The Large Tephra Fall Deposits Related to the Ca. 3.500 BP Pucón Eruption: an Unexpected Revelation, Villarrica Volcano, Southern Andes 39.4°S, Chile

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F u n 2 d 6 la serena octubre 2015 ada en 19 The large tephra fall deposits related to the ca. 3.500 BP Pucón eruption: an unexpected revelation, Villarrica volcano, Southern Andes 39.4°S, Chile

Hugo Moreno* and Virginia Toloza1 *OVDAS-SERNAGEOMIN, Rudecindo Ortega 03850, Temuco, Chile 1Escuela de ciencias ambientales, Universidad Católica de Temuco, Rudecindo Ortega 02950, Temuco, Chile * Contact email: [email protected]

Abstract: The most remarkable Holocene explosive through detailed tephrochronology studies, on sites located deposit of Villarrica volcano is the ca. 3.500 BP Pucón around it but mainly spread on the eastern area, because of ignimbrite eruption that generated a 2,2 km wide summit the prevailing winds. In the studied area there are caldera. Although its volume has been estimated in ca. 5 numerous road-cuts and many of them reach the basement 3 km , surprisingly very little tephra fall has been reported rocks, occasionally covered by moraines of the last related to this big eruption and previously described only as glaciation (Llanquihue Drift). Good exposures of the a “thin scoria-fall deposit”. However, through a detailed different tephra layers were observed, most of them are research on the Quetrupillán Postglacial tephrostratigraphy, falls, flows and surges. Among them a remarkable large scoriaceous tephra fall deposits related to the Pucón scoriaceous deposit was recognized as a guide horizon all ignimbrite eruption were recognized. The dispersion area covers more than 6.500 km2 (considering the 1 cm isopach) over the area. We had complementary new radiocarbon and the deposits reach up ca. 1.7 km3 in volume. The dates on charcoal that allow us to define eruptive ages, tephra fall deposits consist in two main layers: (1) at the together with chemical analyses and grainsize studies. base a rich scoria fall with some lithics and (2) a lithic rich !! level with scoria fall at the top. Between both a thin indurated accrecionary lapilli layer with some lithics and scoria were found all over the area. The main dispersion axis has an East direction, although the lithic rich layer has an ESE axis, that means that the plume shifted from the east to the southeast. Currently that kind of wind direction changes take place in less than one day, therefore the plinian eruption could have lasted only some hours.

Key words: Villarrica volcano, Pucón eruption, plinian tephra fall.

Introduction

Villarrica volcano (39,4°S) is located on the northeast Fig. 1. The Villarrica-Quetrupillán-Lanín volcanic chain extreme of a volcanic chain 45 km long that comprises (39,5°S) and observation sites. three stratovolcanoes, together with Quetrupillán and Lanín volcanoes (Fig. 1). Villarrica is one of the most historically active volcanoes of the Andes, with a record of The Pucón eruption tephra fall: previous Hawaiian to strong Strombolian eruptions since 1558 studies (Moreno and Clavero, 2006). However, large mafic explosive eruptions of basaltic-andesite to andesite As mentioned before, although the Pucón eruption was the composition have been found within the Postglacial (< 14 last large explosive eruption of Villarrica volcano that ky). Among them the so-called Pucón eruption, that took generated a big mafic ignimbrite, former research studies place ∼3.5 ky ago, is the last mayor explosive activity of 3 only mention small tephra deposits related to this eruption. Villarrica that generated a ∼5 km ignimbrite (Clavero, In fact, a very restricted area of only 440 km2 for a 10 cm 1996; Silva et al., 2010) that left a 2,2 km wide summit isopach was reported by Silva et al. (2010). These authors caldera. Even though such a voluminous mafic ignimbrite named the tephra fall as a “basal scoria-fall deposit” generation took place, almost very small tephra fall (basaltic-andesite in composition), because it directly deposits have been described related to this explosive underlies the Pucón ignimbrite, though with some eruption (Silva, 2008; Silva et al., 2010). At 20 km toward assumptions. Also, they only recognized a maximum the southeast, Quetrupillán volcano (39,5°S) was recently thickness of 90 cm on the northern slope of the volcano. our focus to understand its Postglacial explosive activity 570 AT 1 GeoloGía ReGional y Geodinámica andina

