The 1912 eruption that formed Novarupta was the largest to occur during the 20th century. It began on June 6, 1912, and culminated in a series of violent eruptions. Rated a 6 on the Volcanic Explosivity Index, the 60-hour-long eruption expelled of ash, thirty times as much as the 1980 eruption of Mount St. Helens. The erupted magma of rhyolite, dacite, and andesite resulted in more than of air fall tuff and approximately of pyroclastic ash-flow tuff. During the 20th century, only the 1991 eruption of Mount Pinatubo in the Philippines and the 1902 eruption of Santa María in Guatemala were of comparable magnitude; Mount Pinatubo ejected of tephra, and Santa María just slightly less.