Mass movement

Japanese: マス･ムーブメント（英語表記）mass movement

A general term for the phenomenon in which rocks, debris, soil, and other materials that make up a slope move downhill solely due to gravity, and not due to the action of running water, glaciers, wind, or waves. At or near the surface of the earth, rocks are separated from their original masses by weathering, breaking them into smaller pieces that are easier to move. In particular, when the gaps between the debris and soil, or the cracks in the rocks, are filled with water or ice, internal friction decreases and materials on the slope can move, even on gentle slopes. In this case, the water and ice only act as lubricants and do not flow and transport the materials.

Source: Heibonsha World Encyclopedia, 2nd Edition Information

Japanese:

斜面を構成している岩石，岩屑，土砂などが，流水や氷河，風，波などの働きによらず，もっぱら重力に引きずられて斜面下方に移動する現象の総称。地表や地表付近では，風化作用によって岩石がもとの塊から分離し細片化して動きやすくなっている。とくに岩屑や土砂の間隙や岩石の割れ目が水や氷に満たされると，内部摩擦が減少して緩斜面でも斜面物質の移動が起こる。この場合の水や氷は潤滑材の作用をしているだけで，それが流動して物質を運搬しているのではない。

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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