Thèse “Evolution and dynamics of the 3.6 ka BP Pucón eruption of Villarrica Volcano, Chile”

Thèse de doctorat de l’Université du Chili et de l’Université Blaise Pascal, défendue le lundi 8 septembre 2008 par Carolina Silva.

Cette thèse a été préparée sous la direction d’Alfredo Lahsen, de l’Université du Chili, et de Timothy Druitt, Université Blaise Pascal (Clermont-Ferrand, France)

Resumé

The Pucón Ignimbrite (3.6 ka BP, 3.3 km3, 1.8 km3 DRE) is a well preserved sequence of mostly pyroclastic current deposits with basaltic andesitic juvenile composition (54-56% SiO2) emitted by the largest Holocene eruption of Villarrica Volcano (Chile). Fieldwork, together with the determination of physical and chemical parameters (grain-size, counting, density measurements, textural analysis, radiocarbon dating, whole-rock chemical analyses, S, Cl and F glass composition), allowed to reconstruct the facies architecture of the deposit and to understand the dynamics of the eruption.
Prior to the Pucón eruption, subglacial lava domes and/or flows were present near or at the summit and fragmented at the onset of the Pucón eruption.
Following a maximum repose period of 400 years, the Pucón eruption started with a violent strombolian or subplinian fallout (0.1 km3 of magma, VEI=3-4) which rapidly evolved towards a hazardous ignimbrite-forming mechanism (P1 phase). A small base surge was then followed by about ten powerful vulcanian explosions generating multiple pyroclastic flows (20% of the total volume) that covered 180 km2 of the western and northern flanks of the volcano up to 15 km from the summit. A progressive deepening of the fragmentation level accompanied P1. Lithic-rich pyroclastic flows were mostly valley-confined with subordinate surges and followed by the emission in rapid succession (degassing pipes traverse the contacts) of highly-concentrated mostly juvenile-rich pyroclastic flows. During a pause in eruptive activity (several weeks to months ?) the P1 deposits cooled below the temperature necessary for charcoalization of wood ( 200°C) and reservoir replenishment by a more basic magma batch occurred.
During the second phase (P2, less than a few days ?) voluminous pyroclastic flows and surges were deposited all around the volcano (80% of the total volume), covering 530 km2 up to 21 km from the present-day summit. This increase in the eruptive intensity is characterised by the abrupt appeareance of a significant proportion of basement granitoids as free clasts and inclusions in scoria (angular and partiallymelted varieties), suggesting that very rapidly after the start of this phase, a high magma discharge rate caused the fragmentation level to fall within the granitoid basement, probably accompanied by the climactic phase of caldera collapse and stoping of granitoid roof material into a magma reservoir. The turbulent, blast-like leading edge of these flows was followed in rapid succession by high-energy pyroclastic flows, forming thick valley ponds and a complete interfluve record. The unsteady flows of high- to intermediate-concentration (diffuse stratification and huge bed forms) sheared the underlying strata. Then, black juvenile-rich pyroclastic flows to the southeast of the volcano were rapidly followed by at least two lithic-rich surges (degassing pipes traverse the contact) distributed all around the volcano with important slope deposition. During a final waning phase, three lithic-rich pyroclastic flows were emitted on the northern flank, immediately inundated by several lahar waves.
Pucón magma was probably sufficiently rich in volatiles at depth to exsolve and drive magmatic fragmentation. Low-pressure degassing and rapid microlite crystallization produced dense scoria. External water eroded the conduit early on P1 and P2, and then decreased because most of the glacier had melted.
Even infrequent on a human timescale, if a Pucón-sized eruption (VEI=5) occurred today, approximately 15,000-40,000 people would be directly threatened by pyroclastic currents and even more by associated lahars and ash falls. The evolution of the Pucón eruption shows that a violent ignimbritetype episode (P2), of short duration but large magnitude, catastrophic on a regional scale, can occur after a pause of several weeks to months following a period of already important pyroclastic activity (P1). This pause marked an adjustment phase of the plumbing system, rather than the end of the eruption.