How do scientists estimate the size of a volcanic eruption after it happens?
Quick Answer
They use the Volcanic Explosivity Index, or VEI, plus measurements of ash volume, eruption plume height, and how far the tephra spread. The scale is logarithmic, so each step represents a much larger eruption.
The Full Story
Scientists estimate eruption size using the Volcanic Explosivity Index (VEI), which was introduced in 1982 by Chris Newhall and Stephen Self. The index runs from 0 to 8 and combines factors such as erupted material volume, plume height, and eruption duration. A VEI 5 eruption is far larger than a VEI 4 because the scale is logarithmic, not linear. For example, the 1980 Mount St. Helens eruption was VEI 5, while the 1815 eruption of Tambora was VEI 7. Very large eruptions are reconstructed from ash layers, satellite images, ground surveys, and reports of the eruption cloud.
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