We report the newly obtained results of eruptive history of Fuji Volcano, mainly using trench survey at the summit of a scoria cone. Compared with the stratigraphic study of summit eruptions of Fuji Volcano, that of flank eruptions has not obtained enough accurate eruptive history. The trench survey,however, is useful to study the accurate stratigraphy of flank eruption products, and samples for 14C carbon dating. In this volume, five papers are contained. Takada et al. is a report of the first trench survey of scoria cones on the southern flank of Fuji Volcano. Ishizuka et al. constructs the eruptive history of the north to western flank, and discuss the evolution of magma plumbing system of Fuji Volcano. Suzuki et al. is a report of the trench survey on the northwestern foot, including a topics of Omuro eruption. Nakano et al. reports the trench survey on the northeastern foot, and contains the evidences of distant eruption sites from the summit in Oshino village. Kobayashi et al. is a study on eruptive history of Fuji Volcano from AD 700 to AD 1,000 using stratigraphic correlation of KozushimaTenjosan tephra.
The eruptive history on the southern flank of Fuji Volcano, Japan, was studied, mainly using a trench survey at the scoria cone’s summit. The trench survey is useful to obtain the stratigraphy of flank eruption products and samples for 14C carbon dating. The fall horizons of Kawagodaira pumice (Kg) and Kikai-Akahoya tephra (K-Ah) were determined by the content of glass particle and refractor index of glass. Several tephras were newly defined as follows in descending order: A, B, C,D, E, F3, F2, F1, and G. A to D are between Yubune-2 scoria and Zunasawa scoria. E, F3, F2, F1 and G are below the Kawagodaira pumice. F3 and F2 have become good key tephras on the southern flank of Fuji Volcano to correlate scoria beds to each other. They are different from R-II and R-I scorias because their ages and distributions are different from those of R-II and R-I. G is a surge deposit. Finally, we classified the stage of flank eruptions using the results of this survey. Suyamatainai lava flow, Minami-Garanzuka, Obuchi scoria and Kotengu lava flow erupted after Yubune-2 scoria. MinamiGaranzuka is the eruption site of the Mizugatsuka-Hinokimarubi lava flow. The eruption site of Obuchi scoria was determined to be in the vicinity of Kita-Takahachiyama with two craters. Flank eruptions between Yubune-2 scoria and S-10 scoria are Asakizuka and Takayama. Koshirikizuka, just below Zunasawa scoria at the trench site, may be correlated to Nihon land lava flow just below S-10 because of their petrographic resemblance. The flank eruption between S-10 and F3 is Shirotsuka. Flank eruptions between F2 and K-Ah tephra are Higashi-Usuzuka and Hiratsuka. Flank eruptions are slightly older than the K-Ah eruptions, Hinokizuka and Nishi-Usuzuka. Flank eruptions older than the K-Ah eruption are Kita-Takahachiyama, Takahachiyama, Hokuto-Takahachiyama, Kurotsuka, and Azamitsuka.
Eighteen unknown-aged scoria cones on the northern to western slope of Fuji Volcano were trenched to determine their stratigraphic positions. We recognized three key tephra layers derived from large-scale eruptions of Fuji Volcano from 1,400 B.C. to 300 B.C. and detected two widespread tephras of the Kikai-Akahoya at 5,300 B.C. and Amagi-Kawagodaira at 1,400 B.C. within volcanic ash soil layers. Based on the detected levels of the widespread tephras and thickness of volcanic ash soil layers, the eruption ages of the scoria cones could be estimated as follows. 1) Three (Nagayama,Futatsuyama, and Sawarayama) cones formed from 8,300 B.C. to 6,000 B.C. 2) Five (Yumiizuka,Nishiken, Hokusei-okuniwa, Maruyama, and Toyazuka) cones were constructed from 5,300 B.C. to 2,600 B.C. 3) Five (Hachiken’yama, Nishi-kosukemaru, Kosukemaru, Tomine, Usuyama, and Hokuseiusuyama) cones formed from 2,300 B.C. to 1,800 B.C. 4) Four (Hokusei-yumiizuka, Higashiken,Shikanokashira, and Katabutayama) cones were erupted from 1,600 B.C. to 1,300 B.C. The petrographic features and chemical compositions of the products from the scoria cones conform with those from contemporaneous lava flows. During the last 10,000 years, the K2O content and FeO*/MgO ratio of the products from the northern to western flank eruptions decreased during 2,300 B.C. to 1,800 B.C.
