Kurodake volcano in the Taisetsu volcano group was formed approximately 0.2 Ma, producing andesitic lava flows and a dome. The lavas contain numerous mafic inclusions (<20 vol.%) ranging from approximately 1 cm to about 30 cm in diameter. The mafic inclusions exhibit typically rounded to ellipsoidal shapes and have smooth contacts with the host lavas. The mafic inclusions are classified into two types, fine and coarse, based on the size of the groundmass crystals. The groundmass crystals of the fine-type inclusions are composed of acicular minerals (0.1-0.3 mm in length). On the other hand, the groundmass of the coarse-type inclusions is primarily composed of tabular minerals (>85 vol.% and 0.2-0.5 mm in length). The plagioclase core compositions of the host lavas and two types of mafic inclusions vary substantially from An₃₈ to An₉₀. The plagioclase phenocrysts are classified into three groups based on their core compositions: An-rich (type A: An>80), An-poor (type B: An<60), and intermediate (type C: 60<An<80). Type A and type B plagioclases were derived from mafic and silicic magmas, respectively, and type C was derived from a hybrid magma formed by the mixing of the mafic and silicic magmas. The host lavas predominantly contain type B plagioclase phenocrysts, with infrequent types A and C, and most of the plagioclase microphenocrysts and groundmass crystals are type C. In the fine-type inclusions, type A and type B plagioclase phenocrysts coexist, and most of plagioclase microphenocrysts and groundmass crystals are classified into the type C, similar to the host lavas. In the coarse-type inclusions, most of the plagioclase phenocrysts, microphenocrysts, and groundmass crystals are classified as type B. These assemblages in the host lavas and fine-type inclusions can be explained by the mixing of the magmas, whereas the coarse-type inclusions were formed in the silicic end-member magma. Initially, mafic magma containing type A plagioclase was injected into bottom of the silicic magma chamber containing type B. A small amount of mafic magma was mixed with silicic magma to form the host magma. Subsequently, mixing occurred near the boundary between the mafic and silicic magmas, producing the fine-type inclusion forming magma. We presume that the margin of the silicic magma chamber was highly crystalline and the coarse-type inclusions were derived from the margin of the silicic magma chamber.

Identifying and characterizing a precursor volcanic episode that occurs prior to a large silicic eruption is important for the mitigation of volcanic hazards. Our key interest is the types of precursory phenomena that are manifested and how these relate to subsequent large eruptions. A sequence of tephra deposits can represent an invaluable record of precursors to subsequent large eruptions. Here, we report an investigation of the Kuttara-Hayakita (Kt-Hy) tephra (59-55 ka) on Kuttara caldera volcano based on geological field survey, granulometric measurements, and geochemical analyses. The eruption sequence revealed in proximal Kt-Hy is characterized by ejecta extending from earlier sub-Plinian (units Lpfa and Lpdc) to later phreatomagmatic (unit Mpdc) eruptions. Units Lpdc-Mpdc are valley-filling pyroclastic density current deposits. Subsequently, a poor-vesiculated silicic magma was erupted as an extensive and dilute pyroclastic density current (Updc) during the phreatomagmatic phase. On the basis of the inferred eruption sequence and detailed facies interpretations, the source craters for Mpdc and Updc are inferred to have been located in the southern apron of Kuttara stratovolcano and at the present caldera center, respectively. Estimation of the total volume of the Kt-Hy tephra remains uncertain because the evolution of the Kuttara stratovolcano is unclear. A comparison of proximal with distal facies and inferred crater migration suggests that the Kuttara stratovolcano could have grown before and during episode Kt-Hy. Including the volume of the stratovolcano, the estimated volume of Kt-Hy ejecta is approximately 7-8 km³ dense rock equivalent. These features of crater migration and the eruption style of Kt-Hy ejecta are consistent with a typical precursor episode characterized by simple, small-volume phreatomagmatic eruptions associated with stratovolcanoes and small calderas. The geochemical similarity of the precursor eruption products to those of the later large silicic eruption (Kt-3 eruption at 54 ka) may reflect the storage of a large volume of silicic magma during the precursor episode.

