著者

及川 輝樹 八木原 寛 高 美喜男 平野 舟一郎 中野 俊 中尾 茂 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.58, no.4, pp.563-567, 2013-12-27 (Released:2017-03-20)

著者

和田 恵治 佐野 恭平

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.60, no.2, pp.151-158, 2015-06-30 (Released:2017-03-20)

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The Shirataki obsidian-rhyolite field (Shirataki Geopark, Hokkaido) contains many outcrops of densely compact obsidian layers of excellent quality. The Shirataki obsidian lavas (SiO2=76.7-77.4 wt.%) were erupted at ca. 2.2 Ma and formed a monogenetic volcano comprising 10 obsidian-rhyolite lava units. The thickness of the units ranges from 50 to > 150m, and each unit comprises a surface clastic zone, an upper dense obsidian zone, an upper banded obsidian zone, a central thick rhyolite zone, a lower banded obsidian zone, a lower dense obsidian zone, and a lower clastic zone. The dense obsidian is > 98% glass with microlites of mainly magnetite and plagioclase, and rare plagioclase phenocrysts. Obsidian and rhyolite within single lava flows have similar bulk-rock compositions and number density of microlites, although the rhyolite contains glass with perlitic cracks and a large amount of crystalline material (spherulites and lithophysae), while the dense obsidian contains 0.4-0.8 wt.% H2O. These geological and petrological features indicate that the formation of obsidian and rhyolite layers in the lava units was controlled mainly by the timing of the vesiculation and degassing of magmas, in addition to the cooling effect.

著者

早川 由紀夫 中島 秀子

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.4, pp.213-221, 1998-08-31 (Released:2017-03-20)

参考文献数

33

被引用文献数

2

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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.

著者

安部 祐希

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.64, no.2, pp.139-145, 2019-06-30 (Released:2019-07-06)

参考文献数

32

著者

安田 敦 馬場 章 藤井 敏嗣 外西 奈津美

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.64, no.2, pp.83-101, 2019-06-30 (Released:2019-07-06)

参考文献数

52

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Gabbroic inclusions newly found in the Yakeno lava flow, erupted about 1300 years ago on the northwestern flank of Fuji volcano, are divided into two groups; one consists of troctolite and the other consists of olivine gabbronorite and gabbronorite. Troctolite fragments are considered to originate from shallow dykes because of their mineral composition and high porosity. On the other hand, estimated equilibrium conditions of gabbronorite and olivine gabbronorite are 1020-1050°C and 250-380MPa, showing their deep origin; probably they had been caught in the Yakeno magma (i.e. the magma erupted as the Yakeno lava flow) at differentiated small magma bodies just above a substantial basaltic magma chamber of Fuji volcano. Pre-eruptive temperature and water content of the Yakeno magma are evaluated by olivine-liquid thermometer and plagioclase-liquid hygrometer to be 1090°C and 2.8wt%, respectively. So the viscosity of the Yakeno magma is calculated to be several tens Pascal second. Therefore the minimum ascent velocity required to lift the gabbroic fragments is ca.10m/h, implying that the Yakeno lava flow had ascent without significant pause en route to the surface.

著者

伴 雅雄 及川 輝樹 山崎 誠子 後藤 章夫 山本 希 三浦 哲

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.64, no.2, pp.131-138, 2019-06-30 (Released:2019-07-06)

参考文献数

27

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Based on the history of volcanic activity of Zao stage VI, we examined possible courses of future activity of Zao volcano. All activities will start with precursory phenomena. Next, phreatic eruptions from Okama crater or other place inner part of Umanose caldera will occur, and may cause ballistic materials release, ash fall, pyroclastic surge, and lahar. The possibility of small scales edifice collapse and lava flow swelled out is very low but should be included. When the activity progresses, magmatic eruptions will be taken place from Goshikidake, and cause same phenomena as in the phreatic eruptions but larger in scale. The possibility magmatic eruption takes place without preceding phreatic eruption can not be excluded. Rarely, the activity will go up further and resulted in sub-Plinian eruption. Aside from the above sequence, larger scale phreatic eruption from Goshikidake area should be listed, although the possibility of this is very low.

