HR: 12:05h
AN: V31G-08    [PDF]
TI: External Triggering of the Superexplosive Volcanism
AU: * de Silva, S L
EM: desilva@space.edu
AF: Department of Space Studies, University of North Dakota 526 Clifford Hall, Grand Forks, ND 58202-9008 United States
AB: Explosive eruptions of silicic magma are generally thought to be internally triggered by pressure build-up during second boiling that results in the production of a separate gas phase. Resulting overpressures of 20 to 25 MPa are thought to be sufficient to fracture the magma chamber roof and trigger eruption. However, this may not be the trigger for the most catastrophic "superexplosive" eruptions that result in thousands of cubic kilometers of silicic magma being erupted as massive low aspect ratio ignimbrites of the "monotonous intermediate" genre. Exemplified by the Fish Canyon Tuff (San Juan Mountains, Co, USA), the Cerro Galan Ignimbrite (Argentina), or the Atana ignimbrite (Chile), these are typically regionally- extensive, simple cooling units that are crystal-rich (45 vol%), fine-grained, lithic and pumice-poor units, and rarely show evidence for multiple flow units. These features coupled with the lack of basal or intercalated fall deposits suggest that the eruptions developed very quickly to catastrophic proportions without a low discharge rate convective column phase. Typical eruptive conditions that would account for such eruptions are high mass eruption rate, large vents, low gas content and eruption velocity. A further complication is that pre-eruptive viscosities of the magmas (10$^{6}$ to 10$^{8}$ Pa s) are close to the eruptibility barrier for such magmas. A catastrophic mechanism is thus required to trigger these eruptions. Clues to the mechanism include: 1) Intra- vs extracaldera tuff volume - intra-caldera tuff is the dominant volume of erupted material. 2) Paucity of lithics and pumice in the ignimbrites 3) Nature of the eruption sites - complex often trap-door like collapses, faults, fissures These observations suggests that "caldera collapse" occurred very early on in the eruptions, that vent erosion and reaming was not significant and is unlikely to be the cause of the inferred high mass eruption rates, that dense pyroclastic flows resulted in efficient crushing of pumice, and that the roof of the magma chamber may have foundered into the magma chamber along outward dipping faults. The eruption trigger is external and is suggested to be mechanical failure of the thermally weakened low aspect ratio roof. 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 along faults or fissures.
DE: 8400 VOLCANOLOGY
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting