Prior to the onset of magmatic activity at the Nakadake first crater, Aso Volcano (SW Japan) from July 2019 to June 2020, multiple small eruptions occurred between April and May 2019. The May 3-5 eruption was one of the largest events during the pre-magmatic activity period. An ash-fall deposit from the early stage of that eruption (15 : 00-18 : 00 in JST on May 3) was distributed to the south of the source crater, whereas the ash erupted after 20 : 00 on May 3 was dispersed southwestwards. The May 3 15 : 00-18 : 00 ash was composed mainly of fine particles (<0.25 mm in diameter) and fell as accretionary lapilli (<0.8 mm). In contrast, ash after 20 : 00 on May 3 consisted mainly of 0.5 mm grains but lacked silt and clay content. Based on an isomass map, the total discharged mass of the May 3-5, 2019 eruption was about 700 tons. Although lithic (50 %) and altered glass (30-40 %) grains were dominant in both ash-fall deposits, they also included small amounts of black to pale-brown fresh glass shards (2-4 %) inferred to be juvenile material originating from newly ascending magma. After the May 3-5 event, small ash emissions occurred intermittently until July 2019. The proportions of fresh glass shards included in the May-July 2019 ash-fall deposits gradually increased; ash erupted in early July contained 7 % fresh glass grains. Small-scale magmatic activity began on July 26, 2019, and continued to mid-June 2020. The April to early July 2019 ash emissions at Nakadake first crater are inferred to be precursor phenomenon of the late July 2019 to mid-June 2020 magmatic eruptions. It is very important to clarify temporal variations in the mass and component characteristics of erupted materials for understanding the sequence of events and predicting future eruptive activity.

Nakadake volcano, the current active center of the Aso central cones (Kyushu), is one of the most active volcanoes in Japan. It has been active since ca. 22-21 cal ka, and has formed the old edifice (22-21 cal ka), the young edifice (around 5 cal ka) and the youngest pyroclastic cone (until present). The lava flows from the young edifice spread on the flank of the volcano several times around 5 cal ka. These lavas are supposed to give stratigraphic markers for constructing the eruptive history of Nakadake volcano, but the similarity in chemical composition and lithology hampers distinguishing and correlating them. We have conducted a paleomagnetic study to distinguish and correlate the lavas since the paleomagnetic secular variation (PSV) provides a high-resolution age information. If lava units have a temporal difference of more than 50 years, they could be distinguished by their paleomagnetic directions. The samples were collected from 9 lava flows and 8 agglutinate layers (welded scoria-fall deposits) and were subjected to the paleomagnetic and rock-magnetic measurements. These samples, from visual inspection, appear to be influenced by chemical alteration in the surface of the outcrop by sulfides of volcanic gases. To check a rock-magnetic effect of the chemical alteration of the lavas and agglutinates, thermomagnetic analyses were made on chip samples from the top (surface of rock) and bottom (inside of rock) of the collected paleomagnetic cores. The thermomagnetic analyses indicate that the core top and bottom samples show the same behaviors, in spite of the difference in color, and the carriers of magnetization of each core are titanium rich (titanium content, x, is about 0.6) and poor (x is about 0.1-0.2) titanomagnetites. The natural remanent magnetization of each sample shows a simple, single vector component in alternating field demagnetization experiments, which well defines the primary component. Site mean directions can be categorized into three different direction groups. These data suggest that the eruption producing lava flows and/or agglutinates occurred at three different ages. Furthermore, the paleomagnetic directions of one group is not consistent with the directions of the eruptive ages of Nakadake young edifice assigned from the previous stratigraphic studies. Comparing these directions with the paleomagnetic secular variation curve which has been drawn from basaltic volcanoes in the northwestern part of Aso central cones, the ages of the direction groups can be assigned to around 6.0-4.3 cal ka and 3.5 cal ka, respectively. This result demonstrates that paleomagnetic studies can greatly contribute for establishing the eruptive histories of volcanos.