Stratigraphic relations for seven scoria cones on the northwestern foot of Fuji Volcano were re-examined based trenching surveys and outcrop observations. The Yumiizuka scoria cone was formed after the Kikai-Akahoya tephra at 5,300 BC. The Tsugaoyama scoria cone’s eruption age could not be determined due to the accumulated thickness of the Omuro scoria at 1,300 BC. The Shikanokashira and Hokusei-yumiizuka scoria cones were erupted just before the Omuro scoria. Based on the thickness, degree of vesicle, and color of the essential deposits at the Katabutayama scoria cone, Omuro scoria could be derived from not only the Omuroyama but also Katabutayama scoria cones. The detected horizon of the Kozushima-Tenjosan tephra at A.D. 838 and 14C ages suggest that the Tenjin’yama-Igatonoyama and Koriike-Hakudairyuo scoria cones were constructed during A.D. 838-864, and A.D. 410-770, respectively.
Columnar sections of several trenching sites and outcrops on the northeastern foot of Fuji Volcano were prepared with the results of the identification of widespread tephras and radiometric carbon dating. The results are as follows: Older-stage pyroclastic cones (Shin-Fuji) at the Oshino village (Kousu, Usukubo-bashi and Ousu) erupted during 12,000 and 9,000 years ago in ascending order. They are the farthest lateral eruption sites on the northeastern side of the summit crater. Several younger lava flows (Shin-Fuji) were reliably dated. The scoria fall just below the Nakanochaya lava is dated as 1,430±40 yBP. The Gannoana-marubi lava is dated as 1,520±40 yBP and 1,590±60 yBP, and the scoria fall just below the lava is dated as 1,590±40 yBP. The results showed that the above products were effused in a series of eruptions. The Tsuchi-marubi lava is dated as 2,220±40 yBP. The scoria fall just below the Hinoki-marubi-2 and Taka-marubi lava is dated as 1,260±40 yBP. These ages of scoria fall and the proceeding lava effusion support the correlation to the eruption event that occurred in A.D. 800-802 and is recorded in old documents.
We analyzed refractive index, microstructure, vesicularity of glass from an alien tephra in the upper weathered volcanic ash soil horizon of Fuji Volcano, Izu peninsula, Miyakejima, and Niijima Volcanoes. The glass size increases toward Izu Islands. Based on the vesiculality of glass, the Kozushima-Tenjosan tephra (AD 838) can be distinguished from the Niijima-Mukaiyama tephra (AD 886). Thus, we determined from where the tephra of low refractive index on the flank of Fuji Volcano comes from. Next, using the stratigraphic correlation of the tephra, the detailed stratigraphy of Fuji Volcano during the period of AD 700-1,000 was clarified. It was difficult to determine exact age of each eruption by 14C dating with 2 sigma error bar (around ±40 years). However, the KozushimaTenjosan tephra horizon can classify Fuji Volcano products those below the tephra and those above the tephra. The Obuchi scoria (OBC), the Kansuyama scoria (KNS), the Nishi-Futatsuzuka scoria on S and SE flanks, the Takamarubi lava flow (Tam) and the Hinokimarubi 2 lava flow (Hnm 2), the scoria associated with the Oniwa-Okuniwa 2 lava flow (Onw 2) on NE and N flanks are below the Kozushima-Tenjosan tephra. The Fudosawa lava flow (Fud), the Obuchimarubi lava flow (Obu), the Higashi-Usuzuka south lava flow (Hum), the Mizugatsuka-hinokimarubi lava flow (Him) on S and SE flanks, the Kenmarubi 2 lava flow (Ken 2), the Tenjin-Igatono scoria (TNG), the Aokigahara lava flow and the Nagaoyama scoria on the NE and N flanks are above the Kozushima-Tenjosan tephra. The Kozushima-Tenjosan tephra was discovered just below the S-24-6 scoria on E flank. The stratigraphy of E flank is rearranged as follows in ascending order: the S-24-5 scoria, the S-24-5-1 scoria (newly defined), the S-24-5-2 scoria (newly defined), the S-24-5-3 scoria (newly defined), the Kozushima-Tenjosan tephra, the S-24-6 scoria, the S-24-7 scoria, the S-24-9 scoria, the S-24-10 scoria.