Pyroclastic deposits and eruptive heterogeneity of Volcán Antuco (37°S; Southern Andes) during the Mid to Late Holocene (<7.2 ka)

https://doi.org/10.1016/j.jvolgeores.2019.106759Get rights and content

Highlights

  • Tephrostratigraphy of Mid to Late Holocene (<7.2 ka) explosive eruptions of Antuco volcano reconstructed to the east.

  • Recognizance and description of 23 explosive eruptions

  • Three sub-Plinian eruptions are identified and one potential Plinian eruption.

  • Tephras described in terms of componentry, geochemistry, mineralogy and petrology.

Abstract

Volcán Antuco (37°24′ S, 71°22′W; 2979 m asl) is the 13th ranked high threat volcano in Chile with 27 recorded historical eruptions, mostly (~96%) with volcanic explosivity indices (VEI) of ~1–2. An older eruptive record has been reconstructed from sections exposed on the western flank and is intimately related to a well-documented catastrophic sector collapse at ~7.2 cal ka BP. However, very little is known about Antuco's post-collapse eruptive history in other sectors, especially on the eastern flanks where prevalent westerly winds favor optimal eastward tephra transport and deposition. Our study reveals a more complete record of activity that has already been indicated from previous work with at least 23 tephra-forming explosive eruptions, most of them within the last c. 7.2 ka, including 4 events that have generated pyroclastic density currents that have widely inundated the lower eastern flanks. Tephra from these eruptive events are typically composed of scoria, free crystals and lithics, with occasional pumice. The composition of juvenile fragments varies between basalt and trachyandesite (50.2–62.2 wt% SiO2) and show phenocrysts of plagioclase, olivine and pyroxene. Our results show that most of the eruptions of Antuco (c. 79%) are Strombolian to violent Strombolian. These eruptions have an estimated longer repose times (c. 200 year) and are likely higher in magnitude than those registered during historical times. This study also shows that the composition, style and magnitude may change from one eruptive episode to the next. This eruptive variability seems in complete accord with recent findings from other centers in the Southern Volcanic Zone exhibiting similar temporal eruptive diversity and ultimately, has significant implications with respect to hazard assessment.

Introduction

Mafic (i.e. basaltic to basaltic andesite) volcanism generally have Hawaiian or Strombolian eruption style, but can have a combination of or transition in between both end-member styles (Parfitt, 2003) with typical discharge rates from 103 to 105 kg·s−1 (Simkin and Siebert, 1994). Most Strombolian activity consists of short-lived intermittent explosions with ejection of pyroclastic rocks (e.g., Chouet et al., 1974; McGetchin et al., 1974; Blackburn et al., 1976; Heiken, 1978; Francis and Oppenheimer, 2004; Patrick et al., 2007). However, sustained eruption plumes during paroxysmal events could rise to 10 km above the vent being fed by closely spaced individual explosions (Parfitt, 2003), developing the so-called “violent Strombolian” eruptions (Walker, 1973), which are responsible to produce highly vesicular tephra deposits larger in volume than lava flows (e.g., Heiken, 1978; Taddeucci et al., 2004; Pioli et al., 2008; Ruth and Calder, 2014; Barsotti et al., 2015). In some cases, short-lived, high-altitude (~2 km) lava fountains may also produce sustained tephra columns (e.g., Romero et al., 2014; Bonaccorso et al., 2014; Andronico et al., 2014; Romero et al., 2018). In a few cases basaltic to basaltic-andesite eruptions are of Sub-plinian and Plinian styles or types (e.g., Sable et al., 2006; Costantini et al., 2009, Costantini et al., 2011; Romero et al., 2016; Arzilli et al., 2019) with mass discharge rates (MDR) up to 0.7–2.0 × 108 kg·s−1 (Costantini et al., 2009). The most important controlling factor in this eruption heterogeneity is the viscosity of the basaltic magma (e.g. Pioli et al., 2012; Dominguez et al., 2016; Spina et al., 2019). The explosive behavior of mafic volcanoes can be fully substantiated through detailed tephrostratigraphic survey and analysis of their explosive products.

