The Volcanic Explosivity Index (VEI) ranks volcano eruptions by size and power. The scale goes from VEI-0 to VEI-8 and measures ash, lava, and rock ejected.VEI-1 is a gentle eruption that can happen frequently. Italy's Mt. Stromboli has been erupting almost continuously for 2,000 years.VEI-6s are colossal eruptions every 100 years.