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

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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.

著者

石崎 泰男 森田 考美 岡村 裕子 小池 一馬 宮本 亜里沙 及川 輝樹

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.59, no.3, pp.185-206, 2014-09-30 (Released:2017-03-20)

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Nantai volcano (2,486m a.s.l.), a near-conical stratovolcano with a summit crater ~1km wide, is located along the volcanic front of NE Japan. To date, the eruptive history and characteristics of this volcano have been poorly studied, except for an explosive eruption that occurred at ~17 cal. ka BP (Stage 2 eruption). In this paper, we present the results of investigation of the stratigraphy of recent proximal eruption products, the tephrostratigraphy of the northeastern foot of the volcano, and new radiocarbon ages. The results show that at least six eruptions of Nantai volcano have occurred after Stage 2 eruption, and we refer to these as Stage 3 eruptions. We identify four tephra layers and one pyroclastic flow deposit in the soil sections above the Stage 2 pumice flow deposit, at the northeastern foot of the volcano. These are classified in the ascending order as: (1) Nantai-Bentengawara Tephra 4 (Nt-Bt4), (2) Bentengawara Pyroclastic Flow Deposit, (3) Nt-Bt3, (4) Nt-Bt2, and (5) Nt-Bt1. The Nt-Bt2 is phreatic fallout with no juvenile material; the other tephra layers are phreatomagmatic fallouts containing juvenile pyroclasts together with ash aggregates. Six Stage 3 eruption products are identified within and around the summit crater: (1) a blocky lava flow (Osawa Lava) exposed on the northern crater wall, (2) a partly dissected scoria cone and (3) a poorly-preserved tuff ring (and its resedimented deposits) on the steep crater floor, (4) a subaqueous lava and associated lava fragments sandwiched by lacustrine deposits, (5) a tuff breccia containing hydrothermally-altered lava block and clayey matrix, and (6) a stratigraphically uppermost phreatomagmatic tephra (Nantai-Yudonoyama Tephra). All the proximal eruption products, except for the subaqueous lava, can be correlated with the tephra layers and the pyroclastic flow deposit on the northeastern foot based on stratigraphic positions, lithologies, and geochemical affinities. Our study reveals that five tephra-forming eruptions (ca. 14, 12, 8, 7.5, and 7 cal. ka BP) and one non-explosive subaqueous eruption (between 12 and 8 cal. ka BP) occurred during Stage 3, from a discrete eruption center inside the summit crater. Moreover, the tephra-forming eruptions were diverse in style, with strombolian (12 cal. ka BP), phreatomagmatic (14, 8, and 7 cal. ka BP), and phreatic (7.5 cal. ka BP) eruptions. Eruption style was determined primarily by vent position and spatiotemporal variations in local hydrological factors (e.g., the presence or absence of a crater lake, wet lacustrine deposits, and streams).