A 5000-year record of multiple highly explosive mafic eruptions from Gunung Agung (Bali, Indonesia): implications for eruption frequency and volcanic hazards
The 1963 AD eruption of Agung volcano was one of the most significant twentieth century eruptions in Indonesia, both in terms of its explosivity (volcanic explosivity index (VEI) of 4+) and its short-term climatic impact as a result of around 6.5 Mt SO2 emitted during the eruption. Because Agung has a significant potential to generate more sulphur-rich explosive eruptions in the future and in the wake of reported geophysical unrest between 2007 and 2011, we investigated the Late Holocene tephrostratigraphic record of this volcano using stratigraphic logging, and geochemical and geochronological analyses. We show that Agung has an average eruptive frequency of one VEI ≥2–3 eruptions per century. The Late Holocene eruptive record is dominated by basaltic andesitic eruptions generating tephra fall and pyroclastic density currents. About 25 % of eruptions are of similar or larger magnitude than the 1963 AD event, and this includes the previous eruption of 1843 AD (estimated VEI 5, contrary to previous estimations of VEI 2). The latter represents one of the chemically most evolved products (andesite) erupted at Agung. In the Late Holocene, periods of more intense explosive activity alternated with periods of background eruptive rates similar to those at other subduction zone volcanoes. All eruptive products at Agung show a texturally complex mineral assemblage, dominated by plagioclase, clinopyroxene, orthopyroxene and olivine, suggesting recurring open-system processes of magmatic differentiation. We propose that erupted magmas are the result of repeated intrusions of basaltic magmas into basaltic andesitic to andesitic reservoirs producing a hybrid of bulk basaltic andesitic composition with limited compositional variations.
KeywordsAgung Tephrostratigraphy Eruptive history Basaltic andesite Magma mixing Magma mingling
A detailed view of the eruptive history of a volcano provides crucial information for hazard assessment at different timescales. Historical records of volcanic activity typically only span a few hundred years, depending on the geographic location. In the case of frequently erupting volcanoes, these historical records are sufficient for a relatively good understanding of the typical style of activity at a specific volcano in its current state and thus short-to-mid-term hazards, e.g. the case for twentieth century dome-forming eruptions at Merapi (Voight et al. 2000). Even at frequently active volcanoes, however, historical records may be far from complete, as was recently demonstrated for Villarrica volcano in Chile, based on a high-resolution temporal record of tephra and lahar deposits entrained in lacustrine sediments (Van Daele et al. 2014). Mid-to-long-term hazard assessment especially requires information from geological, typically tephrostratigraphic, studies which may highlight (long-term) temporal variations in magma composition, eruptive style and/or frequency and associated hazards (e.g. for Merapi: Andreastuti et al. 2000; Gertisser et al. 2012; Newhall et al. 2000). At dormant volcanoes with only few or no historically documented eruptions, tephrostratigraphy is the primary source of information for volcanic hazard assessment, which feeds into risk management and mitigation plans. In this paper, we present the first tephrostratigraphic record for Gunung (Indonesian for hill or mountain) Agung volcano in eastern Bali, to understand the past and potential future behaviour of this highly explosive basaltic andesite volcano.
