著者
井村 隆介 小林 哲夫
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.135-148, 1991-07-15 (Released:2017-03-20)
被引用文献数
8

This paper presents results of geologic investigation of the eruptive activity in the last 300 years of Shinmoedake, an active volcano in the Kirishima Volcano Group. The recent activity of this volcano is divided into four eruptive episodes : the 1716-1717, 1771-1772, 1822 and 1959 episodes. The most important activity occurred in 1716-1717. During the 1716-1717 eruption, fallout deposits, pyroclastic flows and mudflows were widely dispersed around the volcano. The products of this episode show that the eruption progressed with time from phreatic to magmatic. These field data are in good agreement with historic records of eruptive activity. According to the historic records, the eruptive activity lasted from 11 March, 1716 to 19 September, 1717. The 1771-1772 and 1822 activities produced base surges, pyroclastic flows, fallout tephra and mudflows that were confined to the slope and eastern base of the volcano, but historic records do not reveal the details of these eruptions. The field evidence shows the same phreatic to magmatic sequence as the 1716-1717 activity. However, the eruptions of both episodes were on a smaller scale than the 1716-1717 eruption. The 1959 activity was well described. This episode produced minor gray silty to sandy lithic fallout tephra indicating that only phreatic activity occurred. The fallout was distributed northeast of the vent. In conclusion, the field evidence and historical records show that each eruptive episode of the current activity of Shinmoedake progressed from phreatic to magmatic. The eruptions are frequently accompanied by pyroclastic flows and mudflows.
著者
井村 隆介
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.43, no.5, pp.373-383, 1998-10-30 (Released:2017-03-20)
参考文献数
27
被引用文献数
5

The eruptive sequence of the An-ei eruption of Sakurajima volcano (1779-1782) is revealed by historical records. From the evening of November 7, 1779 (the 29th day of the 9th month in the 8th year of An-ei), Kagoshima and its environs were shaken frequently. At 11 a.m. of the next day, the water in the wells in the island boiled up, spouting at several points and the color of sea became purple. On the noon of the same day, minor white plumes rose up from the Minamidake summit crater. At about 2 p.m., plinian eruption oecurred at the southern upper slope of Minamidake, and several tens of minutes later, at the northeastern flank of Kitadake. The height of eruption column reached about 12000 meters. It is estimated that a pyroclastic flow was generated at 5 p.m. The plinian eruption climaxed from the evening of November 8, to the morning of next day, and later was followed by emission of lava flows. The activity of the southern craters ceased within a few days, but lava emission from northeastern craters lasted for a long period. On November 11, the lava flow from northeastern craters entered into the sea. Since then, submarine explosions occurred repeatedly off the northeastern coast, and it continued to January 18, 1782. Nine small islands produced by this submarine volcanic activity during a year. Submarine explosions caused small tsunamis on August 6 and 15, September 9, October 3 1, November 9, 1780 and April 11, 1781.
著者
中村 一明
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:24330590)
巻号頁・発行日
vol.25, no.4, pp.255-269, 1980-12-01 (Released:2018-01-15)

Rift zones are characteristic features of Hawaiian volcanoes. They are long narrow zones of flank fissure eruptions but are distinct from ordinary flank eruption sites on stratovolcanoes in that eruptions, and therefore dike intrusions, occur repeatedly at the same general place for a long time and thus cause a considerable amount of lateral spreading. This spreading should somehow be accomodated. Moreover, the stress field should remain the same after accomodation in order for a new dike to intrude in the same orientation. The current spreading episode in Iceland (BJORNSSON et al., 1979) between North American and European plates revealed that the sequence of events in the spreading process is similar to that observed for Hawaiian volcanic activities. This implies that the process of plate separation and accretion is nothing but the activity of rift zones. Constructional plate boundaries may be regarded as composed of a chain of rift zones and associated feeding polygenetic centers. Room necessary for repeated dike intrusion is supplied in the case of spreading centers, by the lateral motion (separation) of lithosphere over asthenosphere. In the case of Hawaii, sliding of the volcanic edifice over a deep sea sediment layer may be the analogous mechanism such as appears to have occurred during the 1975 Kalapana earthquake, as studied by ANDO (1979) and FURUMOTO and KOVACH (1979). Kalapana earthquake had been anticipated by SWANSON et al. (1976) as one of the repeated steps as the east rift zone has continuously dilated. Thus, the primary cause for the long, well developed rift zones of Hawaiian volcanoes may be in the existence of thick enough oceanic sediments serving as a potential sliding plane beneath the volcanic edifices. Lack of rift zones in Galapagos shields which grew over the young ocean floor with rough topography is consistent with this view.
著者
田島 靖久 松尾 雄一 庄司 達弥 小林 哲夫
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.59, no.2, pp.55-75, 2014-06-30 (Released:2017-03-20)

