There are ~750 active and potentially active volcanoes in Southeast Asia. Ash from eruptions of volcanic explosivity index 3 (VEI 3) and smaller pose mostly local hazards while eruptions of VEI ≥ 4 ...could disrupt trade, travel, and daily life in large parts of the region. We classify Southeast Asian volcanoes into five groups, using their morphology and, where known, their eruptive history and degassing style. Because the eruptive histories of most volcanoes in Southeast Asia are poorly constrained, we assume that volcanoes with similar morphologies have had similar eruption histories. Eruption histories of well-studied examples of each morphologic class serve as proxy histories for understudied volcanoes in the class. From known and proxy eruptive histories, we estimate that decadal probabilities of VEI 4–8 eruptions in Southeast Asia are nearly 1.0, ~0.6, ~0.15, ~0.012, and ~0.001, respectively.
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 SO
2
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.
Overview of the 2006 eruption of Mt. Merapi Ratdomopurbo, Antonius; Beauducel, Francois; Subandriyo, Joko ...
Journal of volcanology and geothermal research,
07/2013, Letnik:
261
Journal Article
Recenzirano
In the last part of the 20th century and the beginning of the 21st century, Mt. Merapi in Central-Java Indonesia erupted about every 2–5years. Most of the eruptions were low in explosivity, with ...VEI-3 or less. Eruptions usually involve the formation of a lava dome, either in the beginning or in the end of the eruptive episode.
For the 2006 eruption, the precursory signal was first observed in the middle of the year 2005 with a decrease in EDM slope distances to points on the rim, an increase of seismicity and a possible increase of SO2 emissions. Those early events marked the beginning of a more continuous period of inflation, which led to the eruption.
In total, the pre-eruption displacement of the southern rim reached at least 2.4m toward the measuring station in Kaliurang (KAL). From late April until June 2006, a lava dome grew on the summit with a volume that gradually increased until it reached about 4.1millionm3 in 38days. The total of erupted magma was about 5.3millionm3 dense-rock-equivalent (DRE). The dome subsequently collapsed in three steps from June 4 to June 14, leaving an open scar on its southeast side. In this paper we detail the changes of dome morphology that were monitored by taking successive photographs from similar positions. The eruption in 2006 marked a significant change in summit morphology, from west-southwestward opening during the 20th century to the currently southeast orientation. Also, an Mw 6.4 earthquake occurred on 26 May, midway through the eruption, which adds interesting questions about the relationship of the eruption and the earthquake.
EDM data from 2006 and previous eruptions show that the summit remains inflated after each eruption, i.e., no significant deflation occurs following eruptions. The lack of post-eruption deflation suggests that magma remains in the shallow parts of the edifice after the eruption. As a result, the complex of summit lava domes and their intrusive roots grow with time and Merapi's rim and summit become progressively more unstable and prone to collapse.
•We monitor the detail of the growth of lava dome during the 2006 eruption of Mt. Merapi.•The summit rim deformation monitoring is the key for eruption forecasting.•The summit rim at Mt. Merapi is prone to collapse because the rim tends to inflate strongly.•There is no deflation after eruption of Mt. Merapi.
Volcano Alert Levels (VALs) are used by volcanologists to quickly and simply inform local populations and government authorities of the level of volcanic unrest and eruption likelihood. Most VALs do ...not explicitly forecast volcanic activity but, in many instances they play an important role in informing decisions: defining exclusion zones and issuing evacuation alerts. We have performed an analysis on VALs (194 eruptions, 60 volcanoes) to assess how well they reflect unrest before eruption and what other variables might control them. We have also looked at VALs in cases where there was an increase in alert level but no eruption, these we term 'Unrest without eruption' (UwE). We have analyzed our results in the context of eruption and volcano type, instrumentation, eruption recurrence, and the population within 30 km.
We found that, 19% of the VALs issued between 1990 and 2013 for events that ended with eruption accurately reflect the hazard before eruption. This increases to ~30% if we only consider eruptions with a VEI ≥ 3. VALs of eruptions from closed-vent volcanoes are more appropriately issued than those from open-vents. These two observations likely reflect the longer and stronger unrest signals associated with large eruptions from closed vents. More appropriate VAL issuance is also found in volcanoes with monitoring networks that are moderately-well equipped (3-4 seismometers, GPS and gas monitoring). There is also a better correlation between VALs and eruptions with higher population density.
We see over time (1990 to 2013) that there was an increase in the proportion of `UwE’ alerts to other alerts, suggesting increasing willingness to use VALs well before an eruption is certain. The number of accurate VALs increases from 19% to 55% if we consider all UwE alerts to be appropriate. This higher `success’ rate for all alerts (with or without eruption) is improving over time, but still not optimal. We suggest that the low global accuracy of the issuance of VALs could be improved by having more monitoring networks equipped to a medium level, but also by using probabilistic hazard management during volcanic crisis.
