The SO2 is one of the volcanic gases that can use as indicator of volcano activity. Commonly, SO2 emission is measured by COSPEC (Correlation Spectroscopy). This equipment has several disadvantages; such as heavy, big in size, difficulty in finding spare part, and expensive. DOAS (Differential Optical Absorption Spectroscopy) is a new method for SO2 emission measurement that has advantages compares to the COSPEC. Recently, this method has been developed. The SO2 gas emission measurement of Gunung Merapi by DOAS has been carried out at Kaliadem, and also by COSPEC method as comparation. The differences of the measurement result of both methods are not significant. However, the differences of minimum and maximum result of DOAS method are smaller than that of the COSPEC. It has range between 51 ton/day and 87 ton/day for DOAS and 87 ton/day and 201 ton/day for COSPEC. The measurement of SO2 gas emission evaluated with the seismicity data especially the rockfall showed the presence of the positive correlation. It may cause the gas pressure in the subsurface influencing instability of 2006 eruption lava. Keywords: SO2 gas, Merapi, DOAS, COSPEC
SARI Fenomena lain yang muncul bersama semburan Lumpur Sidoarjo adalah semburan gas. Munculnya semburan gas ini menimbulkan pertanyaan, antara lain, jenis dan dampaknya terhadap lingkungan, terutama bagi pendudduk sekitarnya, untuk menjawab pertanyaan tersebut dilakukan penelitian geokimia. Hasil penelitian menunjukkan bahwa gas yang keluar adalah gas hidrokarbon dengan komposisi utama gas metana (CH4) dan gas karbondioksida (CO2). Berdasarkan jenis dan sumbernya, gas tersebut merupakan gas termogenik yang sumbernya diperkirakan adalah bahanorganik pada level di atas “Oil Window”. Dari nilai isotop menunjukkan bahwa gas tersebut mempunyai tingkat kematangan termal yang cukup. Akibat semburan gas hidrokarbon ditengah-tengah pemukiman penduduk tersebut berdampak pada lingkungan, terutama air. Air sumur penduduk menjadi keruh dan berbau menyengat sehingga tidak dapat dipergunakan untuk kebutuhan sehari hari. Kata kunci: Semburan gas, Sidoarjo, dampak lingkungan ABSTRACT Another fenomenon which is ocurred in the mud extrution of Sidoarjo is gas abruptions. At first that are become a big question. Due to this reason, an investigation of geochemistry of the gases are needed. These results show that gas abruptions are dominated by hydrocarbon gases with the main composition of methane (CH4) and is followed by carbondioxide (CO2). Based on the kind and its source, these gases are thermogenic gas which derived from organic matter above the level of “Oil Window”. The carbone isotope of the hydrocarbon gases show a mature of thermal maturity degree. Gases abruptions which are occurred in the area of a densed population, caused the water of people shallow well become turbid with strong bad smell which could not be used for daily used. Keyword: Outpouring of gas, Sidoarjo, environment impact
Abstract Some volcanoes are known for repeatedly producing explosive but short-lived eruptions (< half a day) every decade or so. These eruptions are often preceded by limited unrest signals and short run-up times to eruption (a few hours to months), and thus they are difficult to anticipate. Some well-documented examples are the 1990 and 2014 eruptions of Kelud volcano in Indonesia, or the 2015 Calbuco eruption in Chile. Here we interrogate the rock record and obtain insights into the processes and pre-eruptive conditions that led to the 1990 Kelud eruption, which we integrate with monitoring data (seismicity, lake temperature and hydro-acoustics, sulfur emissions) towards a conceptual model for this type of events. Mineral-melt geothermobarometers indicate that the basaltic andesite magma carried a crystal cargo from as deep as 15–19 km, and reached volatile saturation at 4–9 km with 2–4 wt.% water in the melt. The textures and compositional zoning of orthopyroxene and plagioclase do not support intrusion of more primitive magma as the driver for the eruption, and we instead propose that pre-eruptive fluid accumulation and high-temperature fluid fluxing from depth (likely dominated by CO2) played a major role in priming the eruption to occur. Such pre-eruptive gas accumulation is also supported by mass balance calculation of the emitted excess SO2 gas. Mg-Fe diffusion profiles in reversely zoned pyroxenes constrain timescales of weeks to months before eruption for fluid addition to the reservoir, and such events may be recorded in the monitoring signals, especially in the change of hydroacoustics and water lake temperature, and probably in the seismic swarms. We propose that fluid exsolution and accumulation in the shallow reservoir plays a crucial role in modulating and triggering short-lived explosive eruptions with brief unrest at Kelud and probably other volcanoes worldwide.
