著者
伴 雅雄
出版者
特定非営利活動法人日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.36, no.2, pp.255-267, 1991-07-15
被引用文献数
4

Nasu volcanoes, the southern part of the volcanic front of Northeast Japan arc, comprises 10 small volcanic centers. Minamigassan is one of these centers and is located at the southern end of this volcanoes. A petrological model of magmatic proccess of Minamigassan volcano is presented based on the mineralogy and whole-rock chemistry. The eruption products of Minamigassan volcano can be divided into five units ; E-1, E-2, L-1, L-2, and L-3 units from lower to upper. The caldera collapse occured at the beginning of L-1 unit. The E-1 unit belongs to medium-K tholeiite, E-2 to low-K tholeiite, L-1 to medium-K calc-alkaline, L-2 to medium-K tholeiite, and L-3 to medium-K calc-alkaline categories, respectively. L-1 unit is petrologically similar to E-1 unit. Least squares subtraction calculation for major elements and Rayliegh fractionation model calculation for trace elements indicate that the chemical variation within E-1 and E-2 unit can be interpreted by fractional crystallization of phenocrystic minerals. On the other hand calc-alkaline suites, L-1 and L-3 units suffered magma mixing. These units formed by mixing between basic magma which resembles E-1 unit magma and felsic magma which is derived from early stage magma through crustal contamination or crustal melt. Distinct differences in LIL/HFS ratio among E-1 and E-2 units cannot be explained either by the difference in degrees of partial melting of a common source or by any fractional crystallization process. The difference between E-1 and E-2 units have been originated in chemical difference of source material.
著者
大野 希一 山川 修治 大石 雅之 高橋 康 上野 龍之 井田 貴史
出版者
特定非営利活動法人日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.50, no.6, pp.535-554, 2005-12-30
被引用文献数
1

A cloud height generated by a volcanic eruption reflects the immensity and/or magnitude of the eruption; thus a measuring of the height's temporal variation during the event is very significant in judging whether the activity will become violent or decline. However, when a volcanic eruption occurs during bad weather, we must take information about the cloud's height by means of the pyroclastic deposits. In general, the total time taken for pyroclastic materials to be ejected and deposited at a given distance from the source vent can be divided into three parts as follows : the time for the eruption cloud to ascend and reach its neutral buoyancy level (T_1); the time for the pyroclastic materials to be transported laterally by the eruption cloud (T_2); and the time for pyroclastic materials to fall and be deposited on the ground (T_3). Since T_3 can be calculated from the settling velocity of pyroclastic materials, if the time that the pyroclastic materials fell at a given locality was observed and a given value for T_1 is assumed, the most suitable wind velocity to explain T_2 can be determined. Thus the height at which pyroclastic materials separate from the eruption cloud can be determined by using the vertical profile of wind velocity around the volcano. These ideas were applied to the eruption occurred at 19:44 (JST) on September 23, 2004, at the Asama volcano, which produced a pyroclastic fall deposit with a minimum weight of 7.2×10^6kg. Because this eruption occurred in bad weather, the pyroclastic materials fell as mud raindrops that were aggregate particles saturated by the rainwater. Based on the depositional mass, the number of impact marks of the mud raindrops in the unit area, and the apparent density and the equivalent diameter of these drops during their fall was estimated to be 2.2-3.1mm, which is consistent with the grain-size distribution of pyroclastic materials. According to some experienced accounts, mud raindrops several millimeters in diameter fell at 20:03 in the Kitakaruizawa area (about 9km north-northeast from the source). Assuming 2-5 minutes for T_1 and 11.5-12.0m/s of average lateral wind velocity, the height at which the mud raindrops separated from the eruption cloud can be estimated at 3,430-3,860m (3,610m on average). From this conclusion, the transportation and depositional process of the pyroclastic materials generated on September 23, 2004, at the Asama volcano can summarized as follows : the explosion occurred at 19:44 and the eruption cloud rose to 3,610m while blowing 2.49km downwind from the source. The cloud moved laterally for 4.51km with generating raindrops. At 19:54, mud raindrops separated from the cloud 7.0km north-northeast from the source, then fell to the ground at 20:03 after being blown 2.0km downwind by a lateral wind.
著者
齋藤 和男 亀井 智紀
出版者
特定非営利活動法人日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.40, no.2, pp.99-102, 1995-04-20
被引用文献数
2
著者
諏訪 彰
出版者
特定非営利活動法人日本火山学会
雑誌
火山. 第2集 (ISSN:04534360)
巻号頁・発行日
vol.16, no.2, pp.103-106, 1971-12-01

