HR: 10:35h
AN: V11I-02    [PDF]
TI: New Insights Into Volcanic Hazards in Western Mexico: Multiple Cone-Building Episodes at Arc Stratovolcanoes Revealed by 40Ar/39Ar Geochronology
AU: * Frey, H M
EM: hfrey@umich.edu
AF: University of Michigan Dept. of Geological Sciences, 425 E. University, Ann Arbor, MI 48109-1063 United States
AU: Lewis-Kenedi, K
EM: katelk@umich.edu
AF: University of Michigan Dept. of Geological Sciences, 425 E. University, Ann Arbor, MI 48109-1063 United States
AU: Lange, R A
EM: becky@umich.edu
AF: University of Michigan Dept. of Geological Sciences, 425 E. University, Ann Arbor, MI 48109-1063 United States
AU: Hall, C M
EM: cmhall@umich.edu
AF: University of Michigan Dept. of Geological Sciences, 425 E. University, Ann Arbor, MI 48109-1063 United States
AU: Delgado-Granados, H
AF: Instituto de Geofisica UNAM, Circuitio Exterior C.U., Coyocan, DF 04510 Mexico
AB: The detailed eruptive histories of two andesitic stratocones, Volc\'{a}ns Ceboruco and Tequila, in the western Mexican arc have been documented using $^{40}$Ar/$^{39}$Ar geochronology. The volumes of these volcanoes were obtained with mapping, airphotos, and digital elevation models. The age and volume data constrain the rate and duration of major cone-building events, which bears on the longevity of the underlying upper-crustal magma chambers that fed the eruptions. The results indicate that at each stratovolcano there were two discrete cone-building events, separated by a hiatus. At V. Tequila, six samples from the edifice yielded dates (196 $\pm$ 8, 196 $\pm$ 19, 178 $\pm$ 8, 191 $\pm$ 13, 216 $\pm$ 11, and 198 $\pm$ 11 ka; errors are 1 sigma) with a mean eruption age of 196 $\pm$ 12 ka. Thus the bulk of the main edifice ($\sim$31 km$^{3}$) erupted within 24 kyrs (at the 2 sigma level), leading to a cone-building rate of $>$ 1.3 km$^{3}$/kyr. After a hiatus of $\sim$110 kyrs, $\sim$14 km$^{3}$ of andesite erupted along the NW and SE flanks of V. Tequila at 90 $\pm$ 19 ka. The last activity at V. Tequila produced a $\sim$2 km$^{3}$ parasitic cone at $\sim$60 ka. Since an eruption has not occurred in the last 60 kyrs, V. Tequila is often considered an extinct volcano. This may be the view held by the $>$ 75,000 inhabitants of the town of Tequila located on the northern flanks. A similar history of two discrete cone-building events is found at V. Ceboruco, $\sim$75 km to the NW. Seven samples taken from various parts of the edifice, including the inner caldera wall, indicate an initial cone-building event at $\sim$45 ka in which $\sim$37 km$^{3}$ of andesite erupted. After a hiatus of nearly 44 kyrs, a second eruptive period began $\sim$1000 years ago. The first eruption to occur after the hiatus was Plinian and released 3-4 km$^{3}$ of dacite. In the last 1 kyr, 9.5 km$^{3}$ of andesite and dacite erupted effusively, culminating in the historic 1870 flow. The sobering conclusion, in terms of volcanic hazards assessment, is that the only Plinian eruption to occur happened after a 44 kyr hiatus. Thus prior to the Plinian eruption, it would have been reasonable to conclude that the volcano was dormant and possibly extinct. Both V. Ceboruco and V. Tequila straddle prominent NW-SE faults, along which peripheral domes and cinder cones are also aligned. The location of these two large stratocones suggests that they each overlie a major passageway for magmas ascending from the lower/middle crust into the upper crust. Voluminous batches of magma appear to have collected episodically for relatively short periods in the upper crust, forming chambers. The short duration and recurring nature of voluminous cone-building eruptions have been documented at other stratovolcanoes, such as at Mt. Adams in the Cascade arc. Hildreth and Lanphere (1994) documented three discrete and relatively short-lived, cone-building events at $\sim$500, $\sim$450, and $\sim$30 ka. The emerging concern is that in a region of active subduction and faulting, the concept of an extinct volcano is tenuous. A hiatus of tens or even hundreds of kyrs, during which time no upper crustal magma chamber exists, cannot be used as evidence that the overlying volcano is unlikely to produce future eruptions. A new upper crustal magma chamber may form again along the same fracture system.
DE: 1035 Geochronology
DE: 8414 Eruption mechanisms
DE: 8419 Eruption monitoring (7280)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting