Lithic fragments of precaldera basement rocks in the Plinian fallout deposit of the Bishop Tuff indicate that the eruption began in what is now the south-central part of Long Valley caldera, along or adjacent to the Hilton Creek fault. The earliest ash flows originated there as well, but contrasting lithic contents in several later ash flows indicate ring-fault propagation and incorporation of fragments of locally distinctive, caldera-margin lithologies into successive outflow sheets. The change from a single-vent Plinian mode of eruption to multiple vents along the ring fault took place after emplacement of as little as 20% of the total Bishop ejecta. Approximately two-thirds of the total eruptive volume was trapped within the subsiding caldera.

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