著者

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 (ISSN:04534360)

巻号頁・発行日

vol.48, no.1, pp.157-159, 2003

参考文献数

5

著者

岩森 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.61, no.1, pp.1-22, 2016-03-31 (Released:2017-03-20)

被引用文献数

1

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Earth’s mantle constitutes the largest sub-system of the whole Earth system, involving 70% of the total mass, ~80% of the heat capacity, and more than 50% of the internal heat generation by radioactive decay. Therefore, the mantle and the inherited dynamics may control the whole system to a great extent, e.g., in terms of convective motion (including plate motion as its surface expression) and heat transport from the core to the surface, regulating the core cooling and dynamo that eventually affects the surface environment and life. First the basic structures and dynamics of the mantle convection are described, which demonstrate that the surface cooling dominantly drives the convection, creating buoyancy of several to 10 times greater than that generated near the core-mantle boundary. This estimate for the much larger role of near-surface cooling is consistent with the seismic tomography. Then various types of observations on the structures and dynamics of mantle, particularly three boundary layers (i.e., the near-surface, mid-mantle around 660km discontinuity, and core-mantle boundary) have been reviewed and are compared with the simple estimation. Of these, the ’geochemical probe’ approach, which utilizes composition (in particular the isotopic composition) of young basalts that fingerprint geochemical nature of the mantle materials, has been reviewed in conjunction with convective regimes. The latest result of high spatial resolution has revealed that the mantle can be divided into the eastern and western hemispheres, in terms of an anciently (several hundred million years ago) subducted fluid-component. The spatial pattern is strikingly similar to the hemispherical seismic structure of the inner core. Based on these observations, a model for ‘top-down hemispherical dynamics’ is introduced, as a result of focused subduction towards the supercontinents that existed mostly in the eastern hemisphere from ~900 to 250 million years ago (i.e., Rodinia, Gondwana and Pangea). The cooled domain of mantle may absorb heat from the eastern hemisphere of the core, resulting in faster growth and velocity of the eastern half of the inner core. Such ‘top-down’ dynamics is consistent with the various types observations and arguments (made in the first half of this paper) on mantle convection.

著者

西村 太志 内田 東

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.5, pp.387-391, 2005

参考文献数

11

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We analyze five explosion earthquakes observed at Asama volcano in 2004. The main phase consisting of Rayleigh wave is well explained by a vertical downward single force with peak amplitude of 10¹⁰-10¹¹ N and pulse width of 5-6s. These source parameters are in the range expected from a scaling relation presented by Nishimura and Hamaguchi (1993), which suggests that the 2004 explosions of Asama volcano are typical Vulcanian eruptions. The internal pressure built up beneath the crater is estimated to be 0.2-1.5MPa, which tends to become large after the formation of lava dome in the crater.

著者

宮縁 育夫

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.55, no.5, pp.219-225, 2010-10-31 (Released:2017-03-20)

参考文献数

17

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Komezuka Volcano, located in the northwestern part of the post-caldera central cones of Aso Volcano, SW Japan, is a basaltic monogenetic volcano comprising a scoria cone (370-380m in basal diameter; 80m in height) and lava flows (10.5km2; 5×107m3). We obtained 14C ages of 3,070±40 years BP from a buried soil below silty ash underlying Komezuka lava, which corresponds to 3,370-3,210cal years BP, and 2,760±40 years BP (2,950-2,770cal years BP) from a soil above silty ash overlying Komezuka lava. The age of soil below the lava suggests that the eruption age of Komezuka Volcano is about 3,300cal years BP. The eruption age is consistent with the age of Ojodake Volcano (3,600cal years BP) whose lava underlies Komezuka lava. In the northwestern part of the post-caldera central cones, Late Holocene monogenetic volcanic activity commenced with sub-plinian eruptions and lava extrusion from Kishimadake Volcano at approximately 4,000cal years BP, followed by sub-plinian eruptions and lava extrusion from Ojodake Volcano at 3,600cal years BP, and ceased with strombolian eruptions and lava extrusion from Komezuka Volcano at 3,300cal years BP.

