PYROCLASTIC OBSIDIAN IN THE 1100CE GLASS MOUNTAIN TEPHRA AND THE PULSATORY NATURE OF RHYOLITIC DIKE-FED ERUPTIONS

The 1100CE Glass Mountain (GM) eruption at Medicine Lake Volcano (California, USA) started with the emission of rhyolitic pyroclasts along a 5km-long fissure system. The explosive phase was followed by extrusion of a series (≥10) of small domes from satellite vents along the fissure, and the eruption of a voluminous flow sequence from a central vent. Proximal deposits contain >20 eruptive subunits containing various amounts of pyroclastic obsidian. Subsets of pyroclastic obsidian were selected for microtextural, chemical, and volatile analyses, display abundant evidence for ash aggregation, sintering/remelting, and contain relatively low amounts of H₂O (≤ 0.8 wt.%). Ash aggregates contain both juvenile and foreign material. Vesicles in the glassy regions are not particularly elongate, and microlites show strong to no preferred orientation, contrasting with observations made in obsidian from Newberry or in North Mono eruptions. Altogether, these features suggest the GM pyroclastic obsidian were part of a magma that had slowed or stalled in the conduit, which was pervasively fractured to allow outgassing (formation of tuffisites), and destroyed during subsequent explosive pulses. This interpretation is also supported by field evidence for larger scale outgassing features (meter-wide tuffisites) containing variably rewelded breccia with dense obsidian and vesicular clasts. At GM, pyroclastic obsidian may thus represent the formation of conduit plugs or smaller carapaces in between eruptive pulses, thus illustrating the tendency for these smaller volume dike-fed silicic eruptions to wax and wane to a higher extent than their larger volume, central vent-fed counterparts.