The 2016 eruptions of Niigata-Yakeyama volcano in central Japan consisted of several small eruptions that were accompanied by syneruptive-spouted type lahars. We have reviewed the sequence of the 2016 activity and modeled the eruptive processes based on observations of various volcanic phenomena, including ash fall and lahars, plumes, earthquakes and crustal deformation, and analysis of eruptive products. Eruptions of Niigata-Yakeyama volcano after the 20th century can be categorized into two types; 1) VEI＝0-1 eruptions during which ash fall covered only the summit area and no ballistic blocks were ejected (e.g., 1997-1998 event) and 2) VEI＝1-2 eruptions during which ash fall reached the foot of the mountain with ejected blocks (e.g., 1974 event). We also discuss the characteristics of the 2016 activity by comparing the sequence with those of other events of Niigata-Yakeyama volcano: the 1974 and 1997-1998 eruption events and the 2000-2001 intensified fumarolic event. The 2016 eruptions of Niigata-Yakeyama volcano are divided into the following six stages. Stage I was characterized by the onset of intensified steam plume emission activity (≥200 m). Stage II was characterized by the onset of crustal deformation, slight increase of high frequency earthquakes (approx.>3.3 Hz) and further activation of steam plume emission activity (≥500 m). The crustal deformation observed commenced at the beginning of Stage II and lasted until the end of Stage V. The total inflated volume was estimated to be approximately 7.2×10⁶ m³. Several very small eruptions that provided only a small amount of ash to the summit area also occurred. Stage III was characterized by a rapid increase of high frequency earthquakes accompanied by tilt change, and the onset of low frequency earthquakes (approx.<3.3 Hz). A small eruption was accompanied by a syneruptive-spouted type lahar at this time. Stage IV was characterized by the occurrence of several small syneruptive-spouted type lahars. The occurrence of high and low frequency earthquakes continued, but with decreasing abundance. Stage V was characterized by the highest altitude of steam plume emission (≥1,200 m), while no ash emission nor syneruptive-spouted type lahars were observed. Stage VI was characterized by a gradual decrease in steam plume emission and earthquake activity. The aerial photographs indicate the ash fall distribution, and the maximum scale of the 2016 eruption, which is estimated to be VEI＝1. The assemblage of altered minerals indicates that the volcanic ash originated from volcanic conduits affected by a high-sulfidation epithermal system and no magmatic components were detected. Judging from the depth of the crustal deformation source of magmatic eruptions at other volcanoes, the estimated source of crustal deformation during the 2016 eruption is considered to have been caused by a volume change of the magma chamber. The sequence of the 2016 event can be interpreted as follows: 1) magma supply to the magma chamber, 2) increase in seismicity and fumarolic activity triggered by volcanic fluid released from the new magma, 3) destruction of volcanic conduit by increased fumarolic activity and emission of volcanic ash, and 4) occurrence of syneruptive-spouted type lahars by the “airlift pump” effect. At Niigata-Yakeyama volcano, such small eruptions and fumarolic events have been frequently observed for the last 40 years. We thus consider that the accumulation of magma has progressed beneath the volcano, which is a potential preparatory process for a future magmatic eruption.

This paper presents a revised stratigraphy of the volcanic rocks, pyroclastic materials, and marine deposits transitional from the Pre-Unzen to Unzen volcanoes during the period of 1.9-0.3 Ma in the southern part of Shimabara Peninsula, Kyushu. The geological units, in ascending order, include the Kazusa Formation, Mejima Formation, Minami-Kushiyama Formation, Hojodake basalt, Saishoji Formation, Otani Formation, Kita-Arima Formation, Suwanoike basalt, Ideguchi Formation, Tonosaka andesite, Takaiwasan andesite, and Older Unzen volcanic fan deposits. Among these units, Saishoji and Kita-Arima Formations are shallow marine sediments deposited during a quiet period of volcanic activity, and the Otani Formation, an exotic marker tephra, is intercalated between them. In this study, these formations are newly defined as the Uppermost Kuchinotsu Group (1.0-0.6 Ma), and the Upper Kuchinotsu Group (1.9-1.0 Ma) and Older Unzen volcano (0.6-0.3 Ma), were defined as two active volcanic periods separated by the quiet period (Uppermost Kuchinotsu Group). The continuity of activity ages and similarities in rock chemistry imply that the Suwanoike basalt, Tonosaka andesite, Takaiwasan andesite, and Older Unzen volcanic fan deposits were associated with the Older Unzen volcano. This means that Older Unzen volcano become active after the quiet period of 1.0-0.6 Ma. The Ideguchi Formation, also an exotic marker tephra, and the Otani Formation were excluded from the volcanic activity in this area. The eruption sources of these exotic tephras could have been derived from other regions in Kyushu, but the source was not identified in this study.

