This paper describes the eruption products and structure of Katsuma-Yama volcano. Katsuma-Yama volcano is located in the northern part of Okushiri Island off the Oshima Peninsula, Hokkaido and has three eruption centers: Horonai-Gawa caldera, Katsuma-Yama crater and west Katsuma-Yama crater. Horonai-Gawa caldera is 2km by 1.5km in dimension, and is filled mainly with massive breccias to tuff breccias, fine-grained lacustrine sediments and bedded lapillistone to tuff. Massive breccias to tuff breccias are exposed along the western wall of the caldera and have been recovered by drilling in the central portions of the caldera. The breccias contain fragments mainly of dacite, dacite pyroclastic rocks, andesite, and granodiorite together with minor fragments of perlitic rhyolite. The majority of the rock fragments are quite similar in their constituents and textures to the surrounding basement rocks. Perlitic rhyolite, however, is relatively fresh and cannot be recognized in the surrounding basement rocks. This rock is, therefore, thought to be juvenile, although no eruption products remain outside the caldera. The fine-grained lacustrine sediments are wavy stratified with a wavelength up to a few meters and locally contain sulfur deposits. Bedded lapillistone to tuff comprises mostly fragments of glassy biotite-rhyolite, and projectiles derived from the direction of Mt. Katsuma Yama formed sag structures in the beds. Katsuma-Yama crater occurs at Mt. Katsuma Yama. The crater has a diameter of 740m across and is filled with bedded lapillistone to tuff. The constituents are mostly non- to poorly vesicular polyhedral or platy fragments of glassy biotite-rhyolite and are thought to be phreatomagmatic in origin. Although the outflow deposits partly remain on the western and southeastern flanks of Mt. Katsuma Yama, most of the expected pyroclastic ring or cone has been removed through later erosion. Katsuma-Yama lava of similar composition occurs through the crater infill, spreads over the eroded surface of the outflow deposits, and is distributed mainly on the southwestern flank of Mt. Katsuma Yama and further to Horonai-Gawa caldera. The lava has a thickness of 100m at Mt. Katsuma Yama, thins to the downflow directions with a variable thickness of flow breccias, and is intruded into the infill of Horonai-Gawa caldera, with plastic deformation of the caldera deposits along the contact. West Katsuma-Yama crater opens through Katsuma-Yama lava at its western margin, with a diameter of 180m. The major infillings comprise rock fragments mostly similar to those from the Katsuma-Yama crater. A minor volume of pyroclastic surge deposits from this crater remains on Katsuma-Yama lava in the summit area of Mt. Katsuma Yama, and rests on the brown soil of a few centimeters that covers the eroded surface of the western rim of Katsuma-Yama crater. The eruption volume from Horonai-Gawa caldera is unknown but could be between 1 and 10km³. The eruption volume from Katsuma-Yama crater perhaps slightly exceeds 0.6km³, and the eruption volume from west Katsuma-Yama crater is very small, perhaps less than 0.01km³. Katsuma-Yama lava is dated by fission-track methods to be 0.2-0.7Ma, and no soil occurs between the lava and the overlying pyroclastic surge deposits of the west Katsuma-Yama crater origin. A thin brown soil between the pyroclastc deposits of Katsuma-Yama and west Katsuma-Yama craters represents a short dormancy in volcanic activity. Lacustrine deposits in Horonai-Gawa caldera indicate a high wave-energy setting. Sulfur precipitated in the deposits suggests fumarolic activity in the caldera lake. These facts likely demonstrate post-caldera volcanism, and the plastic deformation of the caldera fill by the intrusion of Kamui-Yama lava suggests that the post-caldera volcanism was succeeded by the activities of Katsuma-Yama and west Katsuma-Yama craters.

