HR: 1340h
AN: V53B-1563    [Abstracts]
TI: Quantitative Analysis of 0-2 Ma Volcanic Vent Distribution in Space, Time, and Composition Centered on Medicine Lake Volcano, Northern California, USA
AU: * Smith, J G
EM: jimsigp@usgs.gov
AF: USGS, 12201 Sunrise Valley Dr., Reston, VA 20192 United States
AU: Donnelly-Nolan, J M
EM: jdnolan@usgs.gov
AF: USGS, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: 525 volcanic vents 0-2 Ma were identified during geologic mapping of approx. 2500 sq km centered on Medicine Lake volcano (MLv), northern Calif. More than 95% of the mapped area consists of volcanic rocks 0-2 Ma. Vent locations were digitized, and each vent assigned to 1 of 5 time slices (0-12 ka, 12-25 ka, 25-120 ka, 120-780 ka, and 0.780-2 Ma) based on radiometric dating and one of 5 chemical composition groups. In this preliminary study, no attempt is made to interpret locations of vents buried by younger units or relate vents to areas or volume of individual volcanic map units. Vent data were analyzed using a GIS. 53% of vents are basalt and mafic andesite (<57% SiO₂ but not classified as low-K tholeiite), 20% are low-K tholeiite (SiO₂ <49%, K2O₂ always <0.5% and commonly <0.39%), 11% are andesite (57-62% SiO₂), 3.5% are dacite (62-70% SiO₂, and 8.5% are rhyolite (SiO₂ >70%). Considering only mapped vents, one vent erupted on average every 3800 yrs during the last 2 m.y. This frequency is clearly too low because vents are unknown or buried for 56 volcanic map units and, therefore, not included in calculating this eruption frequency. If one assumes one vent for each of these 56 units, the interval between vents falls to 3400 years. This frequency is still too low because 1.) Most mapped units with identified vents have multiple vents and 2.) There is a significant decrease in the average time between vent formation in successively younger time slices. About 90% of vents are widely distributed (vent densities <1.3 per sq km) west of -121.48 deg which marks an abrupt N-S trending eastern boundary for all vents in the MLv area. Four conspicuous areas with vent densities between 1.3 and 6.5 vents per sq km form a trend parallel and approximately 5 km W of this abrupt boundary. A second trend of similar vent densities defines a trend of 030. Both trends are present in the 4 youngest time slices, although stronger in some than in others. The two trends intersect near long. -121.5 and 41.8 N in the northwestern part of Lava Beds National Monument, which is also the area of highest vent density, 6.5 vents per sq km. There is no particular concentration of vents in the vicinity of the summit of MLv with vent densities below mean vent density (0.234 vents per sq km, due, at least in part, to subsidence and younger volcanic units covering older ones.
DE: 1819 Geographic Information Systems (GIS)
DE: 8425 Effusive volcanism
DE: 8428 Explosive volcanism
DE: 8486 Field relationships (1090, 3690)
DE: 9350 North America
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005