In 1963, Agung had one of the biggest twentieth century eruptions worldwide (Self and Rampino 2012). After a few days of felt earthquakes and small ash eruptions in February 1963, ~0.1 km3 of andesite lava was extruded until 17th March, when a large explosive eruption generated pyroclastic density currents (PDCs) and lahars and devastated a wide area predominantly north and south of the volcano. A second explosive phase of similar intensity occurred 2 months later, on 18th May, producing more PDCs and lahars. The total death toll of the eruption is estimated between 1100 and 1900 (Self and Rampino 2012; Tanguy et al. 1998; Zen and Hadikusumo 1964). Intermittent, smaller, explosions continued until 20th June 1963 (Piip et al. 1963), and secondary lahars were generated for several months after the end of the eruption. A total estimated magma volume of ca. 0.4 km3 was erupted (Self and Rampino 2012), corresponding to a magnitude 5.0 eruption (Pyle 2000); the bulk volume of pyroclastics (ca. 0.8 km3; Self and Rampino 2012) makes this a high-end VEI 4 event (Newhall and Self 1982). Self and King (1996) inferred from petrological and geochemical data that a basaltic intrusion mixed into an andesitic reservoir. The 1963 AD eruption was one of the first witnessed volcanic eruptions that had a short-lived climatic impact due to vast amounts of sulphur being injected into the higher atmosphere, where it formed 10–12 Mt of H2SO4 aerosols (Self and Rampino 2012). Estimates of the global average temperature decrease as a result of these aerosols in the months to years after the eruption, vary from 0.1 to 0.4 °C (Angell and Korshover 1985; Canty et al. 2013; Rampino and Self 1982; Self et al. 1981; Wigley et al. 2005).
In 1989, weak solfataric activity and a few volcanic earthquakes were reported (Global Volcanism Program, http://volcano.si.edu). Apart from those, Agung has been largely quiet since 1963. Based on ALOS Interferometric Synthetic Aperture Radar data, Chaussard et al. (2013) reported inflation centred on the volcano’s summit at a rate of 7.8 cm/year between mid-2007 and early 2009, followed by slow deflation (and no eruption) at a rate of 1.9 cm/year until mid-2011 (the last acquired data). The 2007–2009 inflation was modelled to result from a shallow source ca. 4.4 km below Agung’s summit, or 1.9 km below the average base level of the volcano, and could reflect the addition of new magma in the shallow storage levels (Chaussard and Amelung 2012). The deflation may be explained by cooling and thermal contraction of the magma (Chaussard et al. 2013) or by passive degassing of magma that was injected up until 2009 (Girona et al. 2014). Based on the evidence of active deformation, in 2012 and 2013, a GPS and new seismic network was installed at Agung by the Indonesian Centre for Volcanology and Geological Hazard Mitigation and the US Geological Survey Volcano Disaster Assistance Program. To date, little significant deformation or seismicity has been detected (J Pallister, personal communication 2015).
Previous studies of Agung have focused mostly on the 1963 AD event (Self and King 1996; Self and Rampino 2012) and its climatic impact (e.g. Rampino and Self 1982). From the geological map by Nasution et al. (2004), it is clear that Agung has a history of repeated eruptions generating lava flows, PDCs and lahars. Unlike at its immediate neighbours, there is no geological evidence to suggest that Agung had a caldera-forming event in its past. This study evaluates the Holocene eruptive frequency–magnitude relationships at Agung using the stratigraphy of pyroclastic deposits, and aims to put constraints on the magma plumbing system from petrological and geochemical data on products from some major eruptions.
Radiocarbon dates of palaeosols between (scoria) fall deposits and charcoal entrained in PDC deposits
Polished thin sections of individual scoria and pumice lapilli from fall deposits, scoriaceous bombs from PDC deposits, and a few pieces of lava, were prepared at High Mesa Petrographics (NM, USA). Bulk geochemical analysis was performed at Activation Laboratories Ltd. (Ontario, Canada). When possible, individual scoria or pumice lapilli were selected for these bulk analyses. In other cases, multiple lapilli were grouped to obtain enough material for a representative bulk analysis. Samples were first powdered in a mild steel mill, after which loss on ignition was determined. Major elements and selected trace elements were analysed with inductively coupled plasma–optical emission spectroscopy (ICP-OES) using lithium-metaborate flux melting to dissolve the powders. Other trace elements were analysed with ICP–mass spectroscopy (ICP-MS). Bulk geochemical data are presented in Supplementary Table 1.
