HR: 1400h
AN: V33A-03    [Abstracts]
TI: Dense Pyroclastic Flows of the 16 -17 August 2006 Eruption of Tungurahua Volcano, Ecuador
AU: * Hall, M L
EM: mhall@igepn.edu.ec
AF: Instituto Geofisico, Escuela Politecnica Nacional Casilla 1701-2759, Quito, Ecuador
AU: Mothes, P A
EM: pmothes@igepn.edu.ec
AF: Instituto Geofisico, Escuela Politecnica Nacional Casilla 1701-2759, Quito, Ecuador
AU: Ramon, P
EM: pramon@igepn.edu.ec
AF: Instituto Geofisico, Escuela Politecnica Nacional Casilla 1701-2759, Quito, Ecuador
AU: Arellano, S
EM: sarellano@igepn.edu.ec
AF: Instituto Geofisico, Escuela Politecnica Nacional Casilla 1701-2759, Quito, Ecuador
AU: Barba, D
EM: dbarba@igepn.edu.ec
AF: Instituto Geofisico, Escuela Politecnica Nacional Casilla 1701-2759, Quito, Ecuador
AU: Palacios, P
EM: ppalacios@igepn.edu.ec
AF: Instituto Geofisico, Escuela Politecnica Nacional Casilla 1701-2759, Quito, Ecuador
AB: The 16-17 August 2006 eruption of Tungurahua volcano in central Ecuador was preceded by 7 years of threatening activity and finally a VEI=2 eruption on 14-15 July 2006. The larger August eruption witnessed tens of pyroclastic flows that descended 17 different channels up to 8.5 km to the volcano's base on the NW, N, W, and SW sides. Tungurahua (5023m) is a steep-sided, low SiO2 andesitic volcano with 2600 to 3200m of relief. The initial, small nuee ardentes began around 1700hr (local time), the larger flows occurred between 2147hr and 0100hr (17 Aug.), and a total of 31 events were indicated by seismic signals. The deposits of three distinct flow cycles are recognized at the NW base of the cone. On the Los Pajaros depositional fan, deposits of flows 1 and 2 are widespread laterally (<600m) and have low-aspect morphologies with low snouts and without levees. Their outer surfaces are covered with accessory > juvenile clasts that mainly range from 15 to 25cm in diameter, conversely their interiors are comprised of 40-42% clasts of 1-25cm size and a matrix (58-60%) of sand-size grains. The earlier flow 1 was accompanied by an ash cloud surge that leveled, but did not scorch, all trees, brush, even metal antenna posts, leaving a 1-10cm thick sandy ash layer upon flow 1's deposit. On the fan as well as in gullies on the upper flanks, flow 3 deposits form long narrow lobes with 1-2m high frontal snouts that are followed by empty flow channels, 5-15m wide, bounded by parallel levees 1-1.5m high. Within these channels subsequent flow lobes are found as remnant pulses. Unlike flows 1 and 2, flow 3 lobes are covered with 0.5-3m cauliflower-shaped, slightly vesiculated bombs that are rarely abraded; the deposit's interior has a 45% sandy matrix. During the climatic eruptive phase continuous lava fountaining, 500-700m high, and crater spilling likely fed a continual stream of fragmented lava onto the cone's upper steep flanks, from which dense pyroclastic mass flows were initiated by gravity. Flows 1 and 2 were more fluidized (due to entrained air and fines), faster, and had wider lateral extents. Flow 3 was poorly fluidized, highly channelized, and behaved more like an inertial granular flow that formed as a continuous avalanche stream that separated into consecutive pulses along the runout channel.
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 9360 South America
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly