Eruptive history of Sundoro volcano, Central Java

Bull Volcanol (2016) 78:81 DOI 10.1007/s00445-016-1079-3

RESEARCH ARTICLE

Eruptive history of Sundoro volcano, Central Java, Indonesia since 34 ka Oktory Prambada 1,2 & Yoji Arakawa 1 & Kei Ikehata 1 & Ryuta Furukawa 3 & Akira Takada 3 & Haryo Edi Wibowo 4 & Mitsuhiro Nakagawa 4 & M. Nugraha Kartadinata 2

Received: 29 March 2016 / Accepted: 15 October 2016 # Springer-Verlag Berlin Heidelberg 2016

Abstract Reconstruction of the eruptive history of Sundoro volcano is needed to forecast the probability of future eruptions and eruptive volumes. Sundoro volcano is located in Central Java (Indonesia), 65 km northwest of Yogyakarta, and in one of the most densely populated areas of Indonesia. On the basis of stratigraphy, radiocarbon dating, petrography, and whole-rock geochemistry, we recognize the following 12 eruptive groups: (1) Ngadirejo, (2) Bansari, (3) Arum, (4) Kembang, (5) Kekep, (6) Garung, (7) Kertek, (8) Watu, (9) Liyangan, (10) Kledung, (11) Summit, and (12) Sibajak. The Ngadirejo (34 ka BP) to Kledung (1 ka) eruptive groups are inferred to have been the stratovolcano building phase. Based on compositions of deposits, one or more magma reservoirs of

intermediate chemical composition are inferred to have existed below the volcano during the periods of time represented by the eruptive groups. SiO2 of juvenile eruptive products ranges from 50 to 63 wt%, and K2O contents range from high K to medium K. The chemical composition and phenocryst content of eruptive products change with time. The lower SiO2 products contain mainly plagioclase, clinopyroxene, and olivine, whereas the more evolved rocks contain plagioclase, clinopyroxene, orthopyroxene, and rare hornblende and olivine. Our work has defined Sundoro’s eruptive history for the period 1–34 ka, and this history helps us to forecast future activity. We estimated that the total amount of magma discharged since 34 ka is approximately 4.4 km3. The average

Editorial responsibility: S. Self Electronic supplementary material The online version of this article (doi:10.1007/s00445-016-1079-3) contains supplementary material, which is available to authorized users. * Oktory Prambada [email protected]; [email protected] Yoji Arakawa [email protected] Kei Ikehata [email protected]

M. Nugraha Kartadinata [email protected] 1

Graduate School of Life and Environmental Sciences, Earth Evolution Science-University of Tsukuba, Chome 1-1-1 Tennodai, Tsukuba, Ibaraki Prefecture 305-8571, Japan

2

Center for Volcanology and Geological Hazard Mitigation (CVGHM), Geological Agency of Indonesia, Ministry of Energy and Mineral Resources, Jl. Diponegoro No. 57, Bandung, West Java 40122, Indonesia

3

Geological Survey of Japan (GSJ), The National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan

4

Division of Earth and Planetary System Science, Hokkaido University, N10W8, Japan Office: Science Bldg. 6, 11F, Sapporo 060-0801, Japan

Ryuta Furukawa [email protected] Akira Takada [email protected] Haryo Edi Wibowo [email protected] Mitsuhiro Nakagawa [email protected]

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eruption rate over this group ranges from 0.14 to 0.17 km3/ kyr. The eruption rate and the frequency of individual eruptions indicate that the volcano has been very active since 34 ka, and this activity in combination with our petrological data suggest the presence of one or more magma reservoirs that have been repeatedly filled and then discharged as eruptions have taken place. Our data further suggest that the volume of the crustal reservoir system has increased with time, such that explosive eruptions are more likely in the future and that they may be larger than the most recent small eruptions. Keywords Sundoro volcano . Petrology medium K . Eruptive group . Eruption rate . Volcanic history . Stratigraphy . Forecasting

Introduction Explosive eruptions cause natural disasters through both local/ regional effects (e.g., loss of life, damage to property, changes in land use) and more widespread effects such as short-term climate change and impacts to international aviation (e.g., Self 2006). Consequently, it is important to know which volcanoes are capable of explosive eruptions and how frequently they will produce large explosive eruptions in the future. In order to do this, we must first know a volcano’s history, i.e., the number and frequency of past explosive eruptions is typically a good guide to future behavior. Second, we need to know how the volcano is evolving, i.e., is it trending toward additional explosive eruptions? We need to base our analysis on geological records in addition to historical records. Such long-term records are important even for frequently active volcanoes with relatively long and detailed historical observations. For example at Merapi volcano, the geological record will provide crucial information on less-frequent, larger, explosive events, information that is needed for probabilistic modeling of future eruption scenarios and which cannot be derived from historical records alone. Basic field volcanology, stratigraphy, geochronology, and petrology are tools we use to determine eruptive history and to clarify and assess trends in volcanic eruption. Once determined, eruptive history and trends in frequency and size of eruptions become important constraints for forecasts of future eruptions, which, in turn, can be used for land use planning and for prioritizing installation of active monitoring networks and other means of mitigating volcanorelated disasters. One Indonesian candidate for this type of investigation is Sundoro volcano (Fig. 1). Sundoro volcano is a stratovolcano with a nearly “perfect” conical morphology that rises to 3136m elevation. It is an active volcano with a long history of eruptions, and it has intensive volcano-tectonic earthquake microseismicity. Sundoro volcano (“Si Ndoro” or “Su Ndoro” comes from the Javanese word “The Master”

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denoting mannerly, thoughtful, and always protecting aspects). This volcano is located in an area having one of the highest population densities in the world (Table S1), with about 877 people/km2 in 1990 (Small and Naumann 2001). A geological map for this volcano was published by the Center for Volcanology and Geological Hazard Mitigation (CVGHM) Geological Agency of Indonesia (Sukhyar et al. 1992). However, until our work, detailed investigations of stratigraphy, petrology, and geochronology were lacking for the Sundoro volcano. Twelve eruptions have been reported since 1806, with the last one having occurred in 1970–1971. There were no historic eruptions recorded before 1806 (Siebert et al. 2010). Eruption styles have been frequently explosive, although the historic eruptions were of low in explosivity (Volcanic Explosivity Index (VEI) 1–2), according to the explosivity scale of Newhall and Self (1982). We note that VEIs for eruptions in Indonesia with minimal information are assigned a default value of VEI 2. Consequently, it is possible that some of these could have been bigger or smaller. Time intervals between historic eruptions range widely but can be grouped as follows: 1–4 years (short), 10–15 years (intermediate), and 60–64 years (long). This eruptive pattern would suggest a high probability for future eruptions in the VEI 1–2 range, or greater, for the near future. However, for a longer term (e.g., more than 100 years) assessment of future eruptive hazards, we need to examine the pre-1806 AD history to be able to include long-term trends in explosivity.

Geological background and summary of deposit type Sundoro volcano lies within the Sunda arc, which includes 80 % of the active volcanoes in Indonesia. It is located only 56 km from the very active Merapi volcano in Central Java. The Sunda arc results from the subduction of the IndoAustralian plate beneath the Eurasian plate at a rate of approximately 6 to 7 cm/year (Hamilton 1979; Hall et al. 2009, Fig. 1a). Earthquake locations show that the subducting slab steepens from nearly horizontal to 45° and then to 70° beneath the Merapi area of Central Java, and the slab can be identified to depths of more than 600 km (Koulakov et al. 2009; Luehr et al. 2013). Tomographic studies indicate a low-velocity body with a volume of more than 50,000 km3 beneath the arc which meets the slab at a depth of 100 km and is interpreted as a region with high magma flux that is capable of developing additional large shallow crustal magma reservoirs (Luehr et al. 2013). Sundoro volcano is one of the most symmetrical stratovolcanoes in Java Island, separated by a 1400-m-high plateau from Sumbing volcano. Parasitic cones are Kembang Hill (2339 m), Kekep Hill (1650 m), Watu Hill (1650 m), Arum Hill (2100 m), and Kebonan Hill (1692 m). The largest among these is Kembang, on the northwest and southern flanks,


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(a)

(b)

Fig. 1 a Map of Sundoro volcano (3136 m a.s.l.), Central Java, Indonesia. Red arrows are convergence vectors. Sundoro belongs to the Sunda arc that was produced by subduction of the Indo-Australian plate

underneath the Eurasian plate at the rate of approximately 7 cm/year (simplified from Hall et al. 2009). b Photograph of Sundoro volcano from 6 km south, showing classic stratovolcano morphology

where lava flows are dominant in the area. The summit crater of Sundoro consists of two rims: an older rim (210 × 161 m) and younger rim (115 × 100 m on Fig. 2c). The younger inset crater was filled with meteoric water until 2010 (Fig. 2a). Following a microseismic crisis in 2011, a persistent sulfur lake formed (Fig. 2b). Phreatic explosion vents are now present (Fig. 2c), and explosions from these vents have produced. Phreatic deposits composed of angular ballistic andesite fragments and fine-grained altered material derived from the summit crater. No juvenile bombs have been identified in these

deposits. At the northeastern flank of Sundoro, hummocks of weathered and fractured andesite lavas and pyroclastics are observed (Sukhyar et al. 1992). We concluded that these hummocks are products of a debris avalanche from adjacent Sumbing volcano. The hummocks are less than 500 m in diameter and 100 m in height, and they tend to be larger in the southern part of the distribution closest to Sumbing. More than 2 m of soil commonly lies on the top of the hummocks suggesting a relatively old age of sector collapse. Petrographic features (phenocryst assemblage and groundmass texture) are

