Volcanic Explosivity Index

A logarithmic scale from 0 to 8 for classifying the size of explosive volcanic eruptions, devised by Christopher Newhall of the United States Geological Survey and Stephen Self in 1982. Each whole step represents roughly a tenfold increase in ejected volume; above a low threshold the formula is VEI equals the floor of the base-ten logarithm of the eruption's tephra volume in cubic kilometers, plus five, with plume height and qualitative descriptors filling in where volume data is thin. Because it is read off a deposit's physical traces rather than measured as the eruption happens, VEI is straightforward to assign for a well-studied modern eruption and much harder to pin down for a prehistoric or poorly documented one, so an ancient eruption's VEI is a reconstruction from tephra volume, ice-core sulfate signals or written accounts, carrying a real error range rather than a single certain number.