A field reconnaissance study of the volcanic geology of Isarog volcano (Luzon, Philippines), complemented with radiocarbon dating, geochemical and petrological data, shows evidence for repeated ...explosive activity generating pyroclastic density currents and block-and-ash flows, including Holocene events. The chemical composition of juvenile material is dominantly andesitic. As there are several indications for magma mixing taking place, the andesitic composition possibly represents a hybrid composition between mixed magmas. In addition to explosive eruptions, unambiguous field evidence is provided for a sector collapse generating a debris avalanche deposited to the NW. The deposit shows characteristic hummocky and ridge topography, highly brecciated jigsaw-cracked blocks and relict internal stratigraphy in transported masses. In a first-order approximation the deposit volume is estimated to be ~6–8km3. The run-out distance is 21–25km — the distal parts of the deposit were transported into San Miguel Bay.
► Isarog has had Holocene eruptions generating pyroclastic density currents. ► Evidence for magma mixing at Isarog is ubiquitous. ► A sector collapse generated a large debris avalanche deposited to the NW of Isarog.
The 1991 eruption of Mount Pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows. Bedload sampling in the Pasig–Potrero River, one of the most heavily ...impacted rivers, revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth, fine-grained bed. Dimensionless bedload transport rates in the Pasig–Potrero River differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress, but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments. The similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply; in arid rivers, this is attributed to a flashy hydrograph, whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery. This work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced.
Despite dense cloud cover, satellite-borne commercial Synthetic Aperture Radar (SAR) enabled frequent monitoring of Merapi volcano's 2010 eruption. Near-real-time interpretation of images derived ...from the amplitude of the SAR signals and timely delivery of these interpretations to those responsible for warnings, allowed satellite remote sensing for the first time to play an equal role with in situ seismic, geodetic and gas monitoring in guiding life-saving decisions during a major volcanic crisis. Our remotely sensed data provide an observational chronology for the main phase of the 2010 eruption, which lasted 12days (26 October–7 November, 2010). Unlike the prolonged low-rate and relatively low explosivity dome-forming and collapse eruptions of recent decades at Merapi, the eruption began with an explosive eruption that produced a new summit crater on 26 October and was accompanied by an ash column and pyroclastic flows that extended 8km down the flanks. This initial explosive event was followed by smaller explosive eruptions on 29 October–1 November, then by a period of rapid dome growth on 1–4 November, which produced a summit lava dome with a volume of ~5×106m3. A paroxysmal VEI 4 magmatic eruption (with ash column to 17km altitude) destroyed this dome, greatly enlarged the new summit crater and produced extensive pyroclastic flows (to ~16km radial distance in the Gendol drainage) and surges during the night of 4–5 November. The paroxysmal eruption was followed by a period of jetting of gas and tephra and by a second short period (12h) of rapid dome growth on 6 November. The eruption ended with low-level ash and steam emissions that buried the 6 November dome with tephra and continued at low levels until seismicity decreased to background levels by about 23 November. Our near-real-time commercial SAR documented the explosive events on 26 October and 4–5 November and high rates of dome growth (>25m3s−1). An event tree analysis for the previous 2006 Merapi eruption indicated that for lava dome extrusion rates >1.2m3s−1, the probability of a large (1872-scale) eruption was ~10%. Consequently, the order-of-magnitude greater rates in 2010, along with the explosive start of the eruption on 26 October, the large volume of lava accumulating at the summit by 4 November, and the rapid and large increases in seismic energy release, deformation and gas emissions were the basis for warnings of an unusually large eruption by the Indonesian Geological Agency's Center for Volcanology and Geologic Hazard Mitigation (CVGHM) and their Volcano Research and Technology Development Center (BPPTK) in Yogyakarta — warnings that saved thousands of lives.
► Remote sensing played a major role in forecasting a large eruption. ► Syn-eruptive changes in Merapi's summit were monitored with satellite radar. ► Unusually rapid rates of lava dome extrusion (>25m3s−1) were estimated. ► High rates of lava extrusion served as warnings for a large eruption.
The Irosin caldera, which is located in the province of Sorsogon, southern Luzon, Philippines, represents the largest extrusion of highly silicic magmas in the Bicol arc at ca. 41
cal ka BP. The 41
...cal ka BP rhyolitic eruption led to a collapse and formation of the 11
km–wide Irosin caldera. This paper presents the results of the stratigraphy, grain assemblage, morphology, and geochemistry of the recently discovered rhyolitic fine ash which is exposed in the crater of Inascan scoria cone, 80
km from Irosin caldera.
