9.1

Volcanic collapse is a common evolution of volcanoes

Once considered rare, volcanic collapse is now recognized to be a common process in the evolution of volcanoes. After the 1980 eruption and collapse of Mount St. Helens in Washington, there was a surge of interest in volcanic edifice failure as it was the first major collapse observed and scientifically documented while it was happening. Now, more than 400 volcanoes world-wide have experienced a collapse in the past. In a paper by Siebert et al. (2006, GSA Spec Paper 412: 1-26) they find 40 debris avalanches associated with Quaternary arcs in Central America alone. The table below summarizes large-volume edifice collapses since 1500 years A.D.:

Large-volume edifice collapses since 1500 years A.D. (Carrasco-Núñez, G., Siebert, L., and Capra, L., 2011)

In the chart above, DB= directed blast; Phreatic= eruption involving water, VEI= Volcanic Explosivity Index.

Volcanic collapses are not as common as other volcanic hazards such as lava flows, pyroclastic flows, and tephra fall, as they usually recur once or twice during the volcano’s lifespan. During the last 500 years, sector collapse has occurred at a rate about 4-5 per century globally. Although many prehistorical events remain undocumented, they have been recorded at less than 1% of all known Holocene eruptions. (Carrasco-Núñez, G., Siebert, L., and Capra, L., 2011)

During the 20th century there have been several well-known examples of volcanic landslides such as Bezymianny and Shiveluch in Russia, Soufrière Hills in Montserrat, and Mount St. Helens in the USA. (Carrasco-Núñez, G., Siebert, L., and Capra, L., 2011).

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