Small Volume Pyroclastic Density Currents of the 22-23 April 2015 Eruption on the NE Flank of Calbuco Volcano, Southern Andes

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F u n 2 d 6 la serena octubre 2015 ada en 19 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: [email protected]

Abstract collapses in the Holocene. These factors, added to the fact that it surroundings have increased its populated areas, During the 22-23 April 2015 eruption of Calbuco volcano a poses important problems in terms of volcanic risks and series of pyroclastic density currents (PDC) were generated how to manage such risks during eruptive events. 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 rd 2 April 2015 eruption associated to the second pulse of the eruption on the 23 of April. These PDCs correspond to both dense and dilute Calbuco volcano started a new eruptive cycle on the 22nd flows. Most of them contain cauliflower scoriaceous bombs, of April at 18:05 local time. Although reported seismicity some up to 1.5m in diameter, only the lower ones contain brownish vesicular pumices similar to the fallout juveniles. increased above background levels only a couple of hours One distinctive PDC deposit contains highly crystalline before the beginning of the eruption, regional reports silicic prismatically jointed bocks (PJB) that show magma (SERNAGEOMIN, 2015a) indicate a rise in the number of mixing textures (glassy, dense and vesicular juvenile VT events beneath the volcano during the previous material mixed together). months. The eruptive column height of the first pulse reached 16 km in a few minutes (SERNAGEOMIN, Key words: pyroclastic density current, Calbuco, 2015b). This first eruptive pulse lasted 1.5 h, with a plume Andes dispersion to the NE. On 23 April at 1:00 local time, a

second eruption generated a column that reached 17 km of

1 Introduction 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 Calbuco is an active and hazardous volcano located in the flows reached 8 km from the vent in the NE and SW flanks southern Andes of Chile (Stern et al., 2009), whose last and lahars reached the Chapo lake in the S flank eruption before the April 2015 event, occurred in AD 1961 (SERNAGEOMIN, 2015e).In the next days the activity (Petit Breuilh, 1999). Its evolution is mainly characterized decreased with sporadic events which generated weak by the extrusion of silicic andesite lavas and domes and plumes (< 2 km high). On 30 April, at 13:08 local time a their associated pyroclastic flows (mainly block-and-ash third pulse was generated with a 3-5 km column high, with flows and blasts), as well as cold and hot lahars (López et a SE dispersion. During the next weeks the eruptive and al., 1992; Hickey-Vargas et al, 1995; Castruccio et al., seismic activity decreased gradually and on 28 May the 2010; Castruccio and Clavero, 2015; Sellés and Moreno, alert level was lowered to yellow. 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 3 Eruptive stratigraphy of the April 2015 deposits and palaeosoil horizons indicate that the two eruption on the NE flank avalanches occurred at ca. 6.500 years BP and ca. 1.200 years BP (Clavero et al., 2008). According to its evolution, The 22-23 April eruption generated a series of pyroclastic geochemistry and historical eruptive activity, Calbuco is and lahar deposits distributed around the volcano. The considered to be one of the most hazardous active following stratigraphic units, included in this work, volcanoes in the Chilean Andes (Petit Breuilh, 1999; correspond to those identified on the N-NE flanks of the Clavero et al., 2008; Castruccio and Clavero, 2015), as it volcano (Fig. 1), in the Río Blanco-Río Hueñu Hueñu- has generated subplinian eruptions in historical times Ensenada area. (1893-1895; 1929 and 1961) as well as suffered sector 198 AT 4 Impacto de las GeocIencIas en la socIedad

In this sector the units identified and shown in Fig. 2 Figure 2 shows the pyroclastic stratigraphy of the sequence correspond initially to a fallout deposit formed by 4 identified on Calbuco’s NE flank associated to the 22-23 subunits (described in more detail in Segura et al., this April eruption, where 8 PDCs, both dense and diluted have congress). Within this fallout deposit, specifically between been recognized. These small-volume deposits show the second and the third units, a series of at least 8 diverse petrographic and structural characteristics, pyroclastic density currents were generated which partially suggesting different emplacement dynamics, probably inundated the Río Frío-Río Blanco valleys (described in related to different generation mechanisms. From base to more detail later in this work). These PDC deposits are top (Figure 2), the deposits are: overlain by the third and fourth subunits of the fallout deposit (Fig. 2), and partially eroded (and dissected) by a Deposit I: PDC deposited on top of layers 0 and 1 (see series of lahar deposits, most of them of secondary origin, Segura et al., this congress) or on top of pre-2015 lahar forming a pyroclastic terrace in the valley up to 20 m thick. 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 Figure1. Location of the study area and the stratigraphic columns layer (E2) has well-developed lamination 2-10 cm thick analyzed in this study. layers formed by fine to medium lapilli with some

