The Chuseri pumice is a widespread Holocene dacitic tephra layer erupted from the Nakano Umi crater in the Towada caldera. The pumice is directly overlain by the Kanegasawa bedded pumice and Utarube ash. The Chuseri pumice. Kanegasawa pumice, and Utarube ash define the Chuseri tephra formation. Volumes are calculated using isopach maps to be 6.5 km^3 for the Chuseri pumice, 1.2 km³ for the Kanegasawa pumice, and 0.9 km³ for the Utarube ash. Plots of the areas enclosed by the isopleths of maximum pumice size and maximum lithic size show the Chuseri pumice to be an average plinian eruption product. The Utarube ash is an ash-fall deposit containing accretionary lapilli and is interbedded with surge deposit near source area. Grainsize characteristics of the Chuseri pumice are described. Like other plinian deposits, the Chuseri pumice does not contain an appreciable amount of fine ash (finer than 1/16mm), at least, as far as 45 km from the source vent. A new radiocarbon date (GaK-9761) on charcoal from within the Chuseri pumice deposit establishes an age for the eruption of 5,390±140 years B.P.

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The 1108 eruption of Asama is the largest among numerous eruptions of the volcano during the Holocene. The magnitude is twice as large as that of the notorious 1783 eruption, which killed about 1,400 people. It is also the oldest written eruption of Asama. Chuyuki, which was written in Kyoto, 300 km SW of Asama, describes that the eruption started on September 29, 1108, by the Julian calendar, and that fields of rice and other crops were severely damaged. Many fatalities are strongly suspected by the distribution of the Oiwake ignimbrite, but no description is given for human loss in Chuyuki. A thin pumice layer intercalated between the 1108 scoria and the 1783 pumice can be correlated to a record of Pele’s hair-fall in Kyoto in 1596. As many as 800 fatalities at the summit in 1598 described in Todaiki cannot be true. Tenmei Shinjo Hen’iki, which describes that a number of villages along the Jabori River were swept away by hot lahars in 1532, is not a contemporary document. It was written in the late 18th century. Fifteen fatalities at the summit in 1721 can be true. After the 1783 eruption, Asama had been relatively quiet for 100 years. During the early and middle 20th century, Asama had been very active with a peak of 398 times vulcanian explosions in 1941. About 30 Iives were lost at the summit, in the 20th century, by 12 explosions among the total about 3,000 explosions.

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We have intended to popularize geological maps, which are restricted professional uses presently. Two geologists, a map designer, and a print coordinator worked together to publish two maps at Asama Volcano and five maps in Izu Peninsula. They are easy to read for everyone because topographic relief is used as a base map. Every map was finished beautiful enough you want to exhibit it on the walls. We sell those maps not only at local shops but through the Internet for 500 yen, a surprisingly lower price than existing geological maps.

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A small-scale explosive eruption of Asama volcano lasted from 02 h 25 m to ca. 06 h, April 26, 1982. It produced a thin but far-reaching ash fall bed and very minor pyroclastic flows on the upper slopes. The main axis of ash-fall deposit extended ESE passing Tokyo to reach the Boso Peninsula. Subordinate axis extended toward the southwest to reach Lake Suwa. The wind above 5, 000 m was mainly responsible for the main axis while the low altitude winds produced the subordinate axis as it blew south to southwestwards. The amount of ash fall ranged from more than 300 g/m² to 100 g/m² at the distance of ca. 10 km from the vent and the total mass erupted was estimated to be about 8 million tons. The ash traveled at an average speed of 12 m/s as far as 200 km from the vent along the main axis. Grain size of the ash regularly decreases clockwise at the distance of about 10 km from the crater reflecting the changing wind direction with altitude. No appreciable change in the medium diameter was found for samples taken in Tokyo (130 km away, Md_φ=3.32) and the one 10 km away (Md_φ=3.26). All samples show marked skewness toward fractions finer than 63 μm suggesting that such fine particles descended in aggregates. Field evidence that in some places ash was incorporated in mud droplets strongly supports this mechanism although it was reported that ash fell apparently in a “dry” state. Ash contained several percent of hydrothermally altered older volcanic materials as well as much water-soluble substances, gypsum, alunite, etc. No clay minerals were found by x-ray diffraction, a fact in strong contrast with the ejecta of 1977-1978 eruption of Usu and 1979 eruption of Ontake volcanoes although both materials had a very similar appearance and clayey physical properties as the present ash. No vesicular, juvenile matelials were identified and the bulk consisted of polyhedral grains of hyalopilitic pyroxene andesite very much similar to recent lavas of Maekawa-yama. No liquid magma but a high-temperature steam jet deep out of the vent may have been responsible for this explosive eruption. High temperature of the erupted material was clearly demonstrated by the glowing deposits observed on the upper slopes immediately after the first phase of eruption. These were mainly laid down by the very small scale pyroclastic flows which overflowed the crater rim and descended for a short distance over the northern and southern slopes.

