Panum Crater is the northernmost volcano in the sequence and is a good example of both a tuff ring and a rhyolite dome. Its structure is twofold; an outer tuff ring (forming a classic crater) and an inner plug, or dome of rhyolite, pumice and obsidian created from lavas.: In this case, heat from the magma feeding Panum flashed groundwater to steam to create the tuff ring before lava reached the surface. Other Mono Craters also were formed in this manner, but their plug domes grew larger than their tuff ring craters. The domes have steep sides and are flanked by slopes of scree consisting of large angular and glass-rich rocks.: Devil's Punch Bowl, located south of the main dome complex, stopped forming at an earlier stage of development. It is a wide and deep explosion pit with a much smaller glass dome on its floor.: