HR: 13:30h
AN: V33A-01 INVITED     [Abstracts]
TI: Volume-Time Relations in Large Silicic Volcanic Fields - Clues to a Thermomechanical Control on Eruption Frequency and Mechanism
AU: * de Silva, S L
EM: desilva@space.edu
AF: University of North Dakota, Department of Space Studies, Grand Forks, ND 58202 United States
AB: In assessing the hazard associated with large calderas it is crucial that the broader context of the formation, evolution, and eruption of these systems be factored into any assessment. The existing paradigm is that as large volumes of intermediate to silicic magma accumulate and evolve at high-level, a pressure build-up during second boiling results in overpressures of 20 - 25 MPa that result in the fracture of the magma chamber roof and consequent eruption. This may not be the case for the largest ignimbrite eruptions. These are typically associated with a regional ignimbrite flare-up and we should consider a more holistic approach that takes into account the spatiotemporal and volume-time evolution of the entire flare-up in understanding the operation of these systems. I illustrate this with a case study of the Altiplano-Puna Volcanic Complex of the Central Andes (APVC). The timing, pattern, and volumes of ignimbrite volcanism in the APVC reveal: 1) Pulsing of the ignimbrite eruptions with an approximate two million year period; 2) A trend to larger volume eruptions climaxing at about 4 Ma; 3) Migration and focusing of activity toward the central part of the APVC with time; and 4) Markedly diminished activity since 4 Ma. These observations suggest that the ignimbrite flare-up is result of progressive thermal (and mechanical) maturation of the crustal column due to intrusion and batholith formation and attendant effects on lithosphere strength. The progressive erosion of crustal strength results in failure of the crust and catastrophic eruption. Examination of available data from other large silicic volcanic provinces through space and time reveal a general pattern similar to that shown by the APVC. This suggests a consistency of process consisting of thermal preparation, catastrophic response, and relaxation in the development of these large volcanic fields. The possibility that the largest ignimbrite eruptions (>1000 km³) are triggered by mechanical failure of a thermally-weakened, low aspect ratio, roof requires a change in paradigm. In this model the roof fails as its tensile strength is exceeded and founders into the magma due to a density inversion produced by concentration of the gas phase into the uppermost part of the magma chamber. Large vents open quickly and catastrophic mass eruption rates are achieved immediately. Fissures and ring faults may progressively unzip allowing dense eruptive curtains or fountains to collapse quickly to generate dense pyroclastic flows with high energy. This type of mechanism may help explain the paucity of classic Valles-type caldera structures and the growing evidence for eruption of large ignimbrites from complex nested sources and along faults or fissures.
DE: 7280 Volcano seismology (8419)
DE: 8414 Eruption mechanisms
DE: 8419 Eruption monitoring (7280)
DE: 8424 Hydrothermal systems (8135)
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly