Abstract
The deposition of large-scale pyroclastic flows (LPF) as part of caldera collapse is a key factor in understanding Crater Lake caldera, Oregon (8×10 km), one of the best examples of crater collapse in the world. The caldera, which formed at 6845 yr B.P., is intermediate in size between a smaller central conduit and a larger ring conduit system. Analyses of lithic components in the ejecta confirm that the climactic ignimbrite was ejected from multiple conduits along a ring fracture. This finding demonstrates that in a caldera such as Crater Lake, the structural resurgence may be too small to act as a shallow stress source; nevertheless, such calderas may have a ring conduit system.
Two types of deformation, ooze-outs and squeeze-outs of fiammes, occur in the Wineglass Welded Tuff (WWT) on the northeast caldera margin, produced by concentric normal faulting and landsliding at the time of caldera collapse. Calculations regarding the conductive cooling history of the WWT and the preclimactic Cleetwood lava flow (CLF) yield the following results: (a) a maximum interval of 9 days from emplacement of the WWT to caldera collapse, and (b) less than 100 years from effusion of the CLF to the onset of the climactic eruption.
The size of LPF units relative to the total eruptive volume appears to be unrelated to caldera size. The flow unit is not necessarily easily identified in the case of an LPF. It is possible that during caldera collapse, the decompression rate of the magma reservoir increases in the case of a smaller number of flow units resulting from a higher eruption rate. We propose that a Valles-type caldera is the most likely candidate in terms of generating this feedback process, since its ring conduit system was stable over time and requires less kinematic energy during caldera collapse than does a funnel caldera.