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A basaltic pyroclastic flow deposit, the Izumikawa pyroclastic flow deposit, occurs at the northeastern foot of Nakadake Volcano, which is the only active central cone of Aso caldera, southwestern Japan. The pyroclastic flow deposit covers a fan-shaped area of about 0.9-1.9km², and the bulk volume is estimated at 4.4-9.4×10⁶m³. The deposit is poorly sorted, and consists of subangular faceted clasts and spherical cauliflower bombs set in a sandy non-cohesive matrix. The deposit forms two different facies: a black reversely graded lower unit and a reddish-gray reversely graded upper unit. The cauliflower bombs, which have slightly vesiculated crusts and denser interiors, are more abundant in the lower unit than in the upper unit. The presence of the cauliflower bombs suggests that the pyroclastic flow was generated by an explosion at the source lava lake or conduit, which was filled with mixture of solidified and molten lavas. The age of the deposit was estimated at ca. 19 cal ka, based on ¹⁴C ages obtained from charred wood fragments in the deposit. Recent tephrochronological studies reveal that Nakadake Volcano became active from ca. 22-21 cal ka and that violent scoria and ash eruptions of Nakadake were concentrated in two periods of ca. 22-21 cal ka and 18-16 cal ka. The age of the Izumikawa pyroclastic flow corresponds an intermediate period between the two violent eruption periods. A similar pyroclastic flow deposit and a basaltic lava flow were also identified. They cover immediately the Izumikawa pyroclastic flow deposit. These facts indicate that multiple violent eruptions producing pyroclastic flows and lava flows occurred in a short period at ca. 19 cal ka. Recent activity of Nakadake has been characterized by ash eruptions, strombolian eruptions and phreatomagmatic explosions. However, the presence of the Izumikawa pyroclastic flow deposit emphasizes the potential hazard induced by bomb-rich pyroclastic flows that may rush down the flanks of Nakadake Volcano.

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We discovered a set of phreatic explosion deposits, herein referred to as the Owakidani tephra group, on the northern slope of Mt. Kamiyama and in the Owakidani fumarolic area of the Hakone Volcano. The tephra group is the product of the volcanic activities since the latest magmatic eruption of Hakone Volcano at around 2.9ka. It comprises five units named Hk-Ow1 to Hk-Ow5 in the ascending order. Both Hk-Ow1 and Hk-Ow2 comprise tephra fall deposits and secondary debris flow deposits. In addition to these deposits, Hk-Ow2 is also associated with surge deposits. Hk-Ow3, Hk-Ow4 and Hk-Ow5 consist of tephra fall deposits. The ash of these tephra fall deposits and the matrix of the secondary debris flows are principally composed of clay, altered lithics and secondary minerals supposed to be of fumarolic area origin. It is possible that Hk-Ow1 and Hk-Ow2 erupted from a fissure on the northeastern ridge of Mt. Kamiyama, while Hk-Ow3, Hk-Ow4 and Hk-Ow5 erupted at Owakidani. No juvenile material was found within the deposits of these eruptions except for Hk-Ow2, while the surge deposit of Hk-Ow2 contained trace amounts of volcanic glass fragment. Although it is considered that the principal nature of the eruptions of the Owakidani tephra group is phreatic, the deformation of the edifice around the source area implies the possibility of magma intrusion to shallow levels. Based on the calendar ages of the Owakidani tephra group and the stratigraphic position of the Kozushima-Tenjosan tephra, we estimated that Hk-Ow3, Hk-Ow4 and Hk-Ow5 erupted in relatively short intervals between the latter half of the 12th and 13th centuries. On the other hand, Hk-Ow1 and Hk-Ow2 erupted at around 3 kyr BP and 2kyr BP, respectively. The eruption ages of the Owakidani tephra group generally correspond to the seismic events that occurred in the Kozu-Matsuda Faults and the Tanna-Hirayama tectonic line. It is suggested that the activity of the Hakone Volcano may be closely related to the tectonic events in this region.

