How do scientists measure a volcano's eruption size using the Volcanic Explosivity Index?

The Volcanic Explosivity Index was introduced in 1982 by volcanologists Christopher Newhall and Stephen Self. It is a logarithmic scale, so a VEI 5 eruption is much larger than a VEI 4 eruption, and a VEI 8 supereruption is in a different league altogether. The system considers how much ash and rock were erupted, how high the eruption column rose, and how long the eruption lasted. For example, Mount St. Helens in 1980 was VEI 5, while the 1815 eruption of Tambora was VEI 7. The scale helps compare eruptions of very different styles and sizes worldwide.