolite dome: Douglas Knob, Yellowstone volcanic field, USA

Abstract

The properties and processes that control the size, duration, and style of eruption of rhyolite magma are poorly constrained because of a paucity of direct observations. Here, we investigate the small-volume, nonexplosive end-member. In particular, we determine the pre-eruptive storage conditions and eruption dynamics of Douglas Knob, a 0.011-km3 obsidian dome that erupted from a 500-m-long fissure in the Yellowstone volcanic system. To determine pre-eruptive storage conditions, we analyzed compositions of phenocrysts, matrix glass, and quartz-hosted glass inclusions by electron microprobe and Fourier-transform infrared analyses. The pre-eruptive melt is a high-silica rhyolite (∼75 wt.% SiO2) and was stored at 760 ± 30 °C and 50 ± 25 MPa prior to eruption, assuming vapor saturation at depth. To investigate emplacement dynamics and kinematics, we measured number densities and orientations of microlites at various locations across the lava dome. Microlites in samples closest to the inferred fissure vent are the most aligned. Alignment does not increase with distance traveled away from the vent, suggesting microlites record conduit processes. Strains of <5 accumulated in the conduit during ascent after microlite formation, imparted by a combination of pure and simple shear. Average microlite number density in samples varies from 104.9 to 105.7 mm−3. Using the magma ascent model of Toramaru et al. (J Volcanol Geotherm Res 175:156–157, 2008), microlite number densities imply decompression rates ranging from 0.03 to 0.11 MPa h−1 (∼0.4–1.3 mm s−1 ascent rates). Such slow ascent would allow time for passive degassing at depth in the conduit, thus resulting in an effusive eruption. Using calculated melt viscosity, we infer that the dike that fed the eruption was 4–8 m in width. Magma flux through this dike, assuming fissure dimensions at the surface represent its geometry at depth, implies an eruption duration of 17–210 days. That duration is also consistent with the shape of the dome if produced by gravitational spreading, as well as the ascent time of magma from its storage depth.