The June 15 eruption had a volcanic explosivity index ( VEI ) of 6 , 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 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
1980 eruption of Mount St. Helens
1980 eruption of Mount St. Helens
Mount Pinatubo