Abstract
Villarrica Volcano in the Southern Andes of Chile has sustained persistent open conduit activity, associated with a dynamic lava lake since at least 1984-85. Increased seismic activity and degassing in August 2014 was followed by increased Strombolian activity starting in February 2015 and culminated with a 1.5 km-high lava fountain on 3 March 2015. This eruption produced tephra fallout, spatter agglutination, clastogenic lavas and mixed avalanche deposits. Field work carried out in 2017 and 2018 focused upon the tephra fallout deposits from the 3 March 2015 paroxysmal eruption. Tephra fall deposits produced by lava fountain and a rapid transition to sustained eruption column at Villarrica are here described for first time. Tephra dispersion was observed mostly southeast of the crater in a narrow, elongated, nearly-elliptic area. Minimum observed tephra loading was estimated to be 80-120 g/m2 at crosswind locations, while maximum load estimation was about 11,500 g/m2, at 7-9 km distance from the vent. At 6-8 km downwind, the tephra fall deposit consisted of a single black scoria layer, fall varied from a lapilli blanket at medial distances (9-19 km) to a few individual particles at distal sites (20-43 km). All samples are vitrophyric in texture, with a mineral assemblage of plagioclase (3-30 %), clinopyroxene (2-5 %), olivine (1-4 %) and scarce orthopyroxene (<1 %) phenocrysts. We estimate that 1.4 x 109 kg, equivalent to ~2.4 x 106 m3 of basaltic andesite tephra (52.8 to 54.7 SiO2 wt. %), were erupted within a period of about 16 minutes at a mass eruption rate of ~1.5 x 106 kg/s. The sustained eruption column height was estimated at ~10.8 km, with magnitude and intensity of 2.11 and 9.13 respectively. We propose that a rejuvenated, volatile-rich magmatic intrusion is the probable triggering factor for the paroxysm. The Villarrica eruption is classified as a short-lived, large-scale lava fountain, similar to intense historical lava fountains at Mt. Etna and Kilauea. As evident by devastating 20th century Villarrica eruptions which also developed similar lava fountain behavior, this eruption style should be considered as frequent and a potential source of volcanic hazard in the future.
Highlights
Lava fountains are powerful, continuous but normally short-lived gas jets caused by fast-rising bubbly melt or the ascent of a bubble foam layer coming from depth (Parfitt, 2004)
During the fieldwork in April 2017 and January 2018, mass per unit area loads of tephra fall deposit were measured at 29 locations, together with the maximum particle size at 21 locations
The mass loading data were used for the construction of an isomass map of the fall deposits (Figure 3A), while satellite images were useful for the reconstruction of tephra dispersal patterns
Summary
Lava fountains ( known as “fire fountains”) are powerful, continuous but normally short-lived gas jets (from less than one to a few hours) caused by fast-rising bubbly melt or the ascent of a bubble foam layer coming from depth (Parfitt, 2004). Episodes with higher lava fountains (200–600 m) accompanied by successively taller eruption columns from 3 to 15 km above the vent, were observed during the eruptions of 1 and 31 January, 1948–1949, 2 March 1964 and 29 December 1971 Most of these eruptions triggered lava effusion, mixed (ice and pyroclast) avalanche generation and massive lahars, killing a total of 100 people and causing major disruption of human activities and damage to infrastructure (González-Ferrán, 1995; Naranjo and Moreno, 2004). During early February 2015, an abrupt increase in radiated infrasound power was observed coincident with increased vigor of Strombolian activity sourced from two small vents within the lava lake and material ejected beyond the summit crater (Johnson and Palma, 2015; Figure 2A). Until February 2017, vent activity was characterized by variations in the magma column level, minor Strombolian activity erupting scoria and spatter a few tens of meters above the crater level, and ash emissions associated with partial collapse of the crater walls
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