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but one of these volcanoes, 1-2-million-year-old Glass Mountain (made of obsidian), were destroyed by the major (VEI-7) eruption of the area 760,000 years ago, which released 600. km3 of material from vents just inside the margin of the caldera. (The 1980 Mount St. Helens eruption was a VEI-5 eruption releasing 1.2 km3.) About half of this material

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