Volcanic mud flow

A phenomenon in which volcanic ejecta that has accumulated on the slopes of a volcano absorbs a large amount of water due to heavy rain and flows down the mountainside. This refers to a debris flow that occurs in volcanic regions. It can sometimes reach speeds of several tens of meters per second and flow downstream in one go, often resulting in disasters that cause widespread destruction in the drainage basin. As the fine volcanic ash layers are impermeable to water, surface water collects in the valleys, absorbing the soil in the flow path and becoming a mudflow. For this reason, it often occurs during eruptions that release a lot of volcanic ash, such as steam explosions. Hot mudflows can also often occur during eruptions.

In the 1926 (Taisho 15) eruption of Mt. Tokachi during the snow season, part of the crater cone collapsed, causing a mudflow caused by melting snow. Approximately 25 minutes after the explosion, the mudflow struck residential areas in Kamifurano town, more than 20 kilometers away, causing devastation including 144 deaths. In the 1985 volcanic eruption of Nevado del Ruiz volcano in Colombia, the glacier at the top of the mountain melted, causing a mudflow in the town of Armero, 40 kilometers from the peak, at the foot of the mountain, killing as many as 23,000 people. Volcanic mudflows can also be caused or promoted by heavy rainfall that occurs when water vapor that rises high into the sky during a volcanic eruption cools and condenses. Furthermore, pyroclastic flows and debris avalanches can flow into rivers and turn into mudflows, such as the Kambara hot cloud that occurred during the major eruption of Mt. Asama in 1783 (Tenmei 3). A phreatomagmatic explosion that occurred on Miyakejima in 2000 (Heisei 12) caused a thick accumulation of fine basaltic volcanic ash on the mountainside, and mudflows occurred repeatedly during heavy rains over the following several years.

[Akira Suwa and Setsya Nakata]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

火山の斜面に堆積(たいせき)した火山噴出物が、大雨などによって多量の水分を含み、山腹を流れ下る現象。火山地域におこる土石流のこと。ときには秒速数十メートルにも達する高速で、下流まで一気に流れ下るので流域地域が広く破壊される災害となることが多い。細かい火山灰の地層が水を通さないので、地表水が谷の部分に集まって流路の土砂を取り込み泥流となる。このため、水蒸気爆発など火山灰を多く放出する噴火中に発生することが多い。しばしば熱い熱泥流が噴火中に発生することもある。

　1926年（大正15）の積雪期に発生した十勝岳(とかちだけ)の噴火では、火口丘の一部が崩れて、融雪による泥流が発生した。爆発の約25分後には泥流が20キロメートル以上も離れた上富良野(かみふらの)町の居住地を急襲し、死者144人などの惨害を出した。1985年にコロンビアのルイス火山（ネバド・デル・ルイス火山）で発生した火山噴火では、山頂部の氷河が溶け、山頂から40キロメートルも離れた麓(ふもと)の町アルメロに泥流が流れ込み、2万3000人もの死者が出た。また、火山泥流は、火山爆発で高空に上昇した水蒸気が冷却・凝結して生じた豪雨で、発生ないし助長されることもある。さらに、1783年（天明3）の浅間山(あさまやま)大噴火の鎌原(かんばら)熱雲のように、火砕流や岩屑(がんせつ)なだれが河川に流入し、泥流に変わるものもある。三宅島(みやけじま)で2000年（平成12）に発生したマグマ水蒸気爆発では、山腹に玄武岩質の細かい火山灰が厚く堆積し、その後数年にわたって大雨の際に泥流が繰り返して発生した。

［諏訪　彰・中田節也］

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