The Kirishima volcanoes located in southern Kyushu are comprised of more than 20 volcanic edifices. The volcanoes occupy an elliptical area of approximately 330km2 with the WNW-ESE direction. Among the different types of volcanic edifices, the typical ones are compound maars and lava flows in Ebinokogen. We studied the volcanic history of Ebinokogen by geological examination of tephra layers and lava flows. After the Karakunidake-Kobayashi plinian eruption, seven tephra were formed in this area. We determined the ages of those tephra and two lava flows. The magmatic eruptions, produced Tamakino B tephra, occurred after Karakunidake-Kobayashi tephra eruption. The first activity in Ebinokogen from about 9.0 cal ka BP generated Fudoike lava flow, and Fudoike-Tamakino A tephra erupted from Fudoike crater. Karakunidake north-Ebino D tephra was generated from the northwest flank of Karakunidake at 4.3 cal ka BP, with debris avalanche and lahars. Phreatic Fudoike-Ebino C tephra erupted from the Fudoike crater at 1.6 cal ka BP. Ioyama-Ebino B tephra eruption started from around the 16th to 17th century with lava flow. Phreatic Ioyama east-Ebino A tephra erupted from Ioyama east crater in 1768 AD. The Ebinokogen area is one of the active regions of Kirishima volcanoes explicated by geophysical observations. Our results indicate cyclical tephra depositions mainly produced by small magmatic and strong phreatic eruptions in this area after the Karakunidake-Kobayashi pyroclastic eruption. Furthermore, the vent locations were found to migrate with each eruption.
著者
井村 隆介
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.37, no.5, pp.281-283, 1992-11-15 (Released:2017-03-20)
参考文献数
5
被引用文献数
3
著者
宮城 磯治 東宮 昭彦
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.47, no.6, pp.757-761, 2003-01-08 (Released:2017-03-20)
参考文献数
16
被引用文献数
3

We developed a new thermometer that uses color of volcanic ashes. A series of heating experiments for basaltic ashes under the atmospheric condition provided a quantitative relationship among color, heating temperature, and heating duration. The higher the heating temperature, the more the redness in color of heated ash. We applied the relationship to estimate heated temperature of the ash that was underlying below or contact with a cauliflower-shaped volcanic bomb ejected from the Miyakejima volcano on 18 August, 2000. The estimated temperature was about 390℃ for the ash underlying 1 cm below the volcanic bomb, and 550℃ for the ash in contact with the bomb. Numerical heat transfer calculations for the volcanic bomb on the ash layer suggested that temperature of its center at the time of landing is about 1,000℃. This is the first concrete evidence that the bomb was essential material and that the 18 August eruption was phreatomagmatic.
著者
及川 輝樹 筒井 正明 大學 康宏 伊藤 順一
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.57, no.4, pp.199-218, 2012-12-28 (Released:2017-03-20)
参考文献数
65