Major and minor element composition of phenocrysts was obtained on thin sections and grain mounts of selected samples with a JEOL JXA 8530-F Field Emission Electron Microprobe (EMP) equipped with five tuneable wavelength-dispersive spectrometers at the Facility for Analysis, Characterisation, Testing and Simulation at Nanyang Technological University, Singapore. Point analyses on pyroxene, olivine and magnetite were acquired using a focused beam, 20 nA beam current and 15 kV accelerating voltage. Beam current was reduced to 10 nA for plagioclase and glass (melt inclusions), which were analysed with a beam diameter of 3 and 5 μm, respectively. Cl and S were measured at a 50-nA current in glass at the end of each analysis. Peak and off-peak counting times varied between 20 and 40 s, depending on expected concentration. Results were quantified using well-characterised natural and synthetic external calibration standards and a modified ZAF matrix correction procedure. A time-dependent intensity correction was applied to analyses of plagioclase and glass to correct for beam modification of the sample during analysis. Amphibole was not present in sufficient quantities for reliable analysis. The groundmass of many samples was too crystal-rich to allow for matrix glass analysis. EMP data are presented in Supplementary Table 2a–e.
Stratigraphy and bulk geochemistry of pyroclastic deposits
The oldest radiocarbon date we obtained on a palaeosol in the tephra fall sequences is 5.16 ± 0.09 cal. ka BP (Sample AG024B; Table 1, Fig. 3). The combined tephra fall sections provide a record of Late Holocene explosive eruptions that were large enough to be preserved. The oldest radiocarbon date on charcoal from a PDC deposit is 5.95 ± 0.05 cal. ka BP (Table 1, Fig. 3). The latter is however separated by a stratigraphic discordance (stream deposits) from the deposits higher up in the sequence. Most PDC deposits studied here were emplaced within the last 1200 years.
Andesitic to (trachy)dacitic pumice fall and PDC deposits interbedded within the scoria fall sequence (Fig. 3) are interpreted to originate from Batur Plinian-style eruptions, as confirmed by their chemical composition (Figs. 4 and 6, Supplementary Table 1; Reubi and Nicholls 2005). Although the major element compositions of some Batur samples are similar to those of some Agung samples, especially in the basaltic andesite range (Fig. 4), certain trace element ratios, e.g. Zr/Nb, are useful to distinguish deposits from Agung and Batur (Fig. 6). We attribute these subtle differences in trace element signature between Agung and Batur to differences in the source rock components and/or the degree of partial melting generating the parent melts beneath both volcanoes (e.g. Reubi and Nicholls 2004b). The trace element signatures confirm that all scoria fall deposits and the few andesitic units we identified in the studied sequences, all downwind of Agung and upwind of Batur, originate from Agung. The latter include pumice at the surface and a PDC unit south of Agung (samples AG001bisH, AG045D; Supplementary Table 1, Fig. 3), which chemically corresponds well with the Aap3 unit defined on the geological map (Doust 2003; Nasution et al. 2004) This andesitic PDC deposit represents the most evolved composition found for Agung and is chemically clearly distinct from the 1843 AD products. It was dated at 0.17 ± 0.10 ka cal BP (sample AG045E; Table 1) and could correspond to the 1808 AD eruption.
A twin fine-grained cream-coloured pumice fall unit which clearly stands out from Agung deposits has proven a useful marker horizon in several key sections. We interpret this unit as the 1257 AD pumice fall deposit from the neighbouring Rinjani–Samalas complex on Lombok. This is consistent with both its geochemical composition and radiocarbon ages bracketing the deposit age (Fig. 3, Table 1, Supplementary Table 1; Lavigne et al. 2013).
Petrography and mineralogy
To quantify mineral compositions using EMP analyses, we selected samples from the three most recent PDC deposits from section AG042-042bis (Fig. 3), spanning the compositional range for PDCs found at Agung, ca. 53–56 % SiO2, including the 1843 AD deposits. For the scoria fall samples, we focus on the major deposits from section AG008-021 as it is the most complete section for the Late Holocene and we can correlate it well to section AG042-042bis (Fig. 3). Again, this selection of samples spans almost the entire compositional range of fall deposits found at Agung, ca. 51.5–57.5 % SiO2.
