Formation of the rhyolite plateau of Yellowstone National Park was one of the major volcanic events in the development of the Rocky Mountains. The volume of rhyolite erupted to form the plateau is estimated to have exceeded 400 cubic miles. The plateau contains many different types of rhyolite extrusive rocks, and volcanic surficial features are well preserved. Only the Taupo-Rotorua district in New Zealand and the Lake Toba area in Sumatra compare with Yellowstone in volume of salic volcanic rocks. The present study of the Yellowstone plateau has been stimulated by the growing interest in the importance of welded tuffs in rhyolitic volcanism and in their origin.