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Costa F.,Nanyang Technological University | Andreastuti S.,Geological Agency | Bouvet de Maisonneuve C.,Nanyang Technological University | Pallister J.S.,U.S. Geological Survey
Journal of Volcanology and Geothermal Research | Year: 2013

To understand the processes that made the 2010 eruption of Merapi much larger and more explosive than most dome-forming eruptions of the past century, we investigated the geochemistry, petrology, and pre-eruptive conditions of magmas erupted in 2006 and 2010. The juvenile rocks of 2010 are plagioclase, two-pyroxene basaltic andesites with seriate textures and minor amounts of reaction-free amphibole, Fe-Ti oxides, and rare crystals of olivine and biotite. The bulk-rock composition, mineral paragenesis, and textures are similar to those of juvenile blocks from the much less explosive eruption of 2006. One of the key differences is that most amphiboles in 2010 don't have breakdown reaction rims, whereas those of 2006 are largely reacted. We acquired >80 X-ray distribution maps of major and minor elements of large areas (>1cm2) and single crystals, backscattered electron images, electron microprobe analyse, and compositional traverses across crystals. The data reveal that both the 2006 and 2010 samples are heterogeneous at various spatial scales, with numerous reaction textures between pyroxenes and amphiboles, dissolution textures, and large variations of crystal sizes, morphologies, and compositions. These features record open-system magmatic processes involving the assimilation of carbonate rocks, and interactions between various parts of Merapi's plumbing system, including a degassed shallow magma system and deep hotter and more volatile rich magma intrusions.The petrological complexity of the samples makes unraveling the pre-eruptive conditions of Merapi magmas a petrological puzzle. We applied five different geothermobarometers and performed thermodynamic modeling with the MELTS algorithm, and we propose that there are at least three crystallization zones or environments below Merapi. A deep reservoir at about 30 (+/-3) km depth is suggested by some amphiboles and high-Al clinopyroxenes. Here is where the high-Al basaltic andesites from Merapi are generated probably by water-rich fractionation of more primitive magmas. Such deep magmas are volatile-rich and at near-liquidus conditions (≥4-6wt.% H2O, ≥0.15wt.% SO2, and an undetermined amount of CO2, at about 1050°C) when they start moving towards the surface. A second crystallization zone is recorded by another type of amphibole at about 13 (+/-2) km. Here high-Al clinopyroxene may also grow together with Ca-rich plagioclase. Assimilation of limestone may also occur at this level as recorded by the very Ca-rich plagioclases found in the cores of some crystals. At this location the water content of the melt must remain high enough to stabilize amphibole (4-6wt.% H2O) but CO2 and SO2 are probably already degassing and contribute to gas changes observed by the monitoring system at the surface. Finally, a shallower part of the system (<10km) is recorded by the lower anorthite plagioclase and low-Al in clinopyroxene, and perhaps also in orthopyroxene. This part of the system is probably crystal-rich and largely degassed, and is the likely source of the high-temperature fumaroles and the volcanic gas plumes that are commonly seen at Merapi.We propose that the 2006 and 2010 eruptions were driven by basically the same processes and magma types. The main difference is the much larger size of the deep and volatile-rich magma replenishment that took place in 2010, which had large effects on the kinetics and dynamics of the plumbing system and processes. In 2006, and perhaps also in most of the typical small dome-forming historical eruptions at Merapi, the direct ascent of deep and gas-rich magmas towards the surface is slowed down and partially arrested by the shallower crystal-rich zones of left-over magma from previous events. However, this was not possible in 2010, where the much larger (up to 10 times) size of the magma intrusions overwhelmed the crystal-rich eruption filter. In 2010 the deep magma probably resided for only a short time at intermediate to shallower depths which allowed it to proceed to the surface still carrying most of its deep gas cargo. The larger magma intrusion probably induced higher rates of crustal carbonate assimilation and production of additional CO2 gas at shallow depths. This contributed to the much faster than usual ascent rates and larger explosivities in 2010 than in 2006. These inferences are supported by the shorter interaction times calculated from the diffusion models of clinopyroxene compositions for the 2010 magmas, by the fact that most amphiboles are not broken down in 2010 as opposed to 2006, and also by the much shorter times of escalating monitoring signals (seismicity and deformation) in 2010 compared to 2006.A puzzling observation is that despite the multiple explosive phases of the 2010 eruption, pumiceous materials are rare, and were only found in the last part of the eruption. This contrasts with the abundant tephra layers and vesiculated deposits of older historical explosive events like 1872, and suggest that syn-eruptive processes in 2010 were also different from standard models. The rarity of expanded pumices in 2010 may be due to rapid degassing and re-welding of magma as it ascended from intermediate depths. Given the near constant bulk composition of Merapi magmas erupted in the last decades, and the similarity of textures and minerals in 2006 and 2010, our study suggests that most Merapi magmas are intrinsically capable of explosive eruptions. Here we propose that whether they do so or not mainly depends on the degree of interaction and magma mass proportions between the upper crystal-rich parts of the system (including carbonates) and the deeper and more gas-rich replenishing magmas. Older historical explosive eruptions at Merapi such as in 1872 were driven by more mafic magmas than those erupted in 2006 and 2010 and thus might be caused by different processes from those discussed here. The still unanswered and vexing questions remain as to why in 2010 a much larger amount of magma was segregated from depths and whether this will happen again in the near future. © 2013 Elsevier B.V. Source