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The Sashikiji 2 (S₂) Member in the products of Izu-Oshima volcano was formed by an explosive eruption accompanied with caldera depression. This member is characterized by breccia called as a “low-temperature pyroclastic flow deposit”. In this paper, the S₂ breccia is re-examined based on stratigraphy, grain fabrics, grain-size distributions and modal compositions. The S₂ Member is divided into six units from S₂-a to S₂-f in ascending order. The S₂-a unit consists of scoria, bomb and aa lava flows from flank fissures. The S₂-b unit is made up of well-bedded ash and fine-lapilli from the summit. The S₂-c unit is composed of matrix-supported breccia, locally filling valley bottoms and containing abundant deformed soil fragments and woods. The S₂-d unit consists of reverse to normal grading, clast-supported breccia with ash matrix, covering topographic relief in the whole island. The S₂-e unit is composed of dune- to parallel-bedded lapilli and ash in the proximal facies. The S₂-f unit is clast-supported breccia with and without ash matrix. New ¹⁴C ages of wood fragments in the S₂ Member have been determined as about Cal AD 340. Although the S₂-c and -d units are previously interpreted to the low-temperature pyroclastic flow deposit, these units are quite different in sedimentological features as follows. The grain fabric measurements have revealed that the S₂-d unit has a-type imbrication showing the longest axis of grains parallel to the flow direction. On the other hand, the S₂-c has random fabric of grains. The grain size distribution of the S₂-d unit shows a bimodal nature having subpopulations at phi -1.0 to 1.0 and coarser than phi -2.5. The bimodal nature and a-type imbrication suggest that the two transport processes overlap; the load of a turbulent suspension is not all in true suspension as the coarser population may travel in a cast-dispersion mass flow. The S₂-c unit shows a polymodal grain size distribution with multi subpopulations from coarse to fine. The poor sorting, massive appearance, valley-confined distribution, and random grain fabric of the S₂-c unit are characteristic of deposition from a cohesive flow without formation of traction-related bedforms or sorting of different grain sizes by turbulence. The modal composition measurements have indicated that the S₂-c and -d units lack essential scoriceous or glassy fragments. This evidence indicates that both units are derived from steam explosions due to outburst of highly-pressurized geothermal fluid within the edifice. The S₂-c unit was plausibly generated by remobilization of phreatic debris around the summit caused by ejection of condensed water from a plume or heavy rainfall. The S₂-d unit was a pyroclastic density current deposit resulted from collapse of a highly-discharged phreatic plume. Estimated velocities of the current are 150 to 30m/s based on suspended grain sizes.

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Asama volcano, located in the central part of Japan, repeated medium to small scale Vulcanian eruptions, from September through December 2004. We analysed the activity between January 2004 and April 2005 using nighttime infrared data from MODIS -Moderate Resolution Imaging Spectroradiometer -onboard Terra and Aqua satellites, in conjunction with the data from ground-based instruments obtained simultaneously and chemical composition of the ejecta. The observed period is divided into four stages, S-I, S-II, S-III and S-IV, based on the relationship between thermal anomalies observed by MODIS and eruptive activity. S-I (second half of August) is the thermally active period preceding to the series of eruptions. This may have been caused by a magmatic supply to the shallow level of the conduit, resulted from dyke intrusion at a deep level (1km bellow sea level) in July, as suggested by the ground deformation monitoring. This magmatic supply probably resulted in initiating the eruptive activity of S-II. S-II (1 September -mid-December) is the thermally active period during the series of eruptions, which is sub-divided into two stages, S-IIa (1 September -10 October) and S-IIb (10 October -mid-December), by the two thermally active pulses in this period. Similar pulses are also recognized in the time-series variation of eruptive amount of S-II, as well as seismicity and SO₂ discharge rate. A deep dyke intrusion observed in the late S-IIa may have resulted in a new supply of magma to the shallow level, which caused the IIb activity. After two months of inactive period of S-III (late December 2004 -February 2005), S-IV (March-at least April 2005), the post eruptive thermally active period, started. Although this stage did not involve eruptive activities, SO₂ discharge rate, level of volcanic glow and height of plume raised. This may have been caused by the third magmatic supply to the shallow level in late January. At the volcanoes possessing open to semi-open passages between the summit crater and shallow level of the conduit like Asama, MODIS may detect pre-eruptive thermal anomaly, which can be utilized for monitoring those distributing in remote areas, such as east Asia.

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