Fundada en 1962

SOC

IEDA

D GEOLOGICA DE CH

ILE

la serena octubre 2015

198

Small volume pyroclastic density currents of the 22-23 April 2015 eruption on the NE flank of Calbuco volcano, Southern Andes Jorge Clavero1,2*, Angelo Castruccio3, Bárbara Droguett1, Andrea Segura3,JL Le Pennec4, O Roche4, P Samaniego4

1 Amawta Geoconsultores Ltda. 2 Escuela de Geología, Universidad Mayor 3 Depto. de Geología, Universidad de Chile 4 IRD, UR 163 Laboratoire Magmas et Volcans, Université Blaise Pascal, Clermont-Ferrand, France *e-mail: jclavero@amawta.cl

Abstract During the 22-23 April 2015 eruption of Calbuco volcano a series of pyroclastic density currents (PDC) were generated and directed to its SSW and NE flanks. To the latter, at least 8 PDC deposits have been recognized interbedded within the fallout generated by the 2 first and main pulses of the eruption. According to the stratigraphy, the flows are associated to the second pulse of the eruption on the 23rd of April. These PDCs correspond to both dense and dilute flows. Most of them contain cauliflower scoriaceous bombs, some up to 1.5m in diameter, only the lower ones contain brownish vesicular pumices similar to the fallout juveniles. One distinctive PDC deposit contains highly crystalline silicic prismatically jointed bocks (PJB) that show magma mixing textures (glassy, dense and vesicular juvenile material mixed together). Key words: pyroclastic density current, Calbuco, Andes 1 Introduction Calbuco is an active and hazardous volcano located in the southern Andes of Chile (Stern et al., 2009), whose last eruption before the April 2015 event, occurred in AD 1961 (Petit Breuilh, 1999). Its evolution is mainly characterized by the extrusion of silicic andesite lavas and domes and their associated pyroclastic flows (mainly block-and-ash flows and blasts), as well as cold and hot lahars (López et al., 1992; Hickey-Vargas et al, 1995; Castruccio et al., 2010; Castruccio and Clavero, 2015; Sellés and Moreno, 2011). A distinctive hummocky terrain is well developed on its lower northern flank associated to two sector collapses that affected the volcano in postglacial times. Radiocarbon dating (uncalibrated) of some pyroclastic deposits and palaeosoil horizons indicate that the two avalanches occurred at ca. 6.500 years BP and ca. 1.200 years BP (Clavero et al., 2008). According to its evolution, geochemistry and historical eruptive activity, Calbuco is considered to be one of the most hazardous active volcanoes in the Chilean Andes (Petit Breuilh, 1999; Clavero et al., 2008; Castruccio and Clavero, 2015), as it has generated subplinian eruptions in historical times (1893-1895; 1929 and 1961) as well as suffered sector

collapses in the Holocene. These factors, added to the fact that it surroundings have increased its populated areas, poses important problems in terms of volcanic risks and how to manage such risks during eruptive events. 2 April 2015 eruption Calbuco volcano started a new eruptive cycle on the 22nd of April at 18:05 local time. Although reported seismicity increased above background levels only a couple of hours before the beginning of the eruption, regional reports (SERNAGEOMIN, 2015a) indicate a rise in the number of VT events beneath the volcano during the previous months. The eruptive column height of the first pulse reached 16 km in a few minutes (SERNAGEOMIN, 2015b). This first eruptive pulse lasted 1.5 h, with a plume dispersion to the NE. On 23 April at 1:00 local time, a second eruption generated a column that reached 17 km of altitude (SERNAGEOMIN, 2015c, d). This episode lasted 6 h approximately with the same plume dispersion to the NE than the first one. During these 2 pulses, pyroclastic flows reached 8 km from the vent in the NE and SW flanks and lahars reached the Chapo lake in the S flank (SERNAGEOMIN, 2015e).In the next days the activity decreased with sporadic events which generated weak plumes (< 2 km high). On 30 April, at 13:08 local time a third pulse was generated with a 3-5 km column high, with a SE dispersion. During the next weeks the eruptive and seismic activity decreased gradually and on 28 May the alert level was lowered to yellow. 3 Eruptive stratigraphy of the April 2015 eruption on the NE flank The 22-23 April eruption generated a series of pyroclastic and lahar deposits distributed around the volcano. The following stratigraphic units, included in this work, correspond to those identified on the N-NE flanks of the volcano (Fig. 1), in the Río Blanco-Río Hueñu Hueñu-Ensenada area.

