The 1991 eruption rated 6 on the Volcanic Explosivity Index and came some 450–500 years after the volcano's last known eruptive activity. The eruption ejected about of material, making it the largest eruption of the 20th century since that of Novarupta in 1912 and some ten times larger than the 1980 eruption of Mount St. Helens. Ejected material such as tephra fallout and pyroclastic flow deposits are much less dense than magma, and the volume of ejected material was equivalent to about of unerupted material.