HR: 0800h
AN: V41B-1443    [Abstracts]
TI: Stratigraphy and Melt Compositions of the 3.6 and 6.7 ka Plinian Eruptions of Hudson Volcano, Chile.
AU: Carey, S
EM: scarey@gso.uri.edu
AF: GSO/Univ. of Rhode Island, S. Ferry Rd., Narragansett, RI 02882 United States
AU: Scasso, R
EM: rscasso@gl.fcen.uba.ar
AF: Univ. de Buenos Aires Cuidad Univ., Pab 2 1 Piso, Buenos Aires, 1428 Argentina
AU: * Kratzmann, D
EM: davidk@gso.uri.edu
AF: GSO/Univ. of Rhode Island, S. Ferry Rd., Narragansett, RI 02882 United States
AU: Naranjo, J
EM: jnaranjo@sernageomin.cl
AF: Serv. Nacional Geol. y Mineria, Casilla, Santiago, 10465 Chile
AU: Bande, A
EM: rscasso@gl.fcen.uba.ar
AF: Univ. de Buenos Aires Cuidad Univ., Pab 2 1 Piso, Buenos Aires, 1428 Argentina
AB: Fallout deposits from two major Holocene eruptions of Hudson Volcano in southern Chile (3.6 ka and 6.7 ka BP, Naranjo and Stern, 1998) provide new evidence for multiple phases, including subplinian to plinian discharges and episodes of phreatomagmatic activity. Four phases have been identified for the 3.6 ka eruption. The melt was trachydacitic and did not exhibit any significant variation throughout the fall sequence. Phase one (P1) produced a commonly reverse graded, lapilli fall deposit. Phase two (P2) also produced a reverse graded, coarse lapilli fall layer. Phase three (P3) deposited a massive, poorly-sorted, silty-ash layer with pumice and minor accretionary lapilli. The final phase of the eruption (P4) laid down a commonly normal graded, coarse lapilli fall deposit. Phases P1, P2 and P4 represent fallout from high altitude plumes with minor intensity fluctuations, whereas P3 resulted from magma/water interactions and a lower eruption column. Isopach maps show a shift in the main dispersal axis for the 3.6 ka phreatomagmatic ashfall (P3), relative to the lapilli deposits. Phases 1, 2 and 4 trend generally to the east, whereas the axis for the P3 fallout trends northeast. This is likely caused by dispersal of material at different altitudes during the eruption and not a general change in the predominant wind direction. Three major phases (P1 to P3) were identified for the 6.7 ka eruption. The initial phase (P1) produced a commonly reverse graded, coarse lapilli fall deposit. The second phase (P2) produced a thick, distinctive accretionary lapilli-rich, silty-ash layer with accretionary lapilli diameters up to 2.3 cm at 35 kms from the volcano. The final phase (P3) laid down an often normal graded, coarse lapilli fall unit. The melt phase was also trachydacitic in composition and relatively uniform during the eruption, but less evolved than the magma erupted during the 3.6 ka event. The accretionary lapilli layer (P2) has been correlated with a widespread tephra in southern Patagonia, 900km to the south of Hudson volcano with an estimated bulk volume more than 18 km3 making this one of the largest Holocene eruptions in southern South America (Naranjo et al. 2001). The occurrence of extensive, fine grained accretionary lapilli-bearing beds within these two plinian eruption sequences may be related to magma/meltwater interactions triggered by eruption discharge through the summit glacier of Hudson volcano, probably related to the formation of its last superimposed or partially nested caldera (Orihashi et al., 2004).
DE: 8404 Volcanoclastic deposits
DE: 8428 Explosive volcanism
DE: 8455 Tephrochronology (1145)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005