著者

松本 哲一 水垣 桂子 玉生 志郎 小野 晃司 北原 哲郎 品田 正一 笹田 政克

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.38, no.1, pp.1-13, 1993-04-01 (Released:2017-03-20)

参考文献数

21

被引用文献数

1

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Taga Volcano, in northern part of the Izu Peninsula, central Japan, is a large Quaternary strato volcano made of andesite and basalt. The Hata Basalts underlying the Taga volcanic rocks is a pile of pyroclastics with some lava flows mainly of basalts which include the most mafic basalt in the late Tertiary to Quaternary volcanic rocks of north Izu and Hakone region. This Basalts has been treated as the late Tertiary product by previous workers. In order to clarify the volcano-stratigraphic relationship between the Hata Basalts and the Taga volcanic rocks, geologic investigations were carried out in and around the Tanna Basin, and the core and cuttings samples from three 500-600 m drill holes in the Tanna Basin were examined. The Hata Basalts is lithologically characterized by dominant pyroclastics, especially high-temperature oxidized, reddish-brown agglutinate, whereas the products of Taga Volcano are characterized by dominant lava flows. The Hata Basalts is overlain by the Taga volcanic rocks (TV4 and TV5) unconformably at the eastward of the Tanna Basin, and overlain by the Shimo-Tanna Shales and the Taga volcanic rocks unconformably at the westward. Petrography and major elements chemistry suggest that both the Hata Basalts and the Taga volcanic rocks are petrologically very similar to each other. K-Ar dating of two rock samples of the Hata Basalts and one sample of Taga Volcano revealed that they were products of almost the same age, around 600-700 ka. All the samples dated can be correlated to the Brunhes normal polarity epoch. We conclude that the Hata Basalts is the early stage products of Taga Volcano.

著者

長谷川 健 中川 光弘

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.4, pp.269-274, 2014-12-31 (Released:2017-03-20)

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This paper introduces a practical use of outcrop data in determining the correlation, stratigraphy and distribution of large-scale pyroclastic flow deposits (PFL). The studied area is the Akan and Kutcharo volcanic zone in Eastern Hokkaido, Japan, which have had a long and complex history of more than 20 caldera-forming eruptions during the Quaternary. A database of the stratigraphy and glass chemistry for the more than 20 PFL can be established by studying a sufficient number of representative outcrops. We found representative outcrops where stratigraphic relationships between several PFL can be observed at the same time. We analyzed glass chemistry of juvenile pumices (>10 clasts) of the PFL. The database enables to identify all exposed PFL in this area, thus allowing us to draw detailed maps of the distribution for each PFL. The database can be also used for correlation and chrono-stratigraphic determination of reworked volcanic deposits, such as terrigenous marine deposits in Kushiro region, located on the plains at the foot of Akan and Kutcharo volcanoes.

著者

藤縄 明彦 藤田 浩司 高橋 美保子 梅田 浩司 林 信太郎

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.46, no.5, pp.269-284, 2001-11-20 (Released:2017-03-20)

参考文献数

24

被引用文献数

3

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Kurikoma volcano is located at the volcanic front of northeastern Japan arc. The volcano can be divided into 6 volcanic edifices on the bases of the inferred eruption centers, relative preservation of primary micro-topographic features on the eruptive materials, and stratigraphic relations. Lava flow has been dominant through the development history of each edifice, while pyroclastic deposits are conspicuous near the craters of several ones. Newly analyzed 7 K-Ar ages for the representative samples range from ca. 0.53 to around 0.11 Ma, practically reconcilable with the stratigraphy. Based on these data, an internally consistent scenario on the development history is summarized as follows: 1) Magmatic eruption started at about 0.5 Ma to make up the southern volcanic row. South and east to northeast flank of the Higashi-Kurikoma volcanic edifice was probably formed nearly the same time. Following these eruptions from the southern vents, central vents effused lava flows, resulting to build the Higashi-Kurikoma edifice and Kokuzou lavas (part of Kurikoma edifice) around 0.4 Ma. 2) After terminating eruption from the southern and east-north eastern vents, the Higashi-Kurikoma vent had been active until 0.1 Ma, and Kurikoma vent lasted several tens of thousands years ago. 3) Magusadake cone was built through repeated lava effusions from several vents in the western part of the volcano from 0.45 Ma to 0.1 Ma. 4) Viscous magma erupted to form Tsurugidake lava dome as the last event of magmatic eruption so far in the Sukawa horse-shoe shaped crater which was formed in northern portion of the Kurikoma (Okomayama) volcanic edifice.

