A collapsed caldera, 1.6 km in diameter and 450 m in depth, was formed at the summit of Miyakejima Volcano during the 2000 eruption. The collapsed caldera appeared on 8 July, with a minor phreatic eruption, 12 days after seismic activity and magma intrusion occurred northwest of the volcano. Growth of the caldera took from 8 July to the middle of August, with seismic swarms associated with the continuous intrusion of magma northwest of the volcano. The growth rate of the caldera was about 1.4×10 7 m 3 /day, and the final volume of the collapsed caldera was about 6×10 8 m 3 . Major phreatomagmatic eruptions produced a total of about 1.6×10 10 kg (1.1×10 7 m 3 ) of volcanic ash after caldera growth. The caldera structure, and the nature of the eruptive materials of the first collapse on 8 July, suggest that the surface subsidence was caused by the upward migration of a steam-filled cavity, with stoping of the roof rock above the magma reservoir. The diameter of the stoping column was estimated to be 600-700 m from circumferential faults that developed in the caldera floor, and the collapse of the caldera wall enlarged the diameter of the caldera to 1.6 km. The total volume of the caldera and the horizontal diameter of the stoping column gave a subsidence of the caldera floor of 1.6-2.1 km.

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[1] Recent geophysical observations on basaltic composite volcanoes in Izu-Bonin arc reveal the process of long-distance lateral magma transport within arc crust. To clarify a long-distance magma transport system of the basaltic arc volcano from geological and petrological aspects, we investigated 20-km-long submarine volcanic chains (Hachijo NW chain and Hachijo-kojima chain) and cones on the northeastern slope (NE edifices) as well as subaerial satellite cones nested Hachijo Nishiyama volcano in the northern Izu arc front. Basalts from Hachijo NW chain have more primitive composition than those from other edifices. The composition of the Hachijo NW chain basalts is controlled by fractional crystallization, while plagioclase accumulation occurred in NE edifices and subaerial satellite cones. Trace element and isotopic characteristics indicate that the same basaltic primary magma is involved in all sections of the volcano. This leads us to consider that magma was transported long distances between the Nishiyama volcano and the Hachijo NW chain. Primitive magma was laterally transported NNW for at least 20 km in the middle to lower crust (10-20 km deep) from Nishiyama volcano with only minimal crustal level modifications and formed Hachijo NW chain. On the other hand, magmas experienced crystal fractionation and accumulation at shallow magma chamber beneath Nishiyama volcano seems to have been transported in a short distance (<5 km) and formed NE trending edifices and subaerial satellite cones. The long-distance magma transport seems to be controlled by a regional extensional stress regime, while short-distance transport may be controlled by local stress regime affected by load of main volcanic edifice.

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The 1914 Taisho eruption of Sakurijima volcano\ud was Japan’s highest intensity and magnitude eruption of the\ud twentieth century. After a 35-year period of quiescence, the\ud volcano suddenly rewoke a few days before the eruption, when\ud earthquakes began to be felt on Sakurajima Island. The eruption\ud began on January 12, 1914, from two fissures located on\ud opposite sides of the volcano, and was characterized by a complex\ud time evolution and changes in eruptive styles. The eruption\ud began with a subPlinian explosive phase in which two\ud convective columns rose from the two fissures. Both plumes\ud were sustained for at least 2 days. This resulted in deposition of\ud a widely dispersed tephra sequence. After this phase, the eruption\ud evolved to a final, waning phase, shifting toward effusive\ud activity that lasted until April 1914. During the first weeks,\ud effusive activity was also accompanied by ash emission. The\ud complex sequence of even t s , characterized by\ud contemporaneous explosive and effusive activity, is typical of\ud several recently observed mid-intensity eruptions, such as during\ud the 2011 eruption of Cordón Caulle, Chile. The stratigraphic\ud sequence of the eruptive deposits from the Taisho eruption\ud comprises alternating coarse-to-fine lapilli beds with ash beds\ud dispersed toward the ESE and SE. These deposits can be\ud subdivided into three lapilli-bearing units (Units T1, T2 and\ud T3, which correspond to the subPlinian phase) and one ashbearing\ud unit (Unit T4, which corresponds to the final ash\ud venting, accompanying the first day/weeks of lava flow activity).\ud Grain size analyses from each unit reveal a marked\ud polymodal distribution generally described by the sum of two\ud or three Gaussian subpopulations. Both the modes and the\ud relative amounts of the coarse subpopulations vary with distance\ud from vent, with those of the fine subpopulation remaining\ud nearly constant. Within the vertical sequence, component\ud analysis shows a progressive increase in lithic fragments, suggesting\ud that conduit enlargement continued until the final\ud stages of the eruption. The estimated volume of the tephra\ud deposit of the subPlinian phase of the eruption is\ud 0.33 ± 0.11 km3 (dense rock equivalent (DRE) volume\ud = 0.09 ± 0.03 km3). The height of the eruption column\ud was also assessed by using four different isopleth maps compiled\ud based on different strategies for the characterization of\ud the largest clasts. Themaximumheight attained by the eruption\ud column is estimated at 15.0 ± 1.2 km above the vent, resulting\ud in a maximum mass discharge rate of 3.6 ± 1.2 × 107 kg s−1\ud (calculated taking into account the strong effect of wind advection).\ud Finally, different classification schemes were applied to\ud classify the eruption, which generally straddles the fields between\ud Plinian and subPlinian