Tuff
A hot lava flow welded by heat can be highly voluminous, such as the Lava Creek Tuff erupted from Yellowstone Caldera in Wyoming 640,000 years ago. 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—greater than any eruption known in the last 10,000 years.