Plant Succession on the Mount St. Helens Debris-Avalanche Deposit

Debris avalanches occasionally occur with the partial collapse of a volcano, and their ecological impacts have been studied worldwide. Examples include Mt. Taranaki in New Zealand (Clarkson 1990), Ksudach in Russia (Grishin et al. 1996), the Ontake volcano in Japan (Nakashizuka et al. 1993), and Mount Katmai in the state of Alaska in the United States (Griggs 1918a, b, 1919). Analyses have shown that as many as 18 previously undetected debris avalanches have flowed from the Hawaiian island volcanoes (Moore and Clague 1992). Following the debris avalanche at Mount Katmai in Alaska, Griggs (1918c) found that the deposit depth influenced plant survival. As a volcano collapses, glaciers, rocks, soil, vegetation, and other material are moved with great force down the mountain. Debris avalanches are typically cool and can bury surfaces with as much as 200 m of material. They tend to follow the original topography, have abrupt edges, and produce steep, undulating topography that can persist for many millennia.