Understanding the role of various factors influencing eruption style is challenging, but it can aid in adapting different hazard mitigations and crisis responses for explosive or effusive events. Here, we focus on the role of magma storage conditions in controlling eruption styles at basaltic andesite volcanoes, and how they can be related to monitoring data. We study the cycle of explosive (1990, sub-Plinian) → effusive (2007, dome) → explosive (2014, sub-Plinian) eruptions from Kelud (Kelut) volcano, Indonesia. We conducted petrological analyses of the eruption products and phase equilibria experiments using pumice and explored a range of temperatures, pressures, oxygen fugacity, and volatile contents. We show that we can reproduce the main mineral assemblage (plagioclase ± pyroxenes ± magnetite ± amphibole ± olivine) and phenocryst content (30–50 wt%) of the magmas from the three eruptions at T = 975 ± 39°C, p = 175 ± 25 MPa, f O 2 = nickel–nickel oxide buffer, and about 4–6 wt% water in the melt (ca. 3 to 5 wt% and ca. 4 to 7 wt% for the 1990 and 2014 eruptions, respectively). However, geothermobarometric results also indicate that some crystals of amphibole were sourced from higher pressures. We infer from a synthesis of our data and historical observations that the high phenocryst content of the 2007 dome (∼70 wt%) likely resulted from slow magma ascent toward the surface alongside progressive degassing and re-equilibration at a lower volatile content (∼1 wt% water in the melt). Mass balance calculations on the sulfur budget of the 1990, 2007, and 2014 magmas show that the explosive events contained an excess fluid phase at pre-eruptive conditions, and we propose that this led to their higher explosivity compared to the 2007 dome. The accumulation of excess fluids during decadal-long repose depends on how plugged the volcanic system is, or its ability to passively release magmatic fluids, prior to eruption. Such condition could be inferred from monitoring records, including changes in gravity of the plumbing system over time, and thus contribute to better anticipate eruptive style.
This is the first study discussing the dynamics of two caldera-forming eruptions in the Banda volcanic complex (BVC) in the marine conservation zone of Banda, Maluku, Indonesia. The first and second caldera episodes are, hereafter, termed as Banda Besar and Naira, respectively. The formation of Banda Besar caldera (ca. 8 × 7 km) ejected homogeneous rhyolitic magmas (bulk-rock, 73.1–73.8 wt.% SiO2) in the following three stages: (1) sub-Plinian (BB-5a), (2) intra-sub-Plinian flow (BB-5b), and (3) caldera collapse (BB-5c and BB-5d). The BB-5a stage produced a reversely graded white pumice fall layer with moderate lithics (2–11%), which originated from a sub-Plinian eruption with an estimated plume height of 22–23 km. Subsequently, intensive erosion of wall rock (13–25%) causes conduit enlargement, leading to the partial collapse of the eruption columns, forming intra-sub-Plinian flow deposits (BB-5b). It is likely that conduit size surpassed the minimum threshold value for a buoyant plume during the final phase of the second stage, causing the complete formation of a pumice-rich pyroclastic density current (PDC) during the early-third stage (BB-5c). Finally, the evacuation of voluminous magma from the reservoir yields the first caldera collapse during the late-third stage, producing a lithic-dominated PDC with minor pumices (BB-5d). The formation of the Naira caldera (ca. 3 × 3 km) ejected homogeneous dacitic magmas (bulk-rock, 66.2–67.2 wt.% SiO2) in the following three stages: (1) early sub-Plinian (N-2a and 2b), (2) late sub-Plinian (N-2c, 2d, 2e), and (3) caldera collapse (N-2f). This research distinguishes the sub-Plinian into two stages on the basis of different vent locations (assumed from the isopach map). In particular, this research suggests that the early sub-Plinian stage (N-2a and 2b) erupted from the northern vent, producing 14 and 8 km eruption plume heights, respectively. Additionally, the late sub-Plinian stage (N-2c, 2d, 2e) was generated from a newly-formed conduit located in the relatively southern position, producing 12–17, 9, and 6 km eruption plume heights, respectively. Conduit enlargement is expected to occur during at both sub-Plinian stages, as lithic portions are considerably high (10–72%) and ultimately generate PDCs during the third stage (caldera collapse; N-2f). Because most of the erupted materials (for both caldera-forming eruptions) are emplaced in the ocean, estimating the erupted volume becomes difficult. However, with the assumption that the caldera dimension represents the erupted volume of magma (Vmagma), and that the total erupted volume (Vtotal) is a summation of Vmagma and the now-vanished pre-caldera island (Vvanished, represented by average lithic fractions), the first and second caldera might produce (at least) 35.2 and 2.4 km3 of erupted materials, scaling them as VEI (volcano explosivity index) 6 and 5, respectively. That VEI is more than enough to initiate a secondary hazard in the form of tsunamis triggered by volcanic activities.
