SECONDARY EXPLOSION PITS ON THE BANCO BONITO RHYOLITE LAVA FLOW, NEW MEXICO

Rootless craters pepper the surface of the Banco Bonito Rhyolite Lava Flow (68 ka), one of the youngest eruptive elements of the Valles Caldera system in New Mexico. Their formation mechanism is not well understood, nor is the implication for the eruptive history of the Valles Caldera. Here we investigate the mechanism that formed these rootless craters and quantify the associated explosive conditions using remote mapping and targeted fieldwork.

The Banco Bonito (up to 149 m thick) exhibits textural zonation with depth, ranging from coarsely pumiceous to glassy. This variation with depth provides insight into the explosion mechanism. A porous horizon described as a coarsely vesicular pumice, capped by an impermeable obsidian layer, may have acted as a reservoir for steam accumulation and explosive overpressurization. Classified airborne laser scanning data, collected in a 2010 survey and gridded at 1 m raster resolution as a digital terrain model, provide an opportunity to morphometrically characterize each explosion pit and probe this potential explosion mechanism.

Preliminary mapping reveals >50 explosion pits ranging from 50 to 184 m in diameter and 10 to 62 m in depth, with volumes on the order of 104 to 106 m³. We observe a positive correlation between pit depth and diameter, perhaps reflecting the greater gas mass required to drive an explosion from greater depths. Nearly every pit was ringed by a debris apron up to 10 m in height and composed of coarse ejecta from the interior of the lava flow. Preliminary measurements show that debris aprons make up ~10 - 20% of the evacuated pit volume, suggesting the remaining material was ejected beyond the apron. The measured depths were used to estimate explosion pressures; results show that explosion pressures were 1 MPa or greater, with final values varying according to source depths at individual craters and tensile strength of the impermeable obsidian cap. We also modeled the explosions that formed the pits as thermals (instantaneous releases of buoyancy) to estimate rise heights of corresponding explosion plumes. Initial estimates suggest rise heights would have ranged from 1.5 to 7 km above the lava flow surface, with results strongly dependent upon lava flow temperature at the time of the explosions and the total lofted volume of fragmented rhyolite.