ERUPTION DYNAMICS AND MAGMA WITHDRAWAL DURING THE PLINIAN PHASE OF THE BISHOP TUFF ERUPTION, LONG VALLEY CALDERA

Variations in eruption column height, magma discharge (intensity), and geochemistry have been documented for the plinian phase of the Bishop Tuff eruption from the Long Valley caldera, California. Although exposures of the complete Bishop fall deposit are sparse, their widespread relationships, knowledge of the plinian vent location, and a reasonable estimate of the main dispersal axis allow for the construction of minimum clast isopleths. Column height and intensity are estimated to have increased from 23 km and 7 x 10(7) kg s-1 to a peak of 45 km and 7.5 x 10(8) kg s-1 at the end of the plinian phase. Variations in geochemistry of the plinian deposit reveal complex magma withdrawal processes during the Bishop plinian phase. Concentrations of many incompatible trace elements increase from the base to midlevel and then decrease to the top of the plinian deposit. This trend is mirrored by the variation in pumice density, which in turn reflects vesicularity. Based on the physical volcanology and compositional variations in the plinian deposit, a model is proposed for the temporal progression of magma withdrawal during the plinian phase. Relatively less-evolved magma was first tapped by the plinian phase from the side of the magma chamber, with the conduit location being controlled by the Hilton Creek fault system. As eruption intensity increased, more-evolved magma from the chamber roof was drawn down and tapped. Less-evolved magma was again tapped for the duration of the plinian phase, possibly as withdrawal from the roof zone became less efficient. Renewed eruption of slightly more evolved magma by the first post-plinian ignimbrites suggests that the vent migrated along the ring fault system to an area that had not experienced previous magma withdrawal.