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.
著者
須藤 茂 阪口 圭一 松林 修 鎌田 浩毅 加藤 完 山本 隆志
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
巻号頁・発行日
vol.29, pp.S253-S265, 1984

Temperature measurements of the lava of 1983 in Miyake-jima in the Ako district were started fifty days after the eruption and have been continued since then. The following three kinds of temperature data have been obtained (Fig. 2). 1. Temperatures at 20 cm depth along a graveled temporary road on the clinkery surface of the lava using mercury and alcohol thermometers. 2. Temperatures at 0.5 to 2.5 m depth in iron pipes inserted into the clinker layer using thermocouples and mercury thermometers. The pipe holes were distributed along the temporary road and at scattered stations on the surface of the lava. 3. We drilled a borehole (DH-1) which penetrates through 5.5 m-thick lava into the previous ground. Temperature was measured at 10 points in the hole using thermocouples. For comparison, similar measurements in the Awabe district were made in pipes with depths up to 2.5 m (Fig. 3). These pipes were buried in the holes dug into the massive part of the lava for electric poles. The temperature data at 20 cm depth and in the pipe holes (Figs. 5-10) indicate that isothermal surfaces in the clinker layer are very complicated. This complexity is explained by rising plumes of hot vapor irregularly present in the lava field. The vapor is produced by degassing process in the massive part of the lava and comes up through newly formed cooling joints. Once a cooling joint is formed, the temperature of the massive part of the lava around the joint fell rapidly because a gas plume effectively transports the heat from the massive part to the surface. But the rate of temperature decrease varies greatly from one station to another. New plumes were formed sporadically and the temperatures of the new plumes were much higher than the decreased temperatures of the older plumes. Some older plumes died out because degassing process ended or the joints were self sealed by sublimates. It is necessary to arrange a number of observation stations and to add stations timely in order to reveal a cooling history of aa lava like the lava of 1983 in Miyake-jima. Around a plume, a convection cell was identified in the clinker layer (Figs. 16-18), which is similar to a hydrothermal convection system usually found in geothermal areas. The change of the temperature-depth profile of DH-1 with time (Figs. 11, 12) clearly shows that the lava heated the underlying previous ground. The peak shape of the profile has become broader and the depth of the maximum temperature has steadily fallen. The change of the temperature-depth profile also suggests that the upper clinker layer prevented rainfall from effective cooling of the massive part of the lava for the first 250 days. During that time, raindrops were evaporated in the clinker layer and did not reach the massive part below the clinker layer. Difference of cooling rate between Awabe lava and Ako lava may be due to the difference of the thickness of the clinker layers (Figs. 15, 19).
著者
鎌田 浩毅 千葉 達朗
出版者
一般社団法人 日本地質学会
雑誌
地質學雜誌 (ISSN:00167630)
巻号頁・発行日
vol.101, no.12, pp.XXXXIII-XXXXVI, 1995-12-01
被引用文献数
2 1
著者
安藤 雅孝 田部井 隆雄 渋谷 拓郎 大倉 敬宏 平原 和朗 鎌田 浩毅 石川 尚人
出版者
名古屋大学
雑誌
基盤研究(A)
巻号頁・発行日
1999

[GPS観測]1.フィリピン諸島の南方の海域(モルッカ海)におけるプレート沈み込み様式を推定するために、インドネシア・スラウェシ島北東端のManadoとミンダナオ島中央のDavaoなど10カ所でGPS観測を行ない、これらの地域の変動速度を(傾斜角30度,固着域下限の深さ60km),南部では東傾斜モデル(傾斜角50度,固着域下限の深さ40km)が得られた。2.マコロード回廊周辺およびフィリピン断層沿いの14カ所でGPS観測を今年度も継続して行った。ユーラシアプレートに相対的な速度場を求めたところ、すべての観測点で西ないし北北西向きに5-9cm/yearの値が得られた。しかし、マコロード回廊の北側と南側ではユーラシアプレートに対する速度が系統的に異なり、マコロード回廊内および回廊の南側が、北側の地域に対して年間2cmの大きさで東ないし北東方向に変位していることが明らかになった。また、マコロード回廊内で2〜4×10E-7の南北ないし北北西-南南東方向の伸長成分が検出された。[地球年代学]フィリピン海溝での沈み込みの開始時期に制約を与えることを目的として,ルソン島ビコール半島の13の火山から37試料を採取し,そのK-Ar年代と化学組成の測定を行った.その結果,ビコール半島のフィリピン火山弧の活動は約7Maにまでさかのぼることが分かった.本研究のデータとSajona et al.(1993,1994)のデータをあわせてみると,沈み込みが北から南へ伝播したというモデルと調和的である.また,パラワンブロックの衝突時期が8-9Maと推定されていることと今回のデータは矛盾しない.[火山地質]1991年ピナツボ山噴火時に形成された火砕流堆積物に対して残留磁化の段階熱消磁実験を行った。結果、ある地点の試料は320-440℃まで温度領域で方向が類似する安定な磁化成分が検出された。これは、火砕流中央部が定着時に最大その温度まで上昇したこと示唆する。また、別の地点の試料のほとんどはマグネタイトのキュリー温度(580℃)までの温度領域で認められる類似した方向をもつ安定な一つの磁化成分を示した。このことは、その温度以上に最下部が上昇していた可能性を示す。