著者

福島 大輔 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.45, no.4, pp.225-240, 2000

参考文献数

38

被引用文献数

2

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The Tarumizu pyroclastic flow deposit with intercalation of Osumi plinian fall deposit of 25 ka eruption is distributed in Tarumizu area, southeast of Aira caldera. The deposit is composed of many flow units and the source is estimated to be at the same position as the vent of plinian eruption based on grain-size variation and depositional structure. From these facts, the Tarumizu pyroclastic flow is considered to be a type of intra-plinian fiow generated by successive partial collapse of the sustained plinian eruption column. Based on the stratigraphic suecessions, the generation of this flow is estimated to be a continuous process from the early to the terminal stage of the plinian eruption. The pyroclastic flow distribution, which is in the same direction with the dispersal axis of plinian fall deposit, may suggests that the wind influenced the direction of partial column collapse. Two lithofacies were identified in the Tarumizu pyroclastic flow deposit: the massive facies and the stratified facies. The former is a 'normal' pyroclastic flow, while the latter is a pile-up of many thin layers of pyroclastic flow and pumice fall. The massive facies are mainly distributed on the lower plain near the coast and differ from the stratified facies that accumulated at a bit higher level, usually along the foot of the mountain. These facts suggest that the main pyroclastic flow accumulated on the lower level and that the corresponding marginal part of the flow eventually reached the higher level exhibiting characteristic thin stratified structure.

著者

西村 裕一 荒牧 重雄

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.44, no.1, pp.51-53, 1999-03-05

参考文献数

6

被引用文献数

1

著者

鈴木 毅彦

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.37, no.5, pp.251-263, 1992

参考文献数

22

被引用文献数

11

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Nasu Volcano is a polygenetic volcano locating on the volcanic front of Northeast Japan Arc. The eruptive history of this volcano during the last 350,000 years is clarified by tephrochronology. On the eastern foot of this volcano, 16 pyroclastic fall deposits are identified. The stratigraphic relations of these deposits with well dated tephra layers derived from other volcanoes provided the chronological framework for the eruptive history. Moreover, the estimated ages and volumes of each deposits, determined by an empirical formula, gave eruption rate of pyroclastic fall deposits. Nasu-Shirakawa Tephra Group (Sr1 to Sr12) erupted from 350 ka to 200 ka. The frequency of explosive eruption during this period is 0.08 times per 1000 years, and eruption rate of pyroclastic fall deposits in dense rock equivalent volume is 0.016 km³ per 1000 years. On the other hand, a K-Ar dating chronology for Nasu Volcano carried out by a previous study shows that no effusions of lava flows occurred during this period. After this period, no obvious explosive eruptions occurred until 55 ka. This non explosive period is most likely corresponds to the stage characlerized by effusions of lava flows which was recognized by the K-Ar dating chronology. The eruption rate of this period is larger than that of former period from 350 ka to 200 ka. The younger stage from 55-50 ka to present is not characterized only by explosive eruptions (occurrences of Kuromori Tephra and Oshima Tephra 1 and 4; Kr, Os 1 and 4) but also by effusions of lava flows, occurrences of pyroclastic flows, formations of lava dome and pyroclastic cone. The frequency of explosive eruption and the eruption rate of pyroclastic fall deposits during this period are 0.06 times per 1000 years and <0.003 km³ per 1000 years, respectively. Both are smaller than those of the period from 350 ka to 200 ka.