Regarding the internal features of the fall deposit, Silva (2008) describes that “in the thickest and most complete exposure on the northern flank, the fall deposit consists of two layers: a lower massive layer composed almost completely of juvenile scoria, and an upper, stratified layer richer in accidental lithics”. Finally and in relation to the eruption itself they stated that “following a maximum repose period of ~400 years, the Pucón eruption started 3 with a violent strombolian or subplinian fallout (0.1 km of Fig. 2. The Pucón eruption fallout interbedded within the magma, VEI=3-4) which rapidly evolved towards a Quetrupillán pyroclastic flows and 14C ages. hazardous ignimbrite-forming mechanism” (Silva et al., 2010) . Internal structure of the Pucón tephra fall ! deposits

Tephrostratigraphic features of the studied In most of the sites, the Pucón tephra fall deposit shows a area: the Pucón eruption tephra fall characteristic internal structure that consists of two main layers separated by an indurated accretionary lapilli level About 60 sites were studied (Fig. 1) and in many of them (Fig. 3). They include a scoria rich basal layer and an columns were done to define the stratigraphy, upper lithic rich layer, which were also described by Silva characteristics and its composition, to correlate each other. (2008) near Villarrica volcano. As the studied area was around the Quetrupillán volcano and mainly toward the east of it, most of the deposits The basal layer is a rich scoria deposit (80%) that starts belong to this volcanic complex. Quetrupillán has mainly with a very thin ash fall, followed by fine to coarse lapilli andesites, dacites and few rhyodacites lavas, together with basaltic-andesitic juvenile scoria fall (∼54% SiO2). It has pyroclastic flows, surges and falls, so it is not so hard to few angular lithics (∼20%) and occasionally big bombs up link up the different layers (commonly with pumice) to this to 25 cm in diameter that fell as far as 32 km east of the volcano. However, within the stratigraphic succession, two volcano. At the beginning the deposit was classified as a remarkable mafic scoria fallout deposits were recognized. typical strombolian scoria deposit, but because of its The youngest consist in a black fine to medium size lapilli extension, thickness (up to 80 cm thick) and the bombs basaltic scoria, interbedded between two pumiceous flow found so far away from the volcano (>30 km), it was deposits and related to the nearby Huililco monogenetic classified as a mafic subplinian type of eruption fallout. volcano. Therefore, its distribution is quite restricted to the The upper layer consists mainly of angular volcanic lithics area immediately toward the east of it because of the with few granitoids from the basement (∼70%) and minor dominant winds. The oldest one is a small sequence layer (∼30%) basaltic-andesitic lapilli size scoria (∼54% SiO2) that appears everywhere in all the area as a genuine guide with some bombs up to 22 cm in diameter, found as far as horizon. At the beginning we thought it was related to the 30 km east of the volcano. In this case, this fallout layer Huililco scoria cone, but because of its large extension, the was classified as a typical vulcanian type eruption deposit increasing thickness towards Villarrica volcano and its that reach 45 cm thick. basaltic-andesitic composition, it was obvious that the vent was not Huililco. To confirm this hypothesis we followed Between both main layers, a thin indurated accretionary the outcrops toward Villarrica volcano eastern foot. lapilli"intercalation was found in all sites. Occasionally, up Around the location of Palguin the same deposit with the to three slim indurated scabs of accretionary lapilli (like classic internal structure was recognized but much thicker cornices) appear interbedded with thin fine to medium (up to ~2,0 m thick) and underlying directly the Pucón lapilli size scoria and lithics merged with ash. ignimbrite deposit. So there was no doubt that the recognized tephra deposits were related to the Pucón eruption of Villarrica volcano. In addition, later we got new 14C radiocarbon dates that also match with the Pucón eruption. Indeed, the tephra deposits are located between two pyroclastic flows from Quetrupillán volcano that gave 3.800±30 yBP the oldest and 2.930±30 yBP the youngest, so both ages are consistent with the Pucón eruption age of ca. 3,5 ky (Fig. 2).