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Fig. 2 a Sundoro summit crater in 2010 filled with meteoric water and lacking solfataric activity. Photo from the CVGHM Sundoro Volcano Observatory’s monthly report. b Sundoro crater in August 2014 with solfatara that formed after a 2011 seismic crisis. Photo by Oktory Prambada 2014. c Summit crater has two rims: an older 210 × 161-m rim and a younger 115 × 100-m rim

(a)

(b) Older Rim

Younger

(c) similar between the constituents of Sumbing volcano and the hummocks. In addition, Sumbing volcano has a large scarp open to the northeast at its summit. For the abovementioned reasons, we conclude that the debris avalanche deposit came from Sumbing and is older than recent products of Sundoro dealt with here. Lava flows on the west and the east flanks of Sundoro are strongly weathered and mantled by thick soils. Lava flow deposits are distributed in various directions. The flow is mainly massive and blocky and displays aphanitic to porphyritic textures. Thicknesses of individual flows vary from 20 cm to 10 m. Pyroclastic density currents (PDCs) traveled at least 13 km from the summit, and their deposits are distributed in various directions. These block-and-ash flow deposits are loose, very poorly sorted, and composed of blocky andesite and basaltic andesite clasts, which are usually angular, have a maximum size of 2-m diameter, and contain large trunks of charcoal that we used for 14C age determination. These flows occur as single cooling units with vertical degassing pipes, which are commonly observed throughout the flow thicknesses. The deposits are homogeneous and poorly sorted; they contain breadcrust blocks and lapilli of pumice and scoria. The

thickness of individual PDC deposits that were well-exposed in multiple quarries typically exceeds 10 m. Pyroclastic fall map units are undifferentiated fall deposits in our work. They are deposited over nearly 75 % area of Sundoro volcano. They are composed of pumice and scoria blocks, lapilli, and ash. Maximum size of clasts ranges from 5 to 60 cm. Pumice clasts are highly vesicular, angular to subangular, and have strongly elongated bubbles. They are strongly weathered. Scoria fall deposits are gray in fresh samples. Scoria fragments are subangular to rounded and contain spherical bubbles. Scoria fragments have maximum long-axis dimensions of 1–3 cm. Variation in extent of the fall deposit map units is a result of both original depositional features and to erosion and deposition of widespread flowage deposits. These deposits are a crucial element in the reconstruction of Sundoro’s eruptive history and are used to correlate different sections with each other (Figs. 3 and 4).

Methods In order to update the volcanic stratigraphy of Sundoro, detailed descriptions of lithofacies and sampling for pyroclastic


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Sundoro volcano Kembang hill Kekep hill Watu hill

Sumbing volcano

Sundoro volcano

Fig. 3 Simplified geological map of Sundoro volcano from Sukhyar et al. 1992, with an insert showing geographic features, locations, and age ranges of 14 C samples (red dots)

deposits and lavas were carried out on all flanks and at the summit of Sundoro (total of 115 locations). Eruptive units were discriminated based on their boundaries with intervening soil zone, degree of reworking, and presence of unconformities. Lateral extent of each eruptive units was determined by correlation of lithological and petrological characteristics and with radiocarbon dating results. Morphological features were also used to discriminate distributions of lava flow lobes and volcanic fans surrounding Sundoro. Such discriminations were aided by the use of satellite imagery (a stereo contour map was generated from ASTER GDEM images with 30-m pixels and complemented by SRTMGL1 1-s DEM created by Awata (2015); Fig. S2). Volumes of lava and PDCs were deduced by multiplying areas and averaged thicknesses observed at outcrops. Averaged thicknesses are separated into the downstream areas of gentle sloping flanks and the upstream area of steep edifice slopes near the summit (Fig. 5). Due to the paucity of exposures on the steep volcanic edifice where thicknesses could be observed, in these areas, we used a default value equal to half of the thickness observed on the gentle slopes. Bulk volumes of pyroclastic fall deposits (tephra) were estimated by the method of Fierstein and Nathenson (1992) as developed from Pyle (1989). Cumulative minimum and maximum eruptive volumes for each unit are plotted versus time to determine