Shinmoedake (Kyushu, Japan), which is one of the Kirishima Volcanoes, experienced several small eruptions in 2010, finally culminating in a sub-plinian eruption on January 26-27, 2011. After this sub-plinian phase, the eruption style shifts to the phase of vulcanian eruption or ash emission. This volcanic activity is still occurring. We here summarize the eruption history of Shinmoedake during the Edo period on the basis of historical records. The eruptions of Shinmoedake during the Edo period occurred in AD 1716-1717 (Kyoho eruption) and AD 1822 (the 4th year of Bunsei eruption). The Kyoho eruption, which was a large-scale (total amount of tephra: 2×1011 kg) eruption, is divided into the following seven stages. Stage 1 (Apr. 10, 1716 to May 7, 1716): small eruptions occurred over two months; Stage 2 (Sep. 26, 1716): falling ash first observed at the foot of Shinmoedake; Stage 3 (Nov. 9 to 10, 1716): the first large eruption was observed, with pumice falling over a wide area; Stage 4 (Dec. 4 to 6, 1716): small eruptions; Stage 5 (Feb. 9 to 20, 1717): the second pumice fall eruption, with an intermittent ash fall eruption thereafter; Stage 6 (Mar. 3, Mar. 8, Mar 13, Apr. 8, 1717): ash fall eruptions; Stage 7 (Sep. 9, 1717): the last ash fall eruption. These eruptions, which continued intermittently over 17 months, were characterized by multiple repetitions of a large eruption. Based on the results of a comparison between the Kyoho eruption and the 2011 eruption, the eruptions from March 30, 2010 to January 26, 2011, were similar to Stages 1 to 3 of the Kyoho eruption; the eruptions after January 26, 2011, were similar to Stages 5 to 6 of the Kyoho eruption. In addition, the relatively large eruption events of Stages 3 and 5 of the Kyoho eruption and the January 26-27, 2011, eruption began without any noticeable precursors. The eruption in the 4th year of Bunsei (AD 1822) was a small eruption that lasted less than a day. The recent eruption sequences, which were also similar to the Edo period eruptions, are divided into a small-scale eruption (the 1959 eruption) and a large-scale eruption (the 2011 eruption). The eruption duration time of the small-scale (total amount of tephra: < 1010 kg) eruption was less than a day. The eruption duration time of the large-scale (total amount of tephra: > 1010 kg) eruption could be a few months or years. Both eruption sequences began with a small eruption. A large-scale eruption can occur a few months after the start of the eruption sequence. This is an important turning point in the eruption sequence of Shinmoedake.
著者
高橋 正樹
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.1, pp.33-42, 1995-03-10 (Released:2017-03-20)
参考文献数
22
被引用文献数
5

There is no positive correlation between the long-term eruption rate of large-scale felsic volcanism and its discharge volume of a single eruptive episode. This means that the storage of voluminous felsic magma at high-level in the crust is caused not by high magma production rate but by continuous accumulation of magma during a long repose time, if the long-term eruption rate reflects the averaged magma production rate. If the cruslal defomation is weak, the magma chamber could be stable in the crust; it is favorable for efficient accumulation of voluminous magma. In fact, the large-volume felsic volcanism occurs exclusively in the region of low crustal strain rate. The low crustal strain rate is considered to be essential for the formation of large-scale felsic volcanism. The large-volume felsic volcanic activity is present in the compressional tectonic stress field as well as in the extensional one; the difference in arrangement of principal stress axes is not related to the occurrence of voluminous felsic volcanism.
著者
前田 美紀 宮地 直道
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.57, no.1, pp.19-35, 2012-03-30 (Released:2017-03-20)
参考文献数
42