In the most evolved samples, orthopyroxene mostly occurs with large cores, often with a patchy appearance, surrounded by relatively broad oscillatory zoned rims (Fig. 8c). Apatite, titanomagnetite and melt inclusions sometimes occur in the patchy cores. The contact between core and rim is usually sharp but can be irregular. In more mafic samples, crystal rims tend to be narrower or even absent and sometimes show signs of dissolution or rounding. Compositions generally vary between En65-74Wo3–4, with Mg# = 67–79 (Fig. 10c, d), except for the cores in the more evolved samples (especially the PDC ones), which tend to be less Mg-rich (as low as Mg# = 61, Fig. 10d) and show a range in composition (En59-71Wo3–4). Orthopyroxene in the 1963 AD products was also commonly found to be reversely zoned by Self and King (1996). In the other samples, core and rim compositions show little or no variation.
Olivine is found in most samples, except in some of the most evolved ones. It occurs as sub- to anhedral crystals, sometimes embayed, and often with reaction rims of orthopyroxene (En68-71Wo4–5, Mg# = 74–78; Fig. 8e, f). Reaction rims are thin in the most mafic sample (AG042D). Sample AG042I, compositionally between AG042D and AG042B, contains embayed olivine with thick reaction rims of orthopyroxene, as well as subhedral olivine without reaction rims. Compositions generally vary from Fo71 to Fo80. Scattered olivines in scoria fall samples show compositions mostly of Fo71–74, with some exceptions (Fo57, Fo62) in one of the most mafic scoria fall samples. Cores are generally slightly more Mg-rich than rims (excluding reaction rims).
A few melt inclusions were found in pyroxene and olivine of the tephra fall samples. The inclusions do not contain vapour bubbles or secondary crystals. The glass composition is generally basaltic andesitic to andesitic, and more evolved than the bulk composition of the host magma (Fig. 4). Sulphur contents vary from 120 to 740 ppm, with a tendency for the higher values to occur in the more mafic melt inclusions. Chlorine contents range from 1900 to 3300 ppm, with higher values in the more evolved melt inclusions. These values are similar to those reported by Self and King (1996) in the 1963 AD products.
Pre-eruptive temperatures of touching pairs or intergrowths of clino- and orthopyroxene were estimated using QUILF (Andersen et al. 1993). In absence of other constraints on pressure, we set the input pressure at 123 MPa, corresponding to 4.4 km depth below the summit (using a crustal density of 2800 kg/m3), i.e. the depth of the modelled source of inflation from 2007 to 2009 by Chaussard and Amelung (2012). Pyroxene pairs were tested for equilibrium following Putirka (2008). Estimated temperatures for touching pairs in equilibrium vary between 976 and 1085 °C, with a general tendency for temperatures to be lower in the more evolved samples. Even the most mafic sample shows relatively low temperatures as well, but we ascribe this to the limited number of touching pairs that were found in each sample (between 3 and 10 pairs per sample).
Based on our dataset representing compositions erupted at Agung during the Late Holocene, we construct a generalised conceptual model of recurring magmatic processes at Agung leading to highly frequent basaltic andesitic explosive activity. All samples show petrographical evidence of systematic magma mingling/mixing, generating the range of erupted magmas. This is particularly evidenced by the complex zoning patterns and wide range of compositions of plagioclase, suggestive of crystallisation, and occasional dissolution, under variable conditions of host melt composition, temperature, water content and/or pressure. Plagioclase crystals tend to look more pristine in andesitic samples than in more mafic samples, and rim compositions in all samples display a more narrow range than cores. Pyroxene compositions are less variable, but their textures are suggestive of a mafic component showing disequilibrium features and a more evolved component with large cores and rims of a different composition. Olivine is unstable and recrystallizing to orthopyroxene in basaltic andesitic samples, but only shows a limited amount of dissolution in more mafic samples. All these textural observations together suggest that mingling of a basaltic (andesitic) magma and an andesitic magma to form a hybrid basaltic andesite is a ubiquitous process at Agung. During mingling, both end-member magmas partially re-equilibrate with the new liquid composition and varying proportions of mingling components may lead to variations in the bulk composition of the hybrid magma over a limited compositional range.
