to swimming in 2006 after geothermal activity in the area increased. There are a number of other hot springs in the area, some of which are open to bathers. Geology Caldera The tectonic causes of the Long Valley volcanism are still largely unexplained and are therefore a matter of much ongoing research. Long Valley is not above a hotspot, such as those which fuel the Yellowstone Caldera or the volcanoes of Hawaii; nor is it the result of subduction such as that which produces the volcanism of the Cascades. The known volcanic history of the Long Valley Caldera area started a few million years ago when magma began to collect several miles below the surface. Volcanic activity became concentrated in the vicinity of the present site of Long Valley Caldera 3.1 to 2.5 million years ago with eruptions of rhyodacite followed by high-silica rhyolite from 2.1 