A mixed eruption took place at Medake, biggest central cone of the composite volcano Akita-Komagatake, in 1970-71. This is the first magmatic eruption of the volcano in the historical age. All the former eruptions on record were phreatic explosions. The new lava is andesite, though older lavas of the central cones are basalts.
著者
和知 剛 土井 宣夫 越谷 信
出版者
特定非営利活動法人日本火山学会
雑誌
火山 (ISSN:04534360)
巻号頁・発行日
vol.42, no.1, pp.17-34, 1997-03-07
被引用文献数
5

The Akita-Komagatake, located in the central part of the Ou Back-Bone Range, northeast Japan, is a Quaternary volcano with two calderas. The tephra, younger than 13,500 y.B.P., erupted from this volcano, are stratigraphically divided into thirteen formations, namely AK-13 to AK-1 in ascending order, on the basis of buried soil and loess intercalated. The tephra consists of three types of lithologies: such coarse-grained essential materials as pumice and scoria (type 1), well-sorted black to brown sandy ash (type 2), and fine-grained ash frequently including accretionary lapilli (type 3). These lithologic types are resulted from different styles of volcanic eruption: Plinian to subplinian style eruption (type 1), vulcanian to phreatoplinian style eruption (type 2) and phreatoplinian style eruption (type 3). ^<14>C measurements on humic soil or woods just below or above the tephra reveal the tendency that the explosive eruptions at the Akita-Komagatake volcano occured during three stages; 13,500 to 11,600 y.B,P., 10,000 to 7,100 y.B.P. and 4,000 to 1,000 y.B.P. In the first stage, AK-13, main part of which is called the Koiwai Pumice, and AK-12, the Yanagisawa Pumice, were erupted. During the eruption of the Koiwai Pumice, the Obonai Pyroclasitic Flow Deposits was produced. The volumes of erupted materials for the tephra including the pyroclastic flow deposit are more than 0-6 km^3, and two calderas are thought to have been formed in this stage. In the second stage, AK-11 to AK-6 were erupted. AK-9, the Arasawa Pumice (newly named in this paper), and AK-8, the Horikiri Pumice, are dominated by pumice and scoria (lithologic type 1), and their volumes of erupted materials are more than 0.1 km^3. The volumes for other tephra in this stage is more than 0.046km^3. Through the second stage to the last stage, the volumes of erupted materials tend to become smaller than 0.046km^3.
著者
岡崎 紀俊 田中 和夫 三品 正明
出版者
特定非営利活動法人日本火山学会
雑誌
火山. 第2集 (ISSN:04534360)
巻号頁・発行日
vol.35, no.4, pp.375-388, 1990-12-28
被引用文献数
1

Surveys of magnetic total intensity and gravity were conducted to reveal the underground structure of Medake, one of the central cones in the cadera of Akita-Komaga-take Volcano, from which about 1.42 million m^3 of lavas flowed in the period of 1970-1971 eruption. Magnetic and gravity anomalies obtained show some characteristic features suggesting the subsurface structure. Long wave length magnetic anomalies show that Me-dake is composed of uniformly magnetized volcanic rocks, lavas and scoriae, On the other hand, short ones found at the summit of Me-dake indicate there is a body with reversal or weaker magnetization than the surroundings. The distribution of the Bouguer anomalies is characterized by a narrow area of higher values of anomalies at the summit and a trend of higher values in northwest of Me-dake and lower ones in southeast. The latter suggests the basement structure of the caldera. The former implies an exislence of more dense intrusive rocks under the ground surface. Both centers of short wave length magnetic anomaly and of the narrow area of higher value of gravity locate at the same place about 100 m northeast of the 1970 crater on the summit. Judging from the topographic features, this place is one of the old craters. To interpret magnetic and gravity anomalies, numerical analyses for demagnetization model and for more dense intrusion model with various shapes were carried out independently. Finally, magnetic anomalies can be expained by a demagnetized vertical pentagonal prism, of which size is 80 m in N-S, 150 m in E-W and 300 m in thickness and of which top is at the depth of 5-10 m. Gravity anomalies can be interpreted by a more dense vertical rectanglar prism, of which size is 30 m×110 m×300 m and the top is at 15 m deep. The zones of high ground temperature and fumaroles are found at the surroundings of the 1970 crater and the area of the higher gravity anomalies. The fumarolic area preceding the 1970 eruption was found at north of the crater. We can infer the processes of the 1970 eruption by using the interpreting subsurface model of Me-dake and the distribution of fumaroles. At the first stage of eruptive activity in 1970, magma intruded into northeast of the 1970 crater. At the second, most of magma passed through the vent conncted with the crater and flowed out. Small quantity of magma remained in the narrow part at northeast of the crater make the present magnetic and gravity anomalies and fumaroles.
著者
佐々木 龍男 勝井 義雄 熊野 純男
出版者
特定非営利活動法人日本火山学会
雑誌
火山. 第2集 (ISSN:04534360)
巻号頁・発行日
vol.26, no.2, pp.113-116, 1981-07-01
被引用文献数
1