BREADCRUST BUBBLES: ESTIMATING EXPLOSIVE ERUPTION EXIT VELOCITY THROUGH TEXTURAL ANALYSIS

Hollow glass spheres with tensional fractures are present in deposits of the ~280 km³ (DRE) 7.05 Ma Rattlesnake Tuff. These rhyolite ash particles, termed “Breadcrust Bubbles” or “BBs,” are typically 400-1000 μm in diameter and compose ~3 wt.% of the 0.6-2.0 mm size fraction of non-welded deposits. Particles typically have 10 to >30 fractures that each accommodate ~1% areal expansion of the sphere surface; these fractures crosscut one another such that they must have formed through a series of discrete brittle failure events. The BBs often have flanges or “fins” protruding from their sides, suggesting that the spheres were once individual cells of a foam. Morphological measurements of BBs collected with SEM and synchrotron-based μ-XRCT show that the shells have thicknesses on the order of ~15-35 μm and the flanges have average thicknesses of ~35 μm. We interpret these particles to record post-fragmentation decompression of individual cells (bubbles) from an incompletely fragmented magmatic foam. As the particles accelerated up the conduit, their thick shells controlled expansion and pressure equilibration. Pressure gradients between the interior and exterior drove viscous or ductile expansion; but at high decompression rates, tensional stress at the particle exteriors exceeded the magma tensile strength, resulting in brittle fracture of the outer shells, followed by ductile deformation. The number of fracture events, the total BB expansion, and the amount of expansion accommodated by fracture opening and viscous flow can be modeled as functions of magma viscosity, the ratio of initial shell thickness to bubble radius, the initial depth (setting the initial pressure), and the rate at which the particles accelerate towards the surface. Given our textural observations and the temperature estimate of ~850°C (Streck and Grunder, 2008), at least some portion of the magma that erupted to form the Rattlesnake Tuff fragmented from a thick-walled foam with a vesicularity of 50-75 vol.% at a depth of 500-2000 m. These particles accelerated up the conduit at a rate of 1-3 m/s² to erupt from the vent at 30-80 m/s. The presence of BBs in tuff deposits can be used to infer shallow fragmentation of thick-walled foams, and these particles provide a unique means of estimating explosive eruption velocity.