Formation mechanism of basaltic pyroclastic flows has not been sufficiently clarified yet because basaltic pyroclastic flows do not occur as frequently as felsic ones. We studied the Osawa pyroclastic flow 3 deposit (OsPfl-3), which took place on the western flank of the Fuji volcano between 2.9 and 3.0 ka. OsPfl-3 has two flow units and one cooling unit, which have a combined volume of 6.2 × 106m3. The flow overlies another unit composed of two scoria fallout deposits (YokSfa-2a and 2b) which sandwich a pyroclastic flow deposit (OtPfl). OsPfl-3 mainly consists of welded blocks and dense blocks with composition and petrographical characteristics of basaltic andesite. Some of the dense blocks have cracks on their surfaces and look like “cauliflower-shaped bomb”. They have a flat surface on one side with concentration of vesicles near the surface. The matrix of OsPfl-3 has dense fragments that are thought to have originated from dense lava blocks and poorly vesiculated scoria. The emplacement temperature of the blocks is estimated to be higher than 580℃ from thermoremanent magnetization measurements. These observations indicate that the blocks in the OsPfl-3 originated from welded pyroclasts, lava flow or lava lake at the summit crater. The sequence of the eruptions that formed OsPfl-3 and underlying deposits are summarized as follows: Stage 1: Deposition of fallout tephras (YokSfa-2a and 2b) and an intercalated pyroclastic flow (OtPfl) which are composed of fairly vesiculated scoria; Stage 2: Formation of lava flow or lava lake at the summit crater, and deposition of pyroclastics on the lava; Stage 3: Occurrence of the pyroclastic flow (OsPfl-3) caused by collapse of lava and pyroclastics. OsPfl-3 is prominently distributed on the western flank. This observation implies that the westward flow from the source lava that filled the summit crater could cross the lower part of the crater rim.
著者
荒牧 重雄 藤井 敏嗣
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:24330590)
巻号頁・発行日
vol.33, no.SPCL, pp.S297-S306, 1988-06-30 (Released:2018-01-15)

Detailed characterization of the whole rock composition of the ejecta of the 1986-1987 eruption of Izu-Oshima volcano by the XRF technique (FUJII et al., 1988) clearly indicates that the ejecta from the central crater of Miharayama (Crater A) are different from those erupted from the fissures (Fissures B and C) on the caldera floor and on the outer slope of the main stratovolcano. This suggests that the conduits which led the A and B, C magmas to the surface were separated physically from each other down to a certain depth. The ejecta from A crater resemble closely to those erupted during the past 1300 years (Y magmas) while the ejecta from B and C fissures are unique in composition among the Izu-Oshima magmas. The A and Y magmas are Fe-enriched island arc type tholeiites that must have been derived from the primary tholeiite magmas through crystal fractionation of olivine, pyroxenes and plagioclase. The B and C magmas can be derived simply through crystal fractionation of A or Y magmas leading to Fe-enriched basaltic andesites, andesites and dacites. This strongly suggests that an isolated pocket of magma starting with a composition of Y underwent strong fractionation to produce volatile enriched ferroandesitic magma. This body of magma was probably activated by the sudden depressurization caused by shattering and fissuring of the crust to form a 1500 m high fire fountains and to produce a sub-Plinian scoria fall deposit. The vent of A crater must have been stable at least during the last 1300 years and directly, or through the subsidiary magma chamber(s), connected to the main chamber above the Moho, where an extensive crystal fractionation has been taking place to produce Y magma from the parent tholeiitic magma. Marked ground depression and extension and migration of seismicity observed during the eruption suggest a possibility that a substantial amount of additional magma was intruded to form a NW-SE trending dike during the peak phase of the eruption. This is in harmony with the Nakamura’s model of the volcano with the NW-SE trending dike swarm which is controlled by the regional compressional stress field. However, gravity and some other grophysical data suggest that the deformation could have been the result of underground cracking without magma injection. Our model is not conclusive on this matter and the expectation that this eruption will eventually lead to the large-scale activity that has been recurring in every 130±50 years is yet to be tested.
著者
早川 由紀夫
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.Special, pp.S1-S15, 1995-12-25 (Released:2017-03-20)

Age of a tephra can be determined by simple stratigraphy, if adequate number of time-markers are provided. Eleven master tephras are chosen as the time-markers for the last one million years. They are Kikai-Akahoya (7.330 ka), Aira-Tanzawa (26.00 ka), Daisen-Kurayoshi (50.00 ka), Aso-4 (87.00 ka), Ata-Torihama (250.0 ka), Kakuto (340.0 ka), Suiendani-TE5 (420.0 ka), Kobayashi-Sakura (540.0 ka), Kaisyo-Toriitoge (650.0 ka), Shishimuta-Azuki (870.0 ka), and Shishimuta-Pink (1000 ka). The present earth surface and Bruhnes/Matuyama boundary (780.0 ka) play a same role as master tephras. Ages of some master tephras are assigned rather arbitrarily, however, it is productive to affix them once to a specific value. A tephra sandwiched between two master tephras is afforded its age by interpolating the thicknesses of loess between them. This technique, loess-chronometry, has the advantage of ability to measure an interval of tens to thousands years in the geologic past, over radiometric dating. More than 900 tephras are presently recorded and linked each other in a computer database including name, source volcano, age, magnitude, stratigraphy, and remarks. An updated version is listed in WWW at "http://www.la.gunma-u.ac.jp/〜hayakawa/English.html".
著者
早川 由紀夫
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.3, pp.357-370, 1991-10-15 (Released:2017-03-20)