Based on detailed fieldwork, petrological and paleomagnetic investigations, we present a revised stratigraphy of deposits from the 7.6 ka eruption at Mashu volcano and the formation process of its summit caldera, eastern Hokkaido, Japan. As previously described, the eruption products consist of an initial phreatomagmatic unit (Ma-j) and the overlying three pumice-fall layers (Ma-i, -h, and -g), which are in turn overlain by pyroclastic-flow deposits (Ma-f). In the present study, we divide Ma-f into 4 subunits: Ma-f1/2, Ma-fAc, Ma-f3a and Ma-f3b in descending order. Ma-f3b is a valley-ponding, pumice-flow deposit with limited distribution. Ma-f3a comprises clast-supported facies (fines-depleted ignimbrite: FDI) and matrix-supported (normal ignimbrite) facies, the two changing across topography. The FDI is characterized by a gray, fines-depleted, lithic-breccia-rich layer with materials incorporated from the substrate. Impact sag structures from large (>50 cm) dacite ballistic blocks were recognized at the base of the Ma-f3a within 10 km from the source. Ma-fAc is a minor eruption unit consisting of accretionary lapilli. Ma-f1/2 is a most voluminous (8.8 km³), widely distributed and weakly stratified ignimbrite. Both Ma-f3a and Ma-f1/2 can be classified as “low aspect ratio ignimbrite (LARI)”. Dacite lithic fragments are ubiquitously observed throughout the sequence and are not considered to be juvenile; they have distinctly different chemical compositions from the pumice fragments in the early pumice-fall (Ma-g～Ma-i) and pyroclastic-flow (Ma-f3b) deposits, but those of pumice clasts in the late pyroclastic-flow units (Ma-f3a and Ma-f2) lie between the two on a FeO*/MgO vs. SiO₂ diagram. The 7.6 ka caldera-forming eruption of the Mashu volcano was initiated by Plinian fall (Ma-j～-g), and then, a small-volume high aspect ratio ignimbrite (Ma-f3b) was deposited by a valley-confined pyroclastic flow that was generated by partial column collapse. After that, a violent pyroclastic flow was generated probably during a strong explosion of a dacite lava edifice on the summit of Mashu volcano. This flow emplaced Ma-f3a. The caldera collapse that followed the explosion generated a climactic pyroclastic flow that emplaced Ma-f1/2. Ma-f3a flow was extremely fast. Ma-f1/2 flow was related to sustained flow due to low settling velocity and high discharge volume. These are supported by field observations and numerical simulation that shows the ability of the flow to surmount high topographic obstacles and spread widely. The 7.6 ka caldera-forming process of Mashu volcano was driven not only by subsidence of roof block but also by violent explosions.

Meakandake Volcano is a post-caldera active stratovolcano located on the south-eastern rim of Akan Caldera, eastern Hokkaido, Japan. Recent eruptive activity has occurred in 1955-1960, 1988, 1996, 1998, 2006, and 2008 at Ponmachineshiri, which is one of several volcanic bodies that form the stratovolcano. These events indicate that Ponmachineshiri has a high potential for future eruptions. In order to better understand the hazards posed by Meakandake Volcano, this study focused on the modern eruptive activity of Ponmachineshiri during the last 1,000 years. The authors conducted field observations at outcrops in the summit area, excavation surveys on the volcanic flanks, component analysis for pyroclastic deposits, and radiocarbon dating for intercalated soil layers. As a result, at least four layers of pyroclastic fall deposits derived from Ponmachineshiri during the last 1,000 years were recognized, ranging from Volcanic Explosivity Index (VEI) levels of 1 to 2. In chronological order, the major pyroclastic fall deposits consist of Pon-1 (10^th to 12^th century; VEI 2), Pon-2 (13^th to 14^th century; VEI 2), Pon-3 (15^th to 17^th century; VEI 1), and Pon-4 (after AD 1739; VEI 1), with small-scale (VEI＜1) phreatic and phreatomagmatic eruption deposits intercalated within Pon-1, Pon-2, and Pon-3 pyroclastic fall deposits. The presence of scoria and minor pumice in the Pon-1, Pon-2, and Pon-3 pyroclastic fall deposits suggests that these eruptions were phreatomagmatic events. On the other hand, the absence of juvenile materials in the Pon-4 pyroclastic fall deposits suggests that the activity was a phreatic eruption. The decreasing proportion of juvenile materials in eruptive deposits over the last 1,000 years is consistent with a reduced magma contribution and indicates that the development of the hydrothermal system is likely to play an important role in future eruption scenarios for Meakandake Volcano.