The Proto-South China Sea (PSCS) is a fully consumed Mesozoic plate believed to have existed in the current location of the South China Sea (SCS). Ophiolites emplaced in the vicinity of the SCS are thought to represent remnants of the PSCS. This includes oceanic fragments in Borneo and Palawan. Cretaceous and Eocene ophiolitic lithologies outcrop in the southern and central portions of Palawan island, respectively. These exposures consist of mantle peridotites, layered and isotropic gabbros, and massive and pillow lavas. In the central Palawan Ophiolite (CPO), mafic dikes intrude peridotites, gabbros and metamorphic rocks, and are marked by distinct bake-and-chill margins. In the southern Palawan Ophiolite (SPO), dikes cut the peridotites and are typified by sharp boundaries. These SPO dikes represent migrating melts formed during the initial stages of back-arc basin (BAB) formation. In this study, petrographic and geochemical characterization of CPO and SPO mafic rocks were conducted to provide insights into the different magmatic processes involved in the formation of the Palawan Ophiolite. CPO and SPO isotropic gabbros are classified as coarse-grained gabbronorite and olivine gabbro. Euhedral to subhedral plagioclases in these samples typically exhibit ophitic to sub-ophitic texture with pyroxenes. The crystallization order for these gabbros is olivine > plagioclase > pyroxenes. CPO mafic dikes are gabbronorites, hornblende gabbros and porphyritic basalt, while SPO dikes are olivine gabbros and troctolite. CPO gabbros, pillow basalts and dikes exhibit transitional mid-oceanic ridge basalt (MORB) to island arc tholeiite signatures. Mineral chemistry analysis of isotropic gabbros and dikes indicate varying clinopyroxene Mg# (=0.64-0.90) and plagioclase anorthite content (An_51-92). Enrichments in large ion lithophile elements (i.e. Pb, Sr) are observed in normal-MORB normalized whole-rock trace element compositions. Based on these, we infer multiple stages of MORB-like magma generation in the PSCS which transpired possibly in a BAB during the Cretaceous and Eocene.

The northern expression of volcanism in the Colombian Andes is a volcano-tectonic province that extends 140 km and comprises large composite volcanoes (e.g., Nevado del Ruiz) and several monogenetic volcanic fields, with compositions ranging from high-Mg basalts to rhyolites. This volcanism is located in a complex tectonic framework generated by the oblique subduction of the Nazca plate and the eastward movement of the Caribbean plate relative to the South American plate. This causes several fault systems (N-S, NE-SW and NW-SE), and a stress field where s₁ trends WNW-ESE. It is recognized that these faults control the propagation of magmas residing 20-30 km deep, however the local control on the emplacement of volcanic edifices is poorly understood. Is the variety of compositions and type of eruptions a result of it? We performed morphometric characterization of these edifices to identify the orientation of the magma-feeding-fractures. We relate the results in a local tectonic context and combine them with petrologic information to understand the relation between them. Preliminary results show that during the Pleistocene, activity was in the north (rhyolites and dacites) and central (dacites) parts of the province. As time progressed, activity focused in the central (andesites) and southern (dacites, trachyandesites and basalts) parts. The most recent activity is focused on the central and south sectors of the province, with mainly dacitic compositions. Morphometric results show NW-SE and NE-SW trends during the Pleistocene and no main trends during the Holocene. This, in addition to the presence of several type of faults in the area, indicate that the trends need to be analyzed for each volcano and not as a group, as magma-feeding-fractures are likely to be impeded where the local and regional principal stresses are not aligned, allowing further fractionation of the magma.

The Tatun Volcano Group (TVG), including more than twenty Quaternary volcanoes, is located 15 km north of Taipei, Taiwan. Prior to the late 1990s, the Tatun Volcano Group was considered an extinct or dormant feature based on the absence of historical evidence for eruptions and outdated dating methods. However, recent geochemical and geophysical studies indicated that the magma reservoir beneath the TVG is existing and the possibility of future eruptions is high. Most recently, a U/Pb age study suggested that the TVG magmatism was initiated around 3 Ma but more active ≤ 0.8 Ma with a flare up ≤ 0.35 Ma. Here, we systematically collected fresh andesite samples from the Tatun Volcano Group and dated these samples by using high-precision 40Ar/39Ar technique. Our preliminary study for one sample collected from Chihsingshan, the highest mountain of this area, yielded an age of 86 ± 4 ka (2 sigma errors). This study, for the first time, provided a precise age for the lava flow in the TVG. Our result refuted the widely accepted age model for the Tatun Volcano Group and suggested that the eruption history should be re-appraised. [This study is supported by HKU Science Faculty RAE Improvement Fund.]

This study investigates historical eruptions recorded at the Mt. Baekdu volcano to make volcanological interpretation of the eruptive events. More than 31 volcanic eruptions were discovered from the Millennium Eruption in 946 AD, most of which were either Plinian or Vulcanian eruptions with volcanic ash dispersed into the regions surrounding the volcano creating fallout ash. The historical eruption of 1702 AD, which is presumed to have erupted from Mt. Baekdu, was a paroxysmal eruption corresponding to the Volcanic Explosivity Index of 5 with a minimum erupted volume of 1.2 km³. This value is based on the empirical formula using an isopach line obtained from two points, in which the recorded fallout ash was more than 3 cm thick and 140 km away from Mt. Baekdu. The 1903 AD eruption was either the phreatomagmatic or Vulcanian eruption that occurred within the Cheonji caldera lake. Based on the historical eruptions, eruption precursors, and volcanic unrest of the volcano between 2002 and 2006, Mt. Baekdu is regarded as an active volcano that has the potential to erupt. Therefore, in order to mitigate the hazard caused by the eruption of Mt. Baekdu, it is necessary to analyze the historical eruption records of Mt. Baekdu and to understand the characteristics of the eruptions through these analyzation.