In the Kikai caldera, a major caldera-forming eruption, the Akahoya eruption (Ah eruption), occurred at 7.3 cal ka BP. It started with a plinian eruption (K-KyP), accompanied by a small intra-plinian Funakura pyroclastic flow (K-Fn). In the second eruptive stage, large Koya pyroclastic flow eruption (K-Ky) occurred, which covered the southern part of Kyushu with widespread co-ignimbrite ash (K-Ah (c)). These series of pyroclastic materials are collectively called Kikai-Akahoya tephra (K-Ah (T)). It has been thought that the Akahoya tsunami (Ah tsunami), occurred in connection with the Ah eruption. However, in outcrops below 50 m elevation in the proximal area of the caldera (~60 km), the K-Ah (T) was either replaced by Ah tsunami deposits of various sedimentary facies or completely eroded away by the same tsunami. The largest tsunami was therefore estimated to be due to the collapse of the caldera rim, which occurred some time after the end of the Ah eruption. On the other hand, in the Yokoo midden at Oita city, approximately 300 km from the caldera, it was considered that the K-Ah (c) was deposited immediately above the sandy tsunami deposit. However, the parent material of these distal Ah tsunami deposit is presumed to be K-Ah (r), which was transported and deposited from hinterland to the estuary, and was then incorporated and redeposited by the subsequent striking Ah tsunami. That is, the particles in the tsunami can be interpreted as separating and settling into two different layers, i.e. the basal sand layer and the upper K-Ah (r) set as the same tsunami deposit, due to differences in density. This interpretation is also supported by the chemical analyses of volcanic glass. Thus, the erosion and deposition either proximal or distal area of the caldera indicate that the largest Ah tsunami occurred some time after the Ah eruption. The caldera rim shows a double depression structure which was formed during the Ah eruption, and there are many channel structures on the caldera rim that suggest intense seawater movement. It is therefore highly probable that the sudden collapse of caldera wall after the Ah eruption is the cause of the tsunami, together with the run-up height near the caldera. However, it is not possible to estimate the time until the collapse that caused the Ah tsunami.

Mt. Hoei (Hoei-zan) is a protuberance on the southeastern flank of the Fuji volcano, Japan. The lateral cone was formed during the Hoei eruption in AD 1707. However, the geological map of the Fuji volcano assigns the material of the protuberance to an older unit in the Hoshiyama Stage (100 to 17 ka); this is because the Akaiwa deposits around the summit have been altered in the same manner as rocks in the Hoshiyama Stage. This assignment has led to a model, unique in the context of modern volcanology, in which Mt. Hoei is an uplifted bulge caused by the intrusion of degassed magma that occurred at the time of the eruption; it thus led us to reinvestigate the geology of Mt. Hoei for the first time since Tsuya (1955). In addition to a geological survey, we obtained paleomagnetic directions from the Akaiwa and fallout deposits in the Goten-niwa erosional valley at the base of Mt. Hoei and compared the former with directions from the spatter cone that formed in the first Hoei crater during the final stage of the Hoei eruption. All the directions agreed well with each other and the archeomagnetic directions reported as corresponding to AD 1707, clearly indicating that the Akaiwa is not a part of the Hoshiyama Stage. We also performed petrographic and whole-rock chemical analyses of the deposits and found a gradual upward compositional change from dacite to basalt corresponding to the distal tephrostratigraphic units Ho-I to Ho-IV. This result shows that the Akaiwa deposits corresponded to the rocks of unit Ho-III, and both paleomagnetic and petrologic investigations strongly suggest that the former formed contemporaneously with the eruption. Therefore, the protuberance is not a bulge caused by the magmatic intrusion but a pyroclastic cone from the Hoei eruption.

When volcanic ash is exposed to water after fell on the ground, various chemical substances will be eluted to water phase. Amounts of water soluble SO₄ and Cl and their Cl/SO₄ ratio of ash are useful for understanding eruptive activities of the volcano. For prompt evaluation of eruptive activities by water soluble components on ash, it would be useful if the analyses are made in-situ or near by the volcano instead of sending the samples to the laboratories far away. For this purpose, we utilized and established a method using a compact handheld absorptiometer for analyses of Cl and SO₄ in ash leachate. The method uses a small digital scale, a handheld absorptiometer and other equipment (PP bottles, syringes filters and etc.). The scale is for ash leachates preparation and the absorptiometer is used for the turbidimetric measurements of SO₄ and Cl. The calibration curves for SO₄ and Cl were linear and parabola for the concentration range of the standard solutions up to 81.9 mg/L and 39.6 mg/L, respectively. Eleven ash samples from Kirishima Shinmoedake and Sakurajima volcanoes were analyzed by turbidimetry method of this study and by ion chromatography method, and were compared for validation of the method. The analyzed concentrations were basically within about 10 % compared to those of ion chromatography, except for samples whose absorbance were smaller than 0.1 unit. We also checked for the interfering components for turbidimetry analyses by checking the compiled ash leachate data of Airys and Delmelle (2012) and came to conclusion that effect of interference can be usually ignored. On the other hand, some of the ash samples with very low water soluble SO₄ and Cl may be under detection limit with the proposed method. As a conclusion, ash leachate analyses for SO₄ and Cl by turbidimetry using a handheld absorptiometer used in this study is an effective method and could be used for prompt evaluation of eruptive activities especially on remotes islands where the chemical laboratories are not available.