HR: 12:05h
AN: V42B-07    [Abstracts]
TI: Implications for Hazards Maps of Identification, Routing, and Initiation of ca. 2 ka Lahars at Misti Volcano, Southern Peru
AU: * Harpel, C J
EM: christopher.harpel@und.edu
AF: Department of Geology and Geological Engineering, University of North Dakota, Box 8358, Grand Forks, ND 58202 United States
AU: de Silva, S
EM: desilva@space.edu
AF: Department of Space Studies, University of North Dakota, 526 Clifford Hall, Grand Forks, ND 58201 United States
AU: Scott, W E
EM: wescott@usgs.gov
AF: US Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Ct., Bld. 10, Ste. 100, Vancouver, WA 98683 United States
AU: Salas, G
EM: guidovolcan@yahoo.com
AF: Departamento de Geologia, Universidad Nacional de San Agustin, Arequipa, 1 Peru
AB: About 2 ka a subplinian eruption at Misti volcano produced extensive volcaniclastic deposits. Our work to make a volcanic-hazard map based on these deposits illustrates some of the complications that arise from genetic interpretations, flow routing, and initiation processes. Earlier workers identified the deposits as pyroclastic flow deposits and evaluated the hazards accordingly (e.g. Thouret et al., 1999; 2001). However, abundant evidence indicates that they are lahar deposits. Needless to say, hazards implications of these different modes of emplacement are such that correct interpretation of the genesis of deposits is a critical first step in hazard assessment. Detailed field mapping shows that the most voluminous lahars were shed onto the southern portion of the ring plain of the volcano. Many of these lahars were channelized into narrow canyons (quebradas) in the ring plain, but, in several instances, lahars filled quebradas, overtopped interfluves and spilled into adjacent quebradas. As a result several quebradas not containing large lahars at their heads had voluminous lahars at their lower reaches. Accurate modeling and portrayal of potential lahar hazards must include an assessment of such diversions and be able to accommodate this behavior. Field mapping of deposits of past lahars can help to identify potential diversion points. Initiation processes of the lahars are not yet definitively understood, but the volume of deposits requires a voluminous source of water. Several lines of evidence point to interaction of hot pyroclasts with snow or ice as the initiation mechanism, but rain may also be involved. Currently, the volcano is in an arid climate, rarely has voluminous snow, and does not have permanent ice. Therefore, the 2 ka lahar deposits may not be a relevant analogue for the assessment of modern lahar hazards. When using mapping and modeling to make a lahar hazards assessment and map, it is critical to correctly identify the origin of deposits, accurately map the distribution of the deposits, and identify the initiation mechanism and consider its relevance under modern conditions.
DE: 8404 Volcanoclastic deposits
DE: 8486 Field relationships (1090, 3690)
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005