The 1991 Ultra-Plinian eruption of Mount Pinatubo was the second largest terrestrial eruption of the 20th century ( surpassed only by the 1912 eruption of Novarupta ) , and the largest eruption in living memory . The eruption produced high-speed pyroclastic flows , giant lahars , and a cloud of volcanic ash hundreds of miles across . Twenty million tons of sulfur dioxide and roughly of tephra are estimated to have been ejected in total , which corresponds to a Volcanic Explosivity Index of 6 . By contrast , roughly of material was ejected in the 1980 eruption of Mount St. Helens ; this corresponds to a VEI of 5
1980 eruption of Mount St. Helens
1980 eruption of Mount St. Helens
Operation Fiery Vigil