8 0 0 0 OA 特集「次世代研究者のための火山学講座」
- 著者
- 鎌田 浩毅 須田 恵理子 齋藤 武士 飯澤 功 酒井 敏
- 出版者
- 社団法人日本材料学会
- 雑誌
- 材料 (ISSN:05145163)
- 巻号頁・発行日
- vol.51, no.2, pp.168-175, 2002-02-15
- 参考文献数
- 28
Volcanic debris avalanche is a large-scale landslide which occurs at unstable volcanic edifice and often creates horseshoe-shaped crater at its source with hummocky deposits of volcanic debris including large blocks inside. Dynamic pressure model has recently been proposed to explain flow behavior of volcanic debris avalanches: Kinetic friction of debris avalanche decreases systematically at threshold of a certain critical speed for flotation. We have done a sliding experiment using flour powder as debris avalanche matrix and metal disks that slide on a powder layer as incorporated large blocks. Sliding traces left on a powder layer showed that a disk was floating over a certain critical speed. Kinetic friction coefficients, calculated by the position of a sliding disk recorded on a digital video, become small at a higher speed which exceeds critical floating speed, and provide smaller coefficient than those of normal surface of a rigid body. The critical floating speed deduced from dynamic pressure model is consistent with our experiment. In order to verify this model as the major driving force to float megablocks incorporated commonly in a volcanic debris avalanche, critical floating speeds for two geological cases at the collapses of 1980 Mount St. Helens and 1984 Ontake, central Japan, are examined and compared with the observed flow velocities. Estimated velocities deduced from the dynamic pressure model correspond fairly well to the actual velocities. This strongly suggests that the model gives the most convincing explanation for transportation and deposition mechanism of volcanic debris avalanche.
1 0 0 0 OA 日光男体火山における約1万年前の火砕流堆積物の発見
- 著者
- 三宅 康幸 齋藤 美由紀 竹下 欣宏 及川 輝樹 齋藤 武士
- 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.54, no.4, pp.163-173, 2009-08-31 (Released:2017-03-20)
- 参考文献数
- 29
- 被引用文献数
- 1
Nantai Volcano is a symmetrical stratovolcano, situated in the southern part of the Northeast Japan arc. Many geologic studies hitherto have suggested that the stratovolcano was formed during the Main stage, and the overlying pyroclastic materials and a lava flow were formed in the Later stage. Because no sedimentary gap is found between any deposits of the Later stage, it is inferred that all of the activity in the Later stage took place successively around 12ky BP (15-14 cal ka BP) and went dormant until now. However, we found a pyroclastic flow deposit named Bentengawara Pyroclastic Flow Deposit (BPFD) at the northeastern flank of the Nantai volcano about 2km from the summit crater. This deposit overlies an 80cm thick deposit of weathered ashy sediments that in turn overlies the Arasawa Pumice Flow Deposit, a member of the Later stage. The lower half of the BPFD consists of volcanic lapilli and ash that is remarkably fine-depleted while the upper half contains abundant scoria of mainly lapilli-block sized clasts. The deposit also includes a small number of breadcrust blocks and occasional accessory lava blocks and fragments of charred wood. The breadcrust blocks consist of a dense outer crust that is significantly fractured and a vesiculated interior. It is noteworthy that the edges of the cracks are sharp and never rounded, suggesting that the vaporization of the inner magma that produced these cracks took place just before or immediately following the settlement of the blocks. Paleomagnetic data from three breadcrust block samples indicate that the magnetic vectors of high temperature components are aligned with our present-day poles. Two pieces of charred wood were measured for their 14C ages with results of 12-11 cal ka BP. The whole rock chemistry of scoria and breadcrust blocks are determined to be significantly different from any of the rocks of the Later stage, but the accessory block in the BPFD has the similar chemistry to the Osawa Lava, the last product of the Later stage. We therefore suggest that the BPFD was deposited after the Later stage with a short (~3ka) dormant period between them. Since the age is possibly around 10ka, the Nantai volcano should be counted as active volcano based on the definition provided by the Meteorological Agency of Japan.