Abstrak Semburan gas bercampur air dan lumpur di Desa Metatu, Kecamatan Benjeng, Kabupaten Gresik, JawaTimur terjadi pada bulan November 2012. Semburan tersebut dipicu oleh akumulasi gas hidrokarbon yang terperangkap di bawah permukaan. Desa Metatu dan sekitarnya merupakan wilayah minyak dan gas bumi yang telah diusahakan sejak zaman Belanda, sehingga banyak dijumpai sumur minyak peninggalan Belanda. Gas hidrokarbon yang memicu semburan Metatu tersebut didominasi oleh gas metana berasal dari oil window seperti yang terdapat di gunung lumpur LUSI, Sidoarjo, tetapi kedua mempunyai perbedaan dalam tingkat kematangannya. Kematangan gas metana dari semburan gas Metatu mempunyai tingkat kematangan yang lebih rendah dari tingkat kematangan LUSI. Kata kunci: Metatu, semburan gas, lumpur Abstract Gases outburst mixed of water and mud in Metatu village, Benjeng subdistrict, Gresik regency, East Java occurred in November 2012. This outburst was trigerred by high pressure of hydrocarbon gases that were accumalted beneath the surface. Metatu and its surrounding is potensially petroleum and natural gases which was developed since Ducth era, so this area plenty of oil wells that were construted by Ducth. Hydrocarbon gases that trigger a gas outburst at Metatu is dominated by methane gas of oil window origin like as in LUSI mud volcano, Sidoarjo, but among them different in maturity degree. Hydrocarbon gas maturity of Metatu gas outburst is lower than LUSI. keywords: Metatu, gases outburst, mud
Volcanic ash provides unique pieces of information that can help understand the progress of volcanic activity at the early stages of unrest and possible transitions towards different eruptive styles. Ash contains different types of particles that are indicative of eruptive styles and magma ascent-related processes. However, classifying ash particles into its main components is not straightforward. Diagnostic observations vary depending on the magma composition and the style of eruption, which leads to ambiguities in assigning a given particle to a given class. Moreover, there is no standardized methodology for particle classification, and thus different observers may infer different interpretations. In order to help improving this situation, we created the web-based platform Volcanic ash DataBase (VolcashDB). The database contains > 6,300 multi-focused high-resolution images of ash particles as seen under the binocular microscope from a wide range of magma compositions and eruptive styles. We quantitatively extracted multiple features of shape, texture, and color in each particle image, and petrologically classified each particle into one of the four main categories: free crystal, altered material, lithic, and juvenile. VolcashDB is publicly available and enables users to browse, obtain visual summaries, and download the images with their corresponding labels, and thus could be used for comparative studies. The classified images could also be used to train Machine Learning models to automatically classify particles and minimize observer biases.