著者

西 潔 石原 和弘

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.48, no.5, pp.407-413, 2003-11-06

参考文献数

11

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A new scheme for hypocenter calculation is proposed for volcanic regions. The scheme finds the global minimum of the travel time residual, and the resulting solution is more stable than that from the conventional Geiger's method, especially when the number of observations is small or the station distribution is unsuited. In the first step (Step 1) of this scheme, the target area js parameterized by the node. The node distance depends on the heterogeneity of the velocity structure; usually about 0.3-1 km is sufficient. Travel times between nodes and stations are stored. In the first half of the second step (Step 2A), a node is sought that minimizes the sum of squares of the travel time residuals for the event. By changing the depth of this node, a set of initial hypocenter candidates is generated. In the following step (Step 2B), with these candidates for the initial hypocenter, precise locations of the event are obtained by nonlinear calculations using the simplex method. The hypocenter with minimum travel time residual is then selected as the most probable hypocenter. For the travel time calculation, the 3-D robust seismic ray tracer known as Fermat (Nishi, 2001) is used. If the velocity structure and the locations of the observation stations do not change, the results of Step 1 are effective for all events. Consequently, repetitions of only Step 2A and 2B are sufficient for every event. Successful outcomes of calculation using synthetic and actual data are obtained with practicable CPU times.

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

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The Kirishima volcanoes located in southern Kyushu are comprised of more than 20 volcanic edifices. The volcanoes occupy an elliptical area of approximately 330km^2 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 16^<th> to 17^<th> 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

安井 真也 高橋 正樹 石原 和弘 味喜 大介

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.52, no.3, pp.161-186, 2007-06-29 (Released:2017-03-20)

参考文献数

49

被引用文献数

4

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The 1914-1915 Sakurajima eruption was the largest eruption in Japan in the 20th century and erupted andesitic magma was about 1.5km3 DRE (Dense Rock Equivalent) in volume. Pumice fall and lava flows were generated from the fissure vents on the western and the eastern flanks of the volcano and pyroclastic cones were formed around the vents. Eruptive style changed with time. It is divided into three stages. After the initial, vigorous, Plinian eruption of about 36 hours (Stage 1), extrusion of lava associated with intermittent ash-emitting eruptions with or without detonations lasted for about 20 days on both sides (Stage 2), followed by an outflow of lava for more than 1.5 years on the eastern side (Stage 3). Consequently, the vast lava fields, which consist of a number of flow units formed on both sides of the volcano. Some units of lava show evidence of welded pyroclastic origin, suggesting clastogenic lava. In the western lava field, surface blocks characteristically consist of pyroclastic materials which show variable degrees of welding even within a single block. Typical eutaxitic textures and abundant broken crystals are also recognized under the microscope. Some flow units can be traced upstream to a pyroclastic cone. These features indicate that many flow units of lava on the western flank are clastogenic, which were generated by the initial, Plinian eruption of Stage 1. In the eastern lava field, evidence of pyroclastic origin is rarely discernable. However, the content of broken crystals varies widely from 20% to 80% in volume. Most lava flows, which were erupted in Stage 2 associated with frequent ash-emitting eruptions, contain broken crystals more or less than 50%. This fact indicates that magma in the conduit experienced repetitive fragmentation and coalescence due to intermittent explosions prior to outflow. Lava flows of Stage 3 contain much smaller amounts of broken crystals indicating gentle outflow of coherent lava. Relatively large-scale lava deltas developed toward the sea in the eastern lava field. Eyewitness account at that time reports that ocean entry of lava from several points started several months after the beginning of Stage 3. Although small-scale breakouts formed at the flow fronts of some lava on both sides, a large volume of the deltas can not be accounted for by secondary breakouts of ponded lava within the precedent flow lobes. It is considered that lava tube system fed lava to form the lava deltas.

著者

後藤 芳彦

出版者

The Volcanological Society of Japan

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.56, no.4-5, pp.137-145, 2011-09-30 (Released:2017-03-20)

参考文献数

22

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北海道東部知床半島の天頂山は,安山岩質溶岩からなる小型の火山で,山頂部には北東-南西方向に配列する爆裂火口列がある.本論では,天頂山の爆裂火口列を形成した降下テフラ(Ten-a)の分布と年代を明らかにした.Ten-aテフラは天頂山の山頂部から知床半島の東海岸に分布し,東北東方向に伸長する分布主軸を示す.テフラは,新鮮~変質した安山岩質の石質岩片と新鮮な軽石からなり,マグマ水蒸気噴火の噴出物であると考えられる.テフラ直下の土壌層から得られた放射性炭素年代値は,1930±40 years BP(1960-1810 cal BP)である.天頂山は約1900年前に噴火し,山頂部に爆裂火口列を形成したと考えられる.