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Izu-Oshima volcano erupted at about 17:20 hours on November 15, 1986, after 12 years quiescence. The first eruptive activity, phase 1, was restricted to fire fountaining and overflow of lave from the south part of Mihara crater (A crater). The phase 2 eruption began at 16:15 hours on November 21, on the caldera floor. In this phase, fissure eruptions took place on the caldera floor (B craters) at 16:15 hours and another on the northwest slope of the somma (C craters) at 17:46 hours. The A crater activated again at 16:44 hours. Coarse scoria fell on the eastern coast of the island from subplinian column on B craters. Lavas from B craters spread on the caldera floor and another lava heading to Motomachi town streamed down from one of C craters. The eruption of C, B and A craters lasted until about 21:00 hours on 21, 01:00 hours on 22 and 04:30 hours on 22 respectively. The whole sequence of phase 2 eruption was broadcasted by television and informed also by other mass communications. In this report, documentation of phase 2 is presented based on these media. Merits and limits of mass media especially TV reports for documentation of eruption are discussed.

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Izu-Oshima Volcano, Japan, erupted early on the evening of November 21, 1986, to form fire fountains along two major fissures running linearly southeast to northwest, one on the caldera floor (eruption B) and the other on the flank outside the northern caldera rim (eruption C). During the eruption B, the individual vents opened in succession towards the southeast and the northwest with a velocity of 0.42 m/s. During the eruption C, the individual vents also opened in succession towards the northwest with a velocity of 0.23 m/s. An additional feature of interest is a temporal and spatial gap between the erupion C. If this gap was caused by subsurface magma movement, its velocity is estimated to be 0.29 m/s. This velocity is so close to the extension velocity of the eruptions B and C that bilateral magma flow over the length of 3.3 km is inferred, though the possibility of accidental coincidence cannot be excluded.

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The eruptive history of the Higashi Izu monogenetic volcano field for the past 32,000 years is revealed by tephrochronology and loess-chronometry. Morphology, color, and size of basaltic tephra grains are widely variable depending on the mode of the eruption; e.g., spinose red scoria are fallout from a strombolian eruption column when a scoria cone is established around the crater; yellowish green lapilli and hard tuff (kata) are products from phreatomagmatic explosions, the former being more magmatic than the latter. In the field, these discriminations are useful not only for identification of each tephra bed, but also for understanding the transition of eruptive styles during one eruptive event. Because vents are closed or sealed at the end of an eruption, an absolutely quiescence occurs between eruptive events in a monogenetic volcano field. This proves the validity of loess-chronometry. Some of the NW-SE or NE-SW trending alignments of volcanoes proved to be created by eruptive fissures; i.e., they are erupted simultaneously. Among them, the 11 km-long Iwanoyama-Iyuzan volcanic chain is the most conspicuous, which was active about 2,000 years ago. Eruptive events more than 10⁹ kg of magma discharge are recognized 13 times during the past 32,000 years, so that the average frequency of eruption in this field is calculated one every 2,500 years. The last is the Iwanoyama-Iyuzan eruption. The discharge rate of magma is 100×l0⁹ kg/ky for the past 32,000 years or 330×10⁹ kg/ky for the past 5,000 years. The rate seems to be accelerated recently, however, it is still an order of magnitude lower than that of a polygenetic volcano such as Izu Oshima. The Kawagodaira eruption of 3,000 years ago is remarkable for two reasons: the largest with 765×10⁹ kg of magma and the first appearance of rhyolite in the field.

