Glass Mountain is a spectacular, nearly treeless, steep-sided rhyolite and dacite obsidian flow that erupted just outside the eastern caldera rim and flowed down the steep eastern flank of Medicine Lake volcano.
Thirteen vents along a northwest-southeast trending fissure were active during this eruption, which occurred about 950 years ago. Tephra produced by explosions early in the eruption deposited pumice and ash several meters thick near the vents, but the deposit thin rapidly to <1 cm some 40 km (25 mi) to the northeast. Ten northwestern vents and the southeastern-most vent produced domes, while lavas of the intervening three vents coalesced to form the Glass Mountain flow, one cubic kilometer (0.25 cubic mile) in volume. The flow consists of three dacitic eastern lobes which grade westward to rhyolite and are overlain by rhyolite lobes. Glass Mountain first erupted dacite containing mafic magmatic inclusions and last erupted rhyolite without inclusions. Granitic inclusions are rare in Glass Mountain, but were probably derived from underlying rock related to Medicine Lake volcano itself.