Fig. 3. Typical internal structure of the Pucón eruption fallout 571 ST 3 METAMORFISMO Y MAGMATISMO EN ZONAS DE SUBDUCCIÓN

layers. The lower layer according to the Ashcalc method gave a volume of 0,8 km3 and the upper layer has almost 3 Dispersion of the Pucón tephra fall deposits 0,6 km considering the area within the 10 cm isopach, so the minimum volume of the Pucón eruption tephra fall is > and isopachs 3 1,4 km . However, there is a much larger area covered by The dispersion of the whole Pucón tephra fallout has a the 1 cm isopach that could not be determined, together main eastern trend axis, although the upper layer has an E- with the interbedded irregular accretionary lapilli layer. A rough estimate of both volumes gave around 0,3 – 0,4 km3, SE trend that suggests a shifting of the plume from the east 3 toward the southeast during the eruption. The lower layer so the total volume of the deposit is near 1,7 km . In covers an area of ≤ 2.100 km2 based on the 10 cm isopach relation to the column height taking into account the MS (Fig. 4) and the upper layer involves an area of ≤ 1.700 and ML of both layers (Walker and Croasdale, 1971; km2 (Fig. 5). The whole tephra fall related to the Pucón Carey and Sparks, 1986), the result was that the lower eruption, over the 10 cm isopach, covers an area of ≤ 2.400 scoria rich deposit reached an altitude between 30 and 40 km2 (Fig. 6) and more than 6,500 km2 within the 1 cm km (Hb, Ht), while the upper lithic rich layer had an isopach. We assume that there was a shifting of the plume elevation between 25 and 35 km (Hb, Ht). because a variation of the layers thickness can be observed along a north-south crosswind profile (Fig. 7). In fact, the Conclusions lower scoria rich layer disappears toward the south and only outcrops the lithic rich layer instead. The results of this study revealed that the Pucón eruption first stage started with an authentic Plinian explosive outburst. The initial outburst was a strombolian eruption that led a thin scoriaceous ash layer followed by a violent mafic Plinian eruption that covered an area of more than 2.000 km2 within the 10 cm isopach. After this powerful eruption ended, large quantities of ash remained in the atmosphere that together with condensed water vapor formed the accretionary lapilli layer. Later (we guess only hours) a second Vulcanian eruption took place leaving the lithic rich layer. The area covered by the whole eruption is more than 6.500 km2 based on the 1 cm isopach.

Acknowledgements

This study was financed by the Red Nacional de Vigilancia Fig. 4. Area and isopachs (over 10 cm) of the lower scoria rich Volcánica. This contribution is sponsored by the layer Subdirección Nacional de Geología of SERNAGEOMIN.

References

Carey, S.; Sparks, R.S.J. 1986. Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns. Bulletin of Volcanology 48, 109–125. Clavero, J. 1996. Ignimbritas andesítico-basálticas postglaciales del Volcán Villarrica, Andes del Sur (39°25'S). Tesis de Magíster (Unpublished), Universidad de Chile, Departamento de Geología: 112 p. Moreno, H.; Clavero, J. 2006. Geología del volcán Villarrica, Región de la Araucanía y de los Lagos. Servicio Nacional de Geología y Minería, Serie Geología Básica, Carta Geológica de Chile 98: 35 p. 1 mapa escala 1:50.000 Silva,C. 2008. Evolution and dynamics of the 3.6 ka BP Pucón eruption of Villarrica volcano, Chile. Tesis de doctorado. Universidad de Chile. Departamento de Geología (Inédito): 245 p. Santiago. Silva P., C.; Druitt, T.H.; Robin, C.; Moreno R., H.; Naranjo S., J.A.. Fig. 5. Area and isopachs (over 10 cm) of the upper lithic rich 2010. The Holocene Pucón eruption of Volcán Villarrica, Chile: Deposit architecture and eruption chronology. Bulletin of layer Volcanology v.72: pp.677-692. Walker, GPL; Croasdale, R, 1971. Two plinian-type eruptions in the Volumes and column height Azores. J. Geol. Soc. 127(1):17–55.

As the interbedded accretionary lapilli layer is quite irregular we calculated the volumes of the lower and upper 572 AT 1 GeoloGía ReGional y Geodinámica andina

Fig. 6. Area and isopachs (over 10 cm) of the whole Pucón eruption tephra fallout

Fig. 7. North-south (crosswind) variation of the layers thicknesses of the Pucón eruption tephra fallout

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