Lava Creek Tuff was at least 1000 times as large as the deposits of the May 18 , 1980 eruption of Mount St. Helens . It had a Volcanic Explosivity Index ( VEI ) of 8 â '' greater than any eruption known in the last 10,000 years . Lava Creek Tuff is known to be at least 1000 times as large as the deposits of the May 18 , 1980 eruption of Mount St. Helens , and it had a Volcanic Explosivity Index ( VEI ) of 8 â '' greater than any eruption known in the last 10,000 years .