The few analysed melt inclusions in olivine and pyroxene from Late Holocene fall deposits, suggest similar pre-eruptive levels of S and Cl in the melt to those reported for the 1963 AD eruption, which produced an estimated 6.5 Mt of SO2 (Self and King 1996; Self and Rampino 2012). If 25 % of eruptions at Agung produce an amount of H2SO4 aerosols of the same order of magnitude (Section “Eruption frequency and rate”), Agung is a significant source of regular, strong atmospheric perturbations.
The valleys south and southeast of Agung are subjected to PDCs every ca. 250–300 years (Figs. 1 and 3). These are minimum estimates influenced by erosion, slope morphology and eruption particularities. The geological map suggests that the N-NE and SW-SE flanks are most prone to PDCs. Lava flows occur on all flanks of the volcano. The most silicic ones are generally restricted to the upper flanks, whereas the most mafic ones have reached the northeast coastline (Nasution et al. 2004).
Most tephra fall deposits are more mafic than the PDC deposits, and it is not clear how the two types of deposits are linked in Agung’s history. The fact that different sequences sample different time intervals complicates interpretations. In 1963 AD, the PDCs were more mafic than the lava flow and early fall deposits (Self and King 1996), and in the case of the 1843 AD eruption, all deposits have a similar geochemical composition. Those two eruptions were both associated with a lava flow, tephra fall and PDCs, but for older deposits, we cannot constrain whether all PDC deposits have associated fall deposits, and vice versa, or whether eruptions typically start with the effusion of a lava flow, as in 1963 AD.
The majority of PDC deposits at Agung resulted from scoria flows, containing scoriaceous breadcrust bombs (e.g. AG042B). Some PDC deposits were identified as block-and-ash flow deposits (e.g. AG042D), which would typically result from gravitational collapse of a growing lava dome, as frequently happens at Merapi in central Java (e.g. Voight et al. 2000). The main juvenile component of these PDC deposits consists of dense decimetre-scale blocks of basaltic andesitic lava, set in a loose gritty ash matrix. Although the crystallinity of blocks originating from a lava dome could be expected to be higher than that of scoriaceous bombs due to slower cooling rates in the former (e.g. Sparks et al. 2000), a qualitative comparison of both types does not reveal significant differences in texture. All blocks and bombs from Agung PDC deposits however display similar crystallinities to blocks from Merapi PDC deposits, which are clearly derived from lava dome collapse (Preece et al. 2013). Despite similarities in overall crystallinity, we did not spot any Ti-magnetite grains with exsolution lamellae that would result from stagnation in a shallow conduit (Turner et al. 2008), as seen for Merapi’s 2010 samples (Preece et al. 2013), in any of the thin sections that were studied with backscatter secondary electron imaging. We cannot rule out that a lava dome could occasionally grow at Agung’s summit, but based on the current dataset, it does not seem likely that lava dome formation is a common process in Agung’s geologically recent history.
Eruption frequency and rate
Fifty-two tephra-fall-producing eruptions are recognised in the last ca. 5.2 ky in combined sections AG008/021 and AG006/006bis (Fig. 3). This represents an average eruptive frequency of one eruption every 100 years, seemingly consistent with the general frequency of eruptions described in historical texts covering the last ca. 500 years (Hägerdal 2006). We assigned an approximate age to each event, assuming the average eruptive frequency of one event per century has not changed significantly over time. This assumption seems valid from a qualitative inspection of the tephra fall sections (Fig. 3).