The recent progress of physical volcanology is reviewed focusing on studies on volcaniclastic flows and their deposits. Pyroclastic flows are high-particle-concentration, laminar currents with gas as a continuous phase. Pyroclastic surges are low-particle-concentration, turbulent currents with gas as a continuous phase. Lahars are flows of debris with liquid water as a continuous phase. Debris avalanches are catastrophic landslides in which a continuous phase is absent or plays no role for the motion. Owing to the upward gas flow, fluidization processes operate in pyroclastic flows. Flow behavior and resultant deposits are remarkably different depending the degree of fluidization, because it effectively reduces the yield strength of the flow. However, the distance traveled is slightly affected by the degree of fluidization. It is determined mainly by the mass incorporated at the source or by the velocity acquired when it wes initiated.
著者
石崎 泰男 森田 考美 鳥山 光
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.62, no.3, pp.95-116, 2017-09-30 (Released:2017-10-11)
参考文献数
32

The ca. 17 cal. ka BP eruption at Nantai Volcano, NE Japan, initially produced widespread Plinian fallout deposit (Nantai-Imaichi Tephra:Nt-I) and two overlying associated scoria flow deposits, i.e., dacitic pyroclast-rich, Shizu Scoria Flow Deposit (SZ) and andesitic pyroclast-rich, Takanosu Scoria Flow Deposit (TKS). A∼2.8m thick outcrop of the Nt-I at Nikko City, 7.5km ESE of the volcano, consists of a basal phreatic fall bed (∼2cm thick) and eleven overlying fall units (units 1-11 in ascending stratigraphic order) defined by componentry, size grading, and chemical composition of the pyroclasts. The total lack of clear boundary structures between each unit suggests that the Nt-I was generated by the pyroclasts falling from continuous eruptive column. Grain size analyses of the Nt-I shows that column height rapidly increased and reached its climax soon after the eruption began, and then oscillated slightly and declined until the end of the Plinian phase. The composition of the pyroclasts shows that the Nt-I resulted from the tapping of a stratified magma chamber, in which dacitic magma capped hybrid andesitic magma. Light-colored, microlite-free, dacitic pumice (DWP) predominates from unit 1 through unit 9. In contrast, moderately vesicular andesitic scoria (AGS) is a major constituent of units 10 and 11. Microlite-rich dacitic obsidian (DOB) is present from unit 1 through unit 3, but was not observed above unit 3. Microlite-rich dacitic scoria (DBS) is present from unit 1 through unit 8, and coexists with DOB in single pyroclast. A plausible explanation for the common eruption of a small amount of microlite-rich pyroclasts along with the predominant DWP is that the microlite-rich pyroclasts represent fragments of the degassed margins of the conduit through which the dacitic magma rose. As the eruption advanced, the passageway may have widened, and the microlite-rich magma along the conduit wall was eroded and ejected along with the DWP. The density of the DWP remained constant from unit 1 through unit 8, and then increased at unit 9. The incorporation of slightly denser, dacitic pyroclast into the column is likely to have destabilized the eruption column. The destabilization caused partial collapse of the column and generated the intra-Plinian Shizu Scoria Flow Deposit, whose particle density is similar to that of unit 9. In contrast, the ejection of dense AGS combined with the incorporation of dense lithics into the eruption column is likely to have destabilized the column, and triggered total column collapse that formed the post-Plinian Takanosu Scoria Flow Deposit.