Antuco is a compound stratovolcano located 90 km east of Los Angeles city in the central part of the Southern Andes of Chile (Fig. 1A). It has exclusively erupted basaltic products (50.9–53.7% SiO2 since the Last Glacial Maxima (Martínez et al., 2018). Antuco lavas are typically richer in Na2O than the average of circum-Pacific basalts (Vergara et al., 1985), and are among the most isotopically primitive Holocene lavas in the Southern Volcanic Zone (Hildreth and Moorbath, 1988; Lohmar, 2000; Hickey-Vargas et al., 2016). At least 27 historical eruptions are reported for the period 1624–1939, mostly VEI 1–2 with Strombolian style, which produced basaltic lava flows and restricted scoriaceous tephra fallout (Petit-Breuilh, 1994; Lohmar, 2000) with low potential of preservation. The latest major eruptive episode at Volcán Antuco occurred during 1852–1853 (Smith, 1915) and was characterized by combined central-vent and parasitic cone-forming activity resulting in the emission of lava flows that inundated the western flanks and the eastward distribution of predominantly black-colored tephra fallout (Fig. 1B). This eruption was later characterized as VEI 3 (Smith, 1915). Despite high-eruptive frequency during historical times (15th to 19th Centuries), its current dormancy for nearly a century and its 13th position in the ranking of high risk volcanoes in Chile (www.sernageomin.cl), very few tephrostratigraphic studies have been previously conducted for this volcano (e.g., Lohmar et al., 1999). The evaluation of the explosive behavior of Volcán Antuco is therefore necessary to characterize and evaluate the threat to the visitors and tourism-related facilities located within Laguna del Laja National Park, as well as to the adjacent critical infrastructure, namely, hydroelectric power stations and an international route to Argentina.

In this study, we present the results of a Late Holocene (i.e. the last c. 7 kyr) tephrostratigraphic survey carried out between March 2015 and November 2018. Both tephra fall and pyroclastic density currents have been studied in terms of their stratigraphy, grain size, componentry, juvenile morphology, petrology and geochemistry, in order to comprehend the style and magnitude of their parental eruptions as well as associated volcanic hazards.

Section snippets

Volcán Antuco

Located in the Biobio Region of southern Chile (37°24′S-71°22′W; 2979 m above the sea level), Volcán Antuco is a composite stratovolcano which has been active during the last 150.4 ka, and has an estimated volume to 62 km3 (Martínez et al., 2018). The main cone is 1 km high, with a volume of 4 km3, and is located in the center of a sector collapse scar triggered by a phreatic eruption at an ancestral edifice (Moreno et al., 2000), which collapsed at 6.2 ka (~7.2 cal ka BP, Lohmar, 2000). The

Field stratigraphy

Tephra thickness and particle size data were obtained from nineteen field sites, and stratigraphic data was obtained from another 30 sites. From these 49 descriptive sites, 22 composite stratigraphic columns were constructed (Fig. 1B; see Supplementary Table 1 for location, distance from the main cone, etc.). All observations occur at distances between 0.9 and 21 km east-southeast of Antuco crater. Thickness, structure, grading and particle size of identified tephras were registered in these

Tephrostratigraphic framework

Preservation of tephra and the overall completeness of the broader tephrostratigraphic framework are dependent on the distance from the vent and strongly influenced by the interaction of physical processes associated with slope, fluvial, glacial and lake environments. Most of these detrital deposits that have interacted with the fresh volcanic products were described and mapped in a series of previous works (e.g., Moreno et al., 1984; Lohmar, 2000; Melnik et al., 2006; Urrutia et al., 2010) and

Tephra correlation and age constrains

Although the stratigraphy established in this work was obtained from subaerial outcrops, an important part of the record was originally deposited in Laguna del Laja, located immediately adjacent to Volcán Antuco. McNamara et al. (2019) have demonstrated that the thickness and grain size of tephra fall deposits in lake cores and nearby terrestrial sections are very similar; however, cores sampled close to fluvial inflows are affected by sediment deposition from the lake's catchment and they

Conclusions

Tephrostratigraphic records found east of Antuco account for at least 23 explosive eruptions in the last <7.2 kyr BP. Any likely influence of other eruptive source(s) within a distance of 200 km south and north from Antuco has been properly evaluated using criteria, such as age, dispersal and major element geochemistry of these tephras, and suggest that these deposits likely belong to Antuco. Even though we have not obtained any absolute age for deposits older than 4 kyr (observed in Section

CRediT authorship contribution statement

Jorge E. Romero: Visualization, Investigation, Data curation, Writing - original draft. Victoria Ramírez: Investigation. Mohammad Ayaz Alam: Supervision. Jorge Bustillos: Investigation. Alicia Guevara: Formal analysis. Alessandro Pisello: Investigation. Daniele Morgavi: Writing - review & editing. Evelyn Criollo: Formal analysis.

Acknowledgements

We thank to Dr. Brent V. Alloway for his detailed review and language assistance in the proof version of this manuscript. Authors are grateful to Mr. Hector Moyano, Mr. Luis Cortes and Dr. Juan Díaz-Alvarado for their assistance and participation in the fieldwork. Dr. Urrutia thanks Conicyt/Fondap/15130015 research funding. The staff of Chile's National Forest Corporation (CONAF), Ana Hinojosa, Miguel Infante, Segundo Necul and Juan Bascur, provided excellent support for the fieldwork within

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