The Bolaven Volcanic Field recently became the most characterized volcanic field in Indochina. This led us to conduct the first quantitative volcanic hazard and exposure assessment of the region, and estimate the Average Recurrence Interval of the field. The volcanic field is covered by basaltic lava flows ranging from ~16 Ma to < 0.05 Ma, however, a key event was inferred to have taken place in the center of the volcanic field at 0.79 Ma, which is the impact of the largest know young meteorite on Earth, the Australasian meteorite. Therefore, understanding the post-impact volcanism of the area, and how it could affect people in the eventuality of a new eruption, was the main goal of this work. This assessment was conducted through four different steps: 1) generation of a vent density map, based on the location of old scoria cones in the field; 2) production of a probabilistic hazard map, based on lava flow simulations through the numerical model MOLASSES; 3) quantification of the exposure of key assets (population, infrastructures, land cover) in the area investigated; and 4) estimation of the Average Recurrence Interval, based on the inferred number of eruptions that took place in the last 790 ka. The results show that in case of an eruption, the socio-economic aspect for local communities would mostly result from impacts affecting the production, supply, and export of coffee and hydropower (the two main businesses in southern Laos). In addition, people’s life can be threatened by phreatomagmatic explosions, in the case of magma coming into contact with surface or groundwater. The ARI was estimated to be ~10,400 years, however, we acknowledge that this is a conservative estimate since there may be a certain number of buried scoria cones not accounted here.

In 2018, an eruption occurred from the Kagamiike-kita crater of Mt. Motoshirane in central Japan. Mt. Motoshirane is one of the pyroclastic cones of the Kusatsu-Shirane volcano. Its location is about 2 km south from Mt. Shirane, which hosts the Yugama crater where phreatic eruptions were repeated in recent years. The eruption was sudden phreatic explosions that resulted in casualties. Since most of the volcanic activities had occurred around the Yugama crater, few studies were conducted at Mt. Motoshirane. A broadband magnetotelluric survey carried out before the 2018 eruption revealed a low resistivity zone at a depth of ~2 km, but shallow structures were not well constrained. X-ray diffraction analysis of the ash from the 2018 eruption suggested that the source depth of the eruption reached the basement rocks. Geological studies inferred that the Neogene basement rocks are exposed around the Kusatsu-Shirane volcano, but there is no information on the depth of the basement rocks below Mt. Motoshirane. In this study, we will show a three-dimensional resistivity structure around the craters erupted in 2018 to clarify the shallow hydrothermal system of Mt. Motoshirane. For this purpose, we conducted an audio-frequency magnetotelluric survey in 2020. The distribution of apparent resistivity was calculated using one of the rotational invariants of the impedance tensor and tended to show high resistivities in the high frequencies and low resistivities in the low frequencies. In particular, apparent resistivity maps at low frequencies showed low resistivity around the craters of the 2018 eruption. We used a three-dimensional inversion code using tetrahedral meshes to accurately express the steep topography around the surveyed area. The obtained model was characterized by features similar to the apparent resistivity distribution. In the presentation, we will show the inferred shallow structure of Mt. Motoshirane and interpret it based on the previous studies.

Tephra plumes can cause a significant hazard for surrounding towns, infrastructure, and air traffic. Ground-based weather radar has been one of the fundamental instruments used to detect the plume and derive eruption characteristics, such as tephra particle size classes and concentration. The current work presents the use of small and compact X-band Multi-Parameter (X-MP) radars for tephra detection and tracking for two eruption events at Merapi Volcano, Indonesia, in May and June 2018. First, we cancelled ground clutter, anomalous propagation, and sidelobe contamination of radar data by using a Bayesian Classifier. Following this step, tephra particle classes and concentration were retrieved from the reflectivity intensity factor of the radar. Two regimes of tephra particle size classes, i.e., ash and lapilli, were detected from the plume. Temporal and spatial tracking of the tephra was performed by applying an advection equation to the detected tephra. Cross-validation was done by field-survey data and a set of control data from radar observation. We discuss the uncertainty of radar-based tephra detection and tracking by applying the mean ensemble approach to the advection model. The presented method and results will complement remote sensing of volcanic plume detection and tracking to support near real-time hazard assessment and risk mitigation at volcanoes.

With global increases in population and air traffic, our ability to forecast when and where the hazard will occur is of prime importance. In the case of volcanic eruptions, forecasting is not limited to when and where an eruption will take place, but also when and where the impact of such eruption will be felt. In the case of explosive volcanic eruption one of the hazards that is cross-boundary is volcanic ash. The dispersal of volcanic ash can be modelled; however, the level of uncertainty drastically depends on our knowledge of the eruption itself. Critical parameters are the duration of the eruption as well as the height of the associated eruptive column. With those 2 parameters you can simulate a range of plausible ash dispersal scenarios. Such parameters could be retrieved by monitoring or research sensors or arrays deployed on the flank of the volcanoes. However, in regions with hundreds of active or potentially active volcanoes this is not always possible. We would like to demonstrate a few recent examples for which remote infrasound stations were used to extract eruption source parameters.