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The eruptive history of Aso volcano for the past 80,000 years is revealed by tephrochronology and loess-chronometry. Around the Aso caldera is a thick accumulation of loess, which is intercalated with numerous Aso tephra layers of limited dispersal as well as three widespread tephra layers of known age that are good marker horizons ; the Akahoya ash (6.3 ka), the Aira-Tn ash (22 ka), and the Aso-4 ignimbrite (70 ka). Loess-chronometry is based on the assumption that, in the Aso region, the accumulation rate of loess has been constant as 12 cm/ky from 80 ka to the present. Most of tephra layers after the caldera-forming Aso-4 eruption are composed of volcanic sand or scoria lapilli of basaltic andesite composition. However the 27 ka Kusasenri dacite (SiO₂ = 67%) pumice is a conspicuous exception. The large volume of 5.85 km³ (bulk) and wide dispersal of this pumice suggests that it is a product of plinian eruption. From October 5 to the end of November 1989, the Nakadake crater of Aso volcano was in eruption. Ash was uninterruptedly emitted from a 500-1,000 m high eruption column coming out of the crater. The average discharge rate of ash was 5 × 10⁷ kg/day. The total mass of ash discharged during the two months reached 3 × 10⁹ kg. The penultimate eruption in recent history was June-August 1979, when 7.5 × 10⁹ kg of ash was discharged. Outside the Aso caldera, the thickness of the 1989 ash is less than 1 cm. It is almost impossible to detect an old ash layer of thickness about 1 cm in a loess cross section, suggesting that sedimentary records 10 km away from a volcano are insufficient to reconstruct past eruptions smaller than 10¹⁰ kg. Eruptions smaller than 10¹⁰ kg can be determined only from proximal deposits. The history of eruptions of Aso volcano over the last few thousand years is tentatively determined from cross sections 2-4 km west of the Nakadake crater. After a 580-1,250 year dormant period, Aso volcano became active about 1,780 years ago. From then, small eruptions each with 10⁹-10¹⁰ kg ash discharge have been repeated 48-88 times up to the present. The duration of each eruption was a few months, and the dormant interval between eruptions averaged 20-37 years.

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Loam is an international scientific term, however, it has been used in a peculiar way in Japan. Japanese loam is a massive, brown, weathered rock unit composed of silt, clay, sand and occasional lapilli. It extensively covers coastal terraces, river terraces, ignimbrite plateaus and other uplands around volcanoes. Loam is not a product of soil forming process operated beneath the earth surface against rock bodies ; but it is a sediment accumulated slowly on the earth surface. Small-magnitude volcanic eruptions play a very minor role for the sedimentation. An eolian reworking process of pre-existing fine-grained deposits by the wind plays a major role. This is proved by following facts : 1) loam has accumulated even during the time when no ash-fall was observed ; 2) a volcano infrequently erupts explosively and the intensity of ash fallout is far lower than the sedimentation rate of loam ; it is about 0.1 mm/year ; 3) loam is hardly thickening toward a volcano. Very small particles carried from continental China by the westerlies at a high altitude are contained in loam, however, in the area around volcanoes their contribution is little for the formation of loam compared with eolian dust carried from nearby bare grounds by local winds at a low altitude. Loam does not accumulate all the year round. Just before and during fresh verdure, occasional strong winds pick up fine particles into the air from a bare ground which is dried up by a high-angle sunlight and high-temperatures. Eventually fine particles will settle down in vegetation. The most favorable season for loam deposition is April to May, in which more than half of an annual amount is achieved. It is convenient and practical to define a single eruption by a tephra layer which is not interbedded with loam. The thickness of loam can be used for the quantitative measurement of geologic time intervals, in years to thousands years, on certain conditions. Lithology of Japanese loam and the mechanism of sedimentation are identical to those of loess in other areas, such as China, northern Europe, northern America and New Zealand. There is no reason to hesitate to designate Japanese loam loess.

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