All but one of these volcanoes, 1–2 million year old Glass Mountain (made of obsidian),[10]:264 were destroyed by the major (volcanic explosivity index (VEI) 7) eruption of the area 760,000 years ago, which released 600 cubic kilometres (144 cu mi) of material from vents just inside the margin of the caldera.[11] (The 1980 Mount St. Helens eruption was a VEI-5 eruption releasing 1.2 km3 (0.29 cu mi).) About half of this material was ejected in a series of pyroclastic flows of a very hot (1,500 °F (820 °C)) mixture of noxious gas, pumice, and volcanic ash that covered the surrounding area hundreds of feet deep. One lobe of this material moved south into Owens Valley, past present-day Big Pine, California.