Welded ignimbrites can be highly voluminous , such as the Lava Creek Tuff erupted from Yellowstone Caldera in Wyoming 640,000 years ago . 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 , greater than any eruption known in the last 10,000 years . The intensity of welding may decrease towards the upper margin of a deposit , towards areas in which the deposit is thinner , and with distance from source . Welded tuff is commonly rhyolitic in composition , but examples of all compositions are known
1980 eruption of Mount St. Helens
1980 eruption of Mount St. Helens
Tuff