The morphology of glass shards obtained at Inascan cone is not only pumice-type but also bubble-wall-type glass shards which are typical of a co-ignimbrite ash. The mineral assemblage of the fine ash is quite similar to those of Irosin pumice. The refractive index, measured using the thermal immersion method, together with geochemical analyses of glass shards from the fine rhyolitic ash deposits and ignimbrite deposits from Irosin caldera, both indicate a strong geochemical similarity between the ignimbrite and fine ash deposits. Thus, the tephra sequence at Inascan scoria cone is interpreted as co-ignimbrite ash falls sourced from the 41
cal ka BP catastrophic eruption that formed the Irosin caldera. The Irosin co-ignimbrite ash-fall deposit, which measures 1.3
m thick and 80
km away from its source volcano, represents the most explosive eruption in the Bicol arc. The identification of the Irosin co-ignimbrite ash-fall deposit is a valuable contribution to the establishment of a chronological framework of widespread tephra in the Philippines as well as a potential regional tephra marker.
The Irosin caldera located at the southeastern tip of Luzon Island in the Philippines was formed by the eruption of 41cal kBP Irosin ignimbrite. Bulusan, a post-caldera volcano, has repeated phreatic ...eruptions during historical times. The special issue on “Geology and Recent Eruptions of Irosin Caldera and Bulusan Volcano, Southern Luzon, Philippines (Part I)” provides various data and discussion mainly on the formation of Irosin caldera. First based on interpretations of volcanic landforms, the evolution of 84 volcanoes in the Philippines is outlined (Moriya, 2014). Fifty-six stratovolcanoes, three caldera volcanoes accompanying four post-caldera volcanoes, three lava domes, four scoria cones including two maars, four lava fields, and ten shield volcanoes are identified. The sequence of caldera-forming eruption at Irosin consists of a precursory fine ash eruption (Malobago lava dome), plinian pumice fallout, and intra-plinian flow deposits (Kobayashi et al., 2014a). Evidence of ground shaking during the plinian phase was also found. The total DRE volume of erupted tephra is estimated to be 30 km3 (VEI = 6). A gravity survey in February 1996 revealed a semi-circular feature with a steep gravity gradient in the Bouguer anomalies, which corresponds clearly to the southern rim of the Irosin caldera (Komazawa et al., 2014). The funnel-shaped depression structure of the gravity basement, which is significantly smaller than that of the topographic depression, was recognized from a three-dimensional analysis of residual gravity anomalies. The mass deficiency was estimated to be 1.1 × 1010 tons, corresponding to 40 km3 of DRE volume. Four thermoluminescence (TL) ages (36 ± 8 ka, 38 ± 10 ka, 33 ± 8 ka and 45 ± 10 ka) are obtained from the matrix and lithic fragments of the ignimbrite and co-ignimbrite ash-falls, respectively (Takashima and Kobayashi, 2014). Of these, the first two ages are in good agreement with a radiocarbon age of 41 cal kBP. A pictorial of representative outcrops is offered to provide an understanding of the geology in and around the caldera (Kobayashi et al., 2014b). Recent activity at the Bulusan volcano is described and discussed in the next issue (Part II).
Here, we review volcanic risk management at Mount St. Helens from the perspective of the US Geological Survey’s (USGS) experience over the four decades since its 18 May 1980 climactic eruption. Prior ...to 1980, volcano monitoring, multidisciplinary eruption forecasting, and interagency coordination for eruption response were new to the Cascade Range. A Mount St. Helens volcano hazards assessment had recently been published and volcanic crisis response capabilities tested during 1975 thermal unrest at nearby Mount Baker. Volcanic unrest began in March 1980, accelerating the rate of advance of volcano monitoring, prompting coordinated eruption forecasting and hazards communication, and motivating emergency response planning. The destruction caused by the 18 May 1980 eruption led to an enormous emergency response effort and prompted extensive coordination and planning for continuing eruptive activity. Eruptions continued with pulsatory dome growth and explosive eruptions over the following 6 years and with transport of sediment downstream over many more. In response, USGS scientists and their partners expanded their staffing, deployed new instruments, developed new tools (including the first use of a volcanic event tree) for eruption forecasting, and created new pathways for agency internal and external communication. Involvement in the Mount St. Helens response motivated the establishment of response measures at other Cascade Range volcanoes. Since assembly during the early and mid-1990s, volcano hazard working groups continue to unite scientists, emergency and land managers, tribal nations, and community leaders in common cause for the promotion of risk reduction. By the onset of renewed volcanic activity in 2004, these new systems enabled a more efficient response that was greatly facilitated by the participation of organizations within volcano hazard working groups. Although the magnitude of the 2004 eruptive sequence was much smaller than that of 1980, a new challenge emerged focused on hazard communication demands. Since 2008, our understanding of Mount St. Helens volcanic system has improved, helping us refine hazard assessments and eruption forecasts. Some professions have worked independently to apply the Mount St. Helens story to their products and services. Planning meetings and working group activities fortify partnerships among information disseminators, policy and decision-makers, scientists, and communities. We call the sum of these pieces the Volcanic Risk Management System (VRMS). In its most robust form, the VRMS encompasses effective production and coordinated exchange of volcano hazards and risk information among all interested parties.
PDF
