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Volcanologists measure the size of a blast mainly according to the volume of ejected material and the height of the eruption. The Volcanic Explosivity Index (VEI) scale ranges from 0 (mild) to 8 (extremely explosive). At the low end (0-1) are mild-mannered and busy volcanoes such as Hawaii’s Kilauea. Mount St. Helens (1980) rated a 5 and Krakatoa (1883) rated a 6. Like the Richter scale for earthquakes, the VEI increases by a factor of 10, meaning the Krakatoa blast was about 10 times greater than the Mount St. Helens. No volcano in modern history has reached a VEI of 8. Tambora in Indonesia scored a 7 in 1815.