long-term trends. This diagram displays each unit volume as a step line following the approach of Bacon et al. (1982) and Kamata and Kobayashi (1997). Charcoal enclosed in PDC deposits and organic soil samples beneath pyroclastic deposits was analyzed from 20 localities by using the accelerator mass spectrometer (AMS) at Paleo Labo Co. Ltd. in Gunma, Japan, and the Institute of Accelerator Analysis Ltd. in Fukushima, Japan (for ID numbers PLD- and IAAA-, respectively, in Table 1). Calibrated ages are shown as years BP (before 1950 AD) by using the IntCal13 standard method (Reimer et al. 2013). Well-preserved and dominant lapilli and blocks in pyroclastic deposits and lavas were taken for petrological analysis to fingerprint eruptive units. Samples were first analyzed in thin section to determine phenocryst assemblages, groundmass textures, and preservation conditions. These petrographic characteristics were used to select the least altered and most characteristic samples of chemical analysis. Thirty-two wellpreserved samples were powdered in mild steel and agate mills and heated at 850 °C for 2 h in a muffle furnace to measure weight loss and determine water contents. Glass beads were made from a mixture of 1.4 g of the sample and 5.2 g of lithium metaborate (LiBO2) and lithium tetraborate (Li2B4O7). Concentration of oxides for major elements was measured by X-ray fluorescence spectroscopy (XRF) at the

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Fig. 4 Correlation of representative Sundoro stratigraphic columns for eruptive groups. Legend summarizes types of deposits. Approximate radiocarbon ages and whole-rock SiO2 ranges are also given

Earth Planetary System Science laboratory of Hokkaido University, Japan (using a MagiX PRO PW2540 vrc spectrometer system) (Table 2). Calibration lines were determined for rock standards (JR-1, JA-1, JA-2, JB-1, JGb-1, HR-1, HA1, HA-1B, HA-1C, and HB-1).

Results Stratigraphy and eruption units We observed lava flows on all sides of the volcano within the area of ca. 9 km from the summit where lava flow levees are prominent and thick lava flows are dominant in summit areas above 1500 m a.s.l. On the western flank, several lava flow lobes are observed; these were extruding from parasitic vents. PDCs at Sundoro consist mainly of monolithologic juvenile blocks and lapilli in an unsorted matrix of fine-grained fragments of the same composition. These deposits are observed on all flanks within ca. 10 km from the summit. Volcanic fans mainly consist of PDCs and secondary deposits. These fans surround and were derived from the summit area of the central volcanic edifices. An additional PDC fan surrounds and was derived from

Kembang Hill. Pyroclastic fall deposits and soil zones are intercalated within the PDC deposits in the fans. We correlated lava flow, PDC, and pyroclastic fall deposits by using a combination of stratigraphic, petrological, and geochronological characteristics (Fig. 4). We obtained 20 radiocarbon ages of charcoal samples from the pyroclastic deposits and from underlying paleosols, as shown in Table 1. Wholerock compositions of lava, juvenile blocks in PDCs, and pyroclastic fall deposits are also used for correlation (Fig. 9). As a result, we discriminated 12 eruptive groups, which are divided by specific stratigraphic positions, significant dormant grouping, and petrological fingerprints (Fig. 6). Each eruptive unit was named for one of the typical localities of Sukhyar et al. (1992). Fig. 5 Composite eruptive deposit distribution map for Sundoro volcano established by deposit physical properties, morphology control, distribution of outcrops, stratigraphic correlation, 14C calibrated ages, and whole-rock compositions. The stratigraphy is divided into 12 eruptive groups. The sort order a to n map is from older to younger eruptive groups. Lava flows are displayed as red areas, PDCs as green areas, phreatomagmatic deposits as yellow areas, and pyroclastic fall deposits are indicated by dashed lines. The contour map generated from ASTER GDEM images with 30-m pixel resolution complemented by SRTMGL1 1-s DEM by Awata 2015


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(b)

(a)

Ngadirejo (34 ka.)

(c)

Bansari (20 ka.)

(d)

Arum (17-19 ka.)

(e)

Kembang (13-17 ka.)

(f)

Kekep (13-17 ka.)

(g)

Garung (11-13 ka.)

(h)

Garung scoria fall (11-13 ka.)

Kertek (9 ka.)

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(i)

(j)

Kertek scoria fall (9 ka.)

(k)

Watu (