199

AT 4 Impacto de las GeocIencIas en la socIedad

In this sector the units identified and shown in Fig. 2 correspond initially to a fallout deposit formed by 4 subunits (described in more detail in Segura et al., this congress). Within this fallout deposit, specifically between the second and the third units, a series of at least 8 pyroclastic density currents were generated which partially inundated the Río Frío-Río Blanco valleys (described in more detail later in this work). These PDC deposits are overlain by the third and fourth subunits of the fallout deposit (Fig. 2), and partially eroded (and dissected) by a series of lahar deposits, most of them of secondary origin, forming a pyroclastic terrace in the valley up to 20 m thick.

Figure1. Location of the study area and the stratigraphic columns analyzed in this study. 4 Pyroclastic Density Current deposits on the NE flank 4.1 Distribution, Stratigraphy and Petrography A series of pyroclastic density currents were generated during the 22-23 eruption of Calbuco volcano, mainly directed towards its NE and S flanks. Here we describe the stratigraphy of those directed to the NE that partially filled up the Río Frío-Río Blanco valleys.

Figure 2 shows the pyroclastic stratigraphy of the sequence identified on Calbuco’s NE flank associated to the 22-23 April eruption, where 8 PDCs, both dense and diluted have been recognized. These small-volume deposits show diverse petrographic and structural characteristics, suggesting different emplacement dynamics, probably related to different generation mechanisms. From base to top (Figure 2), the deposits are: Deposit I: PDC deposited on top of layers 0 and 1 (see Segura et al., this congress) or on top of pre-2015 lahar deposits or bedrock. It is up to 2 m thick and contains abundant (10-35% vol.) brownish vesicular subrounded pumices (<8 cm Ø, similar in petrography to those of the fallout) within a cohesive fine-grained massive ash matrix. It has gas segregation pipes cut by the overlying deposit. Deposit H: It shows a sharp subhorizontal contact with Dep. I (Figure 3) and is up to 80 cm thick. It has gas segregation pipes cut by the overlying deposit. It is formed by “cauliflower” scoriaceous bombs (up to 25 cm Ø) as well as brownish pumices incorporated from the lower deposit (at its base), within a semi-consolidated fine ash matrix. Locally, it shows a laminated fines-depleted base (<20 cm thick). Deposit G: PDC deposit up to 2.8m thick with undulated (syn-depositional deformation structures) upper and lower contacts (Figure 3). It contains large “cauliflower” scoriaceous bombs (up to 80 cm Ø), “silicic” highly-crystalline (40-70% crystals) whitish PJB fragments (up to 25 cm Ø), glassy black fragments (up to 15 cm Ø) within an unconsolidated massive medium ash to fine lapilli gray matrix. Some PJB fragments show internal undulated contacts with glassy bands. Deposit F: PDC up to 1.5m thick with undulated (deformed) base and subhorizontal upper contact. It contains cauliflower scoriaceous bombs (<40 cm Ø), subangular lithic fragments (<20 cm Ø) within a massive medium to coarse ash grey to reddish matrix. Deposit E: Dilute PDC deposit up to 2.5 m thick formed by two distinct layers. The lower (E1) layer shows well-developed lamination, cauliflower bombs up to 40 cm Ø, lithic fragments (<35 cm Ø) within a fine to medium ash crystal-rich matrix with carbonized material. The upper layer (E2) has well-developed lamination 2-10 cm thick layers formed by fine to medium lapilli with some scoriaceous bombs (up to 23 cm Ø). Deposit D: Dense massive PDC deposit with abundant cauliflower scoriaceous bombs (up to 60 cm Ø), subrounded lava lithics (up to 1.4 m Ø) within a fine to medium ash massive matrix. It contains large carbonized to semi-carbonized trunks. Locally, it has high lithic/bombs concentrations (almost clast-supported).

200

ST 11 TERREMOTOS, VOLCANES Y OTROS PELIGROS GEOLÓGICOS

Deposit A: Layered PDC deposit with 3 distinct subunits. The lower one, up to 20 cm thick, is a fines-depleted layer formed by polylithologic lithic-rich fine lapilli fragments. The intermediate layer, up to 6 cm thick, is formed by a discontinuous non-carbonized vegetation-rich paeleosoil horizon deformed and incorporated by the flow. The upper layer, up to 2.5 m thick, has an undulated surface and is formed by large cauliflower bombs (up to 1.1 m Ø), subrounded lithic fragments (up to 1.2 m Ø) within a massive fine to medium ash matrix. Large bombs and lithics increase towards the upper part of the deposit. Deposit S: Formed by 2 distinct layers up to 25 cm thick in total separated by a discontinuous thin laminated ash layer (<1 cm thick). The lower layer (S1) is a fines-depleted, polylithologic, well-sorted fine to medium lapilli slightly laminated layer. The upper one is a fine ash, crystal-rich layer with dispersed (15% vol.) large lithic fragments (up to 15 cm Ø) concentrated towards the upper part of the deposit. It is partially overlain by units 2 and 3 of the fallout deposit. 4.2 Grain size Grain-size distributions for the PDCs identified are shown in Fig. 4. Most fall within the PDC (dilute and dense) fields, although some units show extremely well-sorted distributions, similar to those found on fallout deposits. 4.3 Volume estimate The area covered by pyroclastic flow deposits on the NE flank is approximately 650,000 m2. Taking a mean thickness of 15 m for the whole sequence, then a volume of ~107 m3 is obtained, representing less than 3% of the fallout deposit volume. Taking into account the individual thicknesses observed, it is likely that each individual flow volume is less than 106 m3. By observing aerial photographs (Google Earth), pyroclastic flows on the SSW flank had slightly shorter runouts and probably comparable volumes.