A geochemical study had been carried out on Merapi and Kelud volcanic eruptions. The study was conducted based on the differences on characteristics of explosive and effusive eruptions. One of the factors of causing the difference of eruption character is the magma chemical composition. The geochemical study conducted here includes geochemically of rocks, water, and volcanic gases. The minerals of the explosive and effusive eruption sample materials are the same compound, such as plagioclase, pyroxene (orthopyroxene and clinopyroxene), hornblende, lithic fragments, opaque and glass, but there are texture differences. The explosive eruption materials show smaller size and vesicular which characteristics of high gas content, however the effusive eruption produces the flow texture. The occurrence of normal zoning and oscillatory zoning at the same time in the plagioclase and inconcistence concentration of SiO2 and MgO of pyroxene chrystallization process indicates magma mixing that trigger explosive eruptions. Results of whole rock analysis showed a wider range and a little shift in SiO2 values in explosive eruptions that have implications for viscosity and density values. The composition of the other oxides such as MgO, FeO, Al2O3, CaO, NaO, and K2O showed a consistent trend towards crystal fractionation. Results of trace elements and rare earth element analysis ie Zr/SiO2 ratio, Ba/Sr, La/Yb and Dy/Yb indicated the occurrence of fluid saturation that triggered the explosive eruptions. Melt inclusions (MI) contained bubble/gas of CO2 and H2O as the main compounds. The explosive eruptions contain both of homogeneous and heterogeneous MI, whereas effusive eruption produces homogeneous. Based on the composition of MI, ground mass, and whole-rock showed that the inclusion entrapment occored in recent time and/or rapid cooling process The emission rate of volcanic gas emissions (CO2, HCl, SO2, and H2S) of explosive eruptions increased, that is accompanied by decreasing of the H2O emission rate. In the effusive eruption, lava dome growth correlated with an increasing of HCl concentration and that of CO2/H2O ratio. Moreover, the emission of SO2 gas correlated with rock fall before the multiphase (MP) seismic event. The gases were trapped in the crater lake water caused the higher acidity and increase of temperature rate during the explosive eruption.
Telah dilakukan kajian geokimia terhadap erupsi Gunung Merapi dan Gunung Kelud. Kajian dilakukan berdasar perbedaan karakter erupsi yang bersifat eksplosif dan efusif. Salah satu faktor penyebab perbedaan karakter erupsi adalah komposisi kimia magma. Kajian geokimia yang dilakukan meliputi geokimia batuan, air, maupun gas. Mineral batuan yang terkandung dalam sampel material hasil erupsi eksplosif maupun efusif, mempunyai komponen utama yang sama yaitu plagioklas, piroksen (ortopiroksen dan klinopiroksen), hornblend, fragmen litik, opak dan gelas, tetapi terdapat perbedaan secara tekstur. Material erupsi yang bersifat eksplosif mempunyai ukuran mineral yang lebih kecil dan bersifat vesiculer yang menunjukkan tingginya gas, sedangkan mineral erupsi efusif bertekstur aliran. Zoning normal bersama-sama dengan zoning oskilatori pada mineral plagioklas serta konsentrasi SiO2 dan MgO pada proses kristalisasi piroksen yang tidak konsisten menunjukkan adanya magma mixing yang memicu terjadinya erupsi eksplosif. Hasil analisis unsur mayor batuan menunjukkan rentang SiO2 yang lebih luas pada erupsi eksplosif dan sedikit terjadi pergeseran nilai yang berimplikasi terhadap nilai densitas dan viskositas. Komposisi oksida-oksida yang lain seperti MgO, FeO, Al2O3, CaO, NaO, dan K2O menunjukkan adanya kecenderungan yang konsisten terhadap fraksinasi kristal. Hasil analisis unsur trace dan tanah jarang (Rare Earth Elements, REE), yaitu rasio Zr/SiO2, Ba/Sr, La/Yb, serta Dy/Yb pada erupsi eksplosif terdapat indikasi adanya saturasi fluida yang memicu erupsi eksplosif. Magma yang terinklusi di dalam batuan hasil erupsi (Melt inclusion, MI) mengandung gelembung/gas dengan komponen utama CO2 dan H2O. Erupsi eksplosif memberikan dua tipe MI yaitu bersifat homogen dan heterogen, tetapi erupsi efusif menghasilkan MI bersifat homogen. Berdasarkan komposisi MI, massa dasar, dan whole rock menunjukkan bahwa entrapment inclusion terjadi dalam waktu belum lama dan/atau mengalami proses pendinginan yang cepat. Emisi gas vulkanik, gas CO2, HCl, SO2, dan H2S mengalami peningkatan laju emisi yang disertai dengan penurunan laju emisi H2O pada erupsi eksplosif. Pada erupsi efusif, pertumbuhan kubah lava berkorelasi dengan naiknya konsentrasi HCl dan rasio CO2/H2O. Selain itu gas SO2 yang teremisikan berkorelasi dengan guguran sebelum adanya gempa MP. Gas-gas yang teremisikan yang terperangkap di dalam air danau kawah menyebabkan keasaman dan laju kenaikan temperatur yang lebih tinggi pada erupsi eksplosif dari pada erupsi efusif.
