Debris avalanche - Rock avalanche

Japanese: 岩屑なだれ - がんせつなだれ

...This causes great disasters at the foot of the mountain. A violent explosion causes the mountain to collapse, sending debris avalanches down the mountain, which can mix with water and become muddy mudflows downstream (Mt. St. Helens, Mt. Bandai, etc.). In some cases, no water is involved from the start, and these are large-scale and cause considerable damage.

From [Debris Flow]

…The water in a debris flow does not act as a medium for transporting debris like river water, but rather increases its density by incorporating sediment, making the rock masses easier to float, filling gaps in the sediments, and acting as a lubricant for the debris flowing down by gravity. Debris flow is almost synonymous with debris flow, but in a broader sense it can refer to the entire phenomenon of high-velocity water-containing debris flow, including debris avalanches in which debris slides down steep slopes and mudflows mainly composed of soil and sand. Landslides and debris avalanches can transition into debris flows midway, and debris flows can also change into mudflows. …

From Mass Movement

…
[Rapid Flow]
When a large amount of water is suddenly added to fine-grained sediments such as volcanic ash, sand, and clay, or to a layer of rock debris that contains a lot of these materials, due to rainfall, melting snow, groundwater seepage, or volcanic activity, a high-velocity flow of slope material occurs. Depending on the nature of the material and the amount of water content, they are classified as earth flows, mud flows, debris flows, and debris avalanches. All of them occur temporarily and unevenly. …

From [Landslide]

...They differ from slow-moving landslides in that they occur suddenly, the speed of material movement is extremely fast, and the collapsed parts collapse so much that they no longer retain their original shape. Slope collapse phenomena generally referred to as landslides include debris slides, in which surface rock debris slides down the slope; rock slides, in which rocks are peeled off from the bedrock and slide down the slope; debris avalanches, in which rocks and debris collapse all at once and slide down the slope at high speed; and even the collapse of a volcanic body due to volcanic activity. Large-scale landslides often turn into debris flows or mud flows midway and flow down the valley.

*Some of the terminology that refers to "debris avalanche" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…これは山麓に大きな災害をもたらす。激しい爆発で山体がくずれ，岩屑なだれ(デブリアバランシュdebrisavalanche)が流下し，その下流部では水と混じり泥分の多い泥流になることもある(セント･ヘレンズ火山，磐梯山など)。このように初めから水を伴わない場合もあり，それらは大規模で被害も著しく大きい。…

【土石流】より

…土石流中の水は川の水のように土石を流送する媒体として働いているわけではなく，土砂を含んで密度を増して岩塊を浮きやすくし，堆積物のすき間を埋め，重力によって流下する岩屑のいわば潤滑剤としての役割を果たしている。土石流は岩屑流とほぼ同義であるが，広義には岩屑が急斜面をなだれ落ちる岩屑なだれや，土砂を主体とした泥流を含めて，水を含んだ高速度の土石の流動現象全体をさすこともある。山崩れや岩屑なだれが途中から土石流に移行したり，土石流が泥流に変わることもある。…

【マス･ムーブメント】より

…
［急速な流動］
　火山灰や砂，粘土など細粒の堆積物やそれらを多く含んだ岩屑層に，降雨，融雪，地下水の湧出，火山活動などで急に多量の水が加わると，速度の大きな斜面物質の流動が起こる。物質の性質と含水量の多少によって土砂流earth flow，泥流mud flow，土石流，岩屑なだれdebris avalancheに分けられる。いずれもその発生は一時的かつ偏在的である。…

【山崩れ】より

…発生が突発的で物質の移動速度がきわめて速く，崩れ落ちた部分が原形をとどめないほど崩壊する，などの点で動きの緩慢な地すべりと異なる。　一般に山崩れと総称される斜面崩壊現象には，表層の岩屑が斜面をすべり落ちる岩屑すべり，基盤岩から岩盤が剝がれてすべり落ちる岩石すべり，岩盤や岩屑が一挙に崩れて高速で斜面をすべり下る岩屑なだれ，さらには火山活動による火山体の崩壊などを含んでいる。規模の大きな山崩れでは，途中から土石流や泥流に変わって谷を流下することが多い。…

※「岩屑なだれ」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典第２版について | 情報

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