Activity 5. The Volcanic Explosivity Index (Grades 9-12
The size of a volcanic eruption cannot be measured as readily as the magnitude of an earthquake. To measure how "big" an eruption is, geologists describe the volume of tephra, the eruptive column height, explosive energy, or the distance traveled by ballistic ejecta. However, these parameters can be difficult to determine if no one observes the eruption (McClelland and others, 1989). The volcanic explosivity index was devised by Newhall and Self (1982) to provide a relative measure of the explosiveness of eruptions. Volume of products, eruption cloud height, and qualitative observations are used to determine the explosivity value. The scale is open-ended and ranges from 0, for non-
explosive eruptions, to 7, for history's largest explosive eruption. Strombolian and Vulcanian eruptions are the most common. Plinian and ultraplinian eruptions generate the greatest cloud column heights. The volume of Hawaiian and Strombolian eruptions is 10^4 cubic m or less. Very large eruptions produce one million cubic meters more tephra than non-explosive or small eruptions. The volume of plinian and ultraplinian eruptions is near 10^10 cubic m. 10^10 - 10^4 cubic m = 10^6 cubic m =1,000,000 cubic m. The May 18, 1980, eruption of Mount St. Helens is the only event rated 5 on the volcanic explosivity index for 1975-1985.
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