Figure 4. Grain size distribution of PDC deposits of the 22-23 April eruption on the NE flank of Calbuco volcano.

5 Discussions According to the stratigraphic relationship of the PDC deposits and the fallout, it is possible to suggest that the occurrence of the pyroclastic flows marked a disruption in the eruptive conditions, probably associated to the start of the second, and more energetic, pulse. The PDC deposit characteristics suggest different flow dynamics probably also associated to different origins. Juvenile material variations within the deposits suggest that important magma mixing processes during the evolution of the second pulse of the eruption, which were absent during the first pulse. 6 References Castruccio, A., Clavero, J., Rivera, A. 2010. Comparative study of lahars generated by the 1961 and 1971 eruptions of calbuco and Villarrica volcanoes, Southern Andes of Chile. Journal of Volcanology and Geothermal Research, doi: 10.1016/j.volgeores.2009.12.005. Castruccio, A., Clavero, J. 2015. Lahar simulation at active volcanoes of the Southern Andes: implications for hazard assessment. Natural Hazards, DOI 10.1007/s11069-015-1617-x Clavero, J., Godoy, E., Arancibia, G., Rojas, C. and Moreno, H. 2008. Multiple Holocene sector collapses at Calbuco volcano, Southern Andes. Proceedings of the IVACEI General Assembly 2008-Iceland. Freundt A., Wilson C., Carey S. 2000. Ignimbrites and block-and-ash flow deposits. In Encyclopedia of volcanoes, Sigrurdsson (Ed.), Academic Press, p. 581-599. Hickey-Vargas, R., Abdollah, M.J., Parada, M.A., López, L., Frey, F. 1995.Crustal xenoliths from Calbuco Volcano, Andean Southern Volcanic Zone:implications for cristal composition and magma-crust interaction. Contributions to Mineralogy and Petrology 119: 331-344. López, L., Parada, M.A., Moreno, H., Frey, F. and Hickey-Vargas, R. 1992. A contribution to the petrogenesis of Osorno and Calbuco volcanoes, Southern Andes (41°00’-41°30’S): comparative study. Revistageológica de Chile, 19: 211-226. Petit-Breuilh, M. 1999. Cronología eruptiva histórica de los volcanes Osorno y Calbuco, Andes del Sur (41°-41°30’S). Boletín No. 53, Servicio Nacional de Geología y Minería, Chile, 46p. Stern, C., Moreno, H., Lopez-Escobar, L., Clavero, J., Lara, L., Naranjo, J., Parada, M., Skewes, M., 2007. Chilean Volcanoes. In: Moreno T, Gibbons W (eds) The Geology of Chile, Geological Society of London, London pp 149-180. Sellés, D., Moreno, H. 2011. Geología del volcán Calbuco. Carta Geológica de Chile, Serie Geología Básica No. 130, escala 1:50.000. SERNAGEOMIN, 2015. Reporte Especial de Actividad Volcánica (REAV) Región de los Lagos. a. (RAV) Año 2015 Marzo – volumen 3 b. (REAV) Año 2015 Abril 22 (20:45 HL) c. (REAV) Año 2015 Abril 22 (22:30 HL) d. (REAV) Año 2015 Abril 23 (10:30 HL) e. Volcán Calbuco. 30 de Abril (16:00 HL). Volumen 11.

201

AT 4 Impacto de las GeocIencIas en la socIedad

Figure 2. Stratigraphic columns showing the pyroclastic deposits of the 22-23 April 2015 eruption of Calbuco volcano identified on its NE flank (for location see Figure 1).

Figure 3. Left: three pyroclastic density current deposits, showing sinuous (undulated) contact and subhorizontal contact. Right: detail of the sharp contact (dashed line) between 2 pyroclastic density currents, in which the upper contains pumice fragments incorporated from the lower one (p). A “cauliflower” scoriaceous bomb (CB) is also shown.