With both reference sections AG008/021 and AG006/006bis 8–10 km downwind of Agung and only 3.5 km apart, we use deposit thickness and maximum grain size as a proxy of eruption size relative to that of the 1963 AD eruption (thickness ~45 cm, maximum grain size ~40 mm), and assign an approximate VEI and associated volume to each event. This simplified approach assumes that the sections west of Agung capture the most complete history of tephra-fall-producing eruptions for Agung due to prevailing easterly winds. Sections east of Agung clearly contain much less, and much more poorly preserved, tephra fall deposits, and sections north and south mostly comprise PDC deposits unlikely to provide a complete picture of eruptive histories due to potential erosion in underlying units during emplacement. Deposits which are clearly thicker and/or coarser than 1963 AD at the type locality, e.g. 1843 AD, are assigned VEI 5 and 1 km3 (the lower VEI 5 limit; Newhall and Self 1982). Deposits of similar scale to 1963 AD are assigned VEI 4 with 0.5 km3, except for 1963 AD itself, which is estimated to comprise 0.8 km3 of tephra (Self and Rampino 2012). Events smaller than 1963 AD are further subdivided into VEI 3 (0.05 km3) and VEI 2 (0.005 km3) using an arbitrary deposit thickness threshold of 10 cm.
Reported eruptive rates for explosive activity over the last 0.1–10 ky at basaltic–andesitic subduction zone volcanoes range from 0.1–0.7 km3/ky (e.g. Colima: Luhr and Carmichael 1982; Kuju: Kamata and Kobayashi 1997; Lamongan: Carn 2000; Aso: Miyabuchi 2009). Apparently, Agung has experienced periods of relatively high magma eruptive rates in its recent past, alternating with rates comparable to those seen at other similar volcanoes. The processes controlling the variable eruptive rates remain unclear. A regional tectonic influence resulting in partial blockage of the conduit or deeper plumbing systems seems counterintuitive: we would expect to see (i) more evolved compositions in the higher-activity periods as a result of prolonged magmatic differentiation during calmer periods (e.g. seen at Merapi, Gertisser and Keller 2003b) and (ii) a similar response of reduced/increased activity to regional tectonic stress changes in neighbouring volcanoes. In contrast, it seems more likely that the Late Holocene history of Agung is characterised by periodic increased magma supply rates from depth, followed by limited crustal residence times, and resulting in frequent intense explosive activity of relatively mafic magmas.
The tephrostratigraphic record of Agung volcano shows that its eruptive activity in the Late Holocene was dominated by explosive eruptions generating moderately widespread tephra fall deposits, mainly impacting the east of Bali. Juvenile scoria of basaltic andesitic composition, as well as PDC deposits, are typically confined to the northern and southern slopes. These PDCs were commonly reworked to produce lahars nearer the coastline. The compositional range of eruptive products at Agung is limited to basaltic andesite, and occasionally andesite, e.g. the 1963 AD lava flow and the 1843 AD eruption. The latter is of at least similar magnitude to the 1963 AD events, as are about 25 % of the documented deposits. The complex petrographic relationships record evidence for systematic open-system magmatic differentiation. The most abundant mineral phases show textural and compositional evidence of a repeated history of basaltic intrusions into a slightly more evolved reservoir, possibly andesitic, producing hybrid basaltic andesitic magmas. S-rich melt inclusions suggest that Agung is a potential source of regular and significant atmospheric perturbation by frequent emission of large amounts of SO2. The tephra fall record suggests an average frequency of one explosive eruption per century, with some of the deep valleys along the southern flank of Agung subjected to PDCs every few centuries. At the millennium scale, Agung is characterised by periods of background eruptive rates similar to other subduction zone volcanoes, alternated with periods of increased eruptive rates, ascribed to increased magma supply rates from depth.
We thank CVGHM for logistic support during fieldwork and RISTEK for research permits. We are grateful to Anwar Sidik, I Nengah Wardhana and Dewa Mertheyash from the Rendang Volcano Observatory for their hospitality and help in the field. Ryuta Furukawa is thanked for introductions to key outcrops. Tanya Furman is kindly acknowledged for sharing the work by Doust (2003). Reviews by John Pallister and Mary-Ann del Marmol, and editorial handling by James Gardner were greatly appreciated. Fieldwork and laboratory analyses were funded by the Earth Observatory of Singapore. Data interpretation and writing was performed at Oxford (NERC grant NE/I013210/1) and Ghent universities.
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