scoriaceous bombs (up to 23 cm Ø). 4 Pyroclastic Density Current deposits on the NE flank Deposit D: Dense massive PDC deposit with abundant cauliflower scoriaceous bombs (up to 60 cm Ø), 4.1 Distribution, Stratigraphy and Petrography subrounded lava lithics (up to 1.4 m Ø) within a fine to A series of pyroclastic density currents were generated medium ash massive matrix. It contains large carbonized during the 22-23 eruption of Calbuco volcano, mainly to semi-carbonized trunks. Locally, it has high directed towards its NE and S flanks. Here we describe the lithic/bombs concentrations (almost clast-supported). stratigraphy of those directed to the NE that partially filled up the Río Frío-Río Blanco valleys. 199 ST 11 TERREMOTOS, VOLCANES Y OTROS PELIGROS GEOLÓGICOS

Deposit A: Layered PDC deposit with 3 distinct subunits. 5 Discussions The lower one, up to 20 cm thick, is a fines-depleted layer According to the stratigraphic relationship of the PDC formed by polylithologic lithic-rich fine lapilli fragments. deposits and the fallout, it is possible to suggest that the The intermediate layer, up to 6 cm thick, is formed by a occurrence of the pyroclastic flows marked a disruption in discontinuous non-carbonized vegetation-rich paeleosoil the eruptive conditions, probably associated to the start of horizon deformed and incorporated by the flow. The upper the second, and more energetic, pulse. layer, up to 2.5 m thick, has an undulated surface and is The PDC deposit characteristics suggest different flow formed by large cauliflower bombs (up to 1.1 m Ø), dynamics probably also associated to different origins. subrounded lithic fragments (up to 1.2 m Ø) within a Juvenile material variations within the deposits suggest massive fine to medium ash matrix. Large bombs and that important magma mixing processes during the lithics increase towards the upper part of the deposit. evolution of the second pulse of the eruption, which were absent during the first pulse. 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, 6 References polylithologic, well-sorted fine to medium lapilli slightly Castruccio, A., Clavero, J., Rivera, A. 2010. Comparative study of laminated layer. The upper one is a fine ash, crystal-rich lahars generated by the 1961 and 1971 eruptions of calbuco and layer with dispersed (15% vol.) large lithic fragments (up Villarrica volcanoes, Southern Andes of Chile. Journal of to 15 cm Ø) concentrated towards the upper part of the Volcanology and Geothermal Research, doi: 10.1016/j.volgeores.2009.12.005. deposit. It is partially overlain by units 2 and 3 of the fallout deposit. Castruccio, A., Clavero, J. 2015. Lahar simulation at active volcanoes of the Southern Andes: implications for hazard assessment. Natural 4.2 Grain size Hazards, DOI 10.1007/s11069-015-1617-x Grain-size distributions for the PDCs identified are shown in Fig. 4. Most fall within the PDC (dilute and dense) Clavero, J., Godoy, E., Arancibia, G., Rojas, C. and Moreno, H. 2008. Multiple Holocene sector collapses at Calbuco volcano, fields, although some units show extremely well-sorted Southern Andes. Proceedings of the IVACEI General Assembly distributions, similar to those found on fallout deposits. 2008-Iceland.

4.3 Volume estimate Freundt A., Wilson C., Carey S. 2000. Ignimbrites and block-and-ash The area covered by pyroclastic flow deposits on the NE flow deposits. In Encyclopedia of volcanoes, Sigrurdsson (Ed.), 2 Academic Press, p. 581-599. flank is approximately 650,000 m . Taking a mean thickness of 15 m for the whole sequence, then a volume Hickey-Vargas, R., Abdollah, M.J., Parada, M.A., López, L., Frey, F. 7 3 of ~10 m is obtained, representing less than 3% of the 1995.Crustal xenoliths from Calbuco Volcano, Andean Southern fallout deposit volume. Taking into account the individual Volcanic Zone:implications for cristal composition and magma-crust thicknesses observed, it is likely that each individual flow interaction. Contributions to Mineralogy and Petrology 119: 331-344. volume is less than 106 m3. By observing aerial López, L., Parada, M.A., Moreno, H., Frey, F. and Hickey-Vargas, R. photographs (Google Earth), pyroclastic flows on the SSW 1992. A contribution to the petrogenesis of Osorno and Calbuco flank had slightly shorter runouts and probably comparable volcanoes, Southern Andes (41°00’-41°30’S): comparative study. volumes. 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.

Figure 4. Grain size distribution of PDC deposits of the 22-23 April eruption on the NE flank of Calbuco volcano. 200 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.

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