Fontijn K.,Nanyang Technological University | Fontijn K.,University of Oxford | Fontijn K.,Ghent University | Costa F.,Nanyang Technological University | And 4 more authors.
Bulletin of Volcanology | Year: 2015

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. © 2015, Springer-Verlag Berlin Heidelberg. Source

Gunawan H.,Geological Agency | Pallister J.,U.S. Geological Survey | Caudron C.,Nanyang Technological University
Eos | Year: 2014

"Wet volcanoes" with crater lakes and extensive hydrothermal systems pose challenges for monitoring and forecasting eruptions. That's because their lakes and hydrothermal systems serve as reservoirs for magmatic heat and fluid emissions, filtering and delaying the surface expressions of magmatic unrest. ©2014 The Authors. Source

Darmawan S.,Padjadjaran University | Junursyah G.M.L.,Geological Agency
Near Surface Geoscience 2013 | Year: 2013

A region of east Indonesia with a big geothermal energy potential is Akesahu in the province of North Maluku. Akesahu is located on the island of Tidore. An integration investigation included geological, geochemical, and geophysical survey has been completed to determine of subsurface mapping in this area. There are seven hot springs are found in this area as a surface manifestation of geothermal system. Furthermore, Hg and CO2 mapping has been supporting an investigation due to of ability to indicate a heat fluid leak and structure zone mapping. The presences of these hot spring and also the other supporting data from geological and previously geophysical data could be help on magnetotelluric data interpretation. Based on these previous survey data, data compilation are consistent shown a reservoir prospect in Akesahu area is in the northeastern of Tidore Island. At least, there are three layers shown in LINE-01 and LINE-02 MT section. In addition, the presences of other structures or weak zones that appear on MT seem consistent with the interpretation based on the other data sets and it's proved by 1-D magnetotelluric data which show the top reservoir is 900 m. Source

Junursyah G.M.L.,Geological Agency | Harja A.,Padjadjaran University
AIP Conference Proceedings | Year: 2014

The Manglayang mountain is one of volcanoes located in Bandung Basin area, it is limiting the Lembang active faults in the eastern part. Relationship between those can be interpreted by the information of volcanic deposit thickness and it has conducted by Magnetotelluric method (MT). This method utilizing the sources of natural electromagnetic wave that induce subsurface rocks and produce secondary fields which can be measured orthogonally on the surface. The results are presented in the form of resistivity rock variation resulting by one dimensional (1D) inversion. Interpretation of volcanic deposit thickness in the eastern of Manglayang Mountain indicated reaches 443 m (55-115 Ωm). It is unconformable on soft sediment (3-13 Ωm) with thickness 1,671 m. Those products covered the basement rock (11-42 Ωm). The subsurface geological conditions suggest that the dominated by low resistivity values (<1000 Ωm) suspected correlated with the soft rock as clay, sand, high porosity rocks, and the weathered rock. It is very different from western part of Manglayang mountain which consist of igneous rock dominantly, so that the relationship with Lembang active faults can interpreted separated by geological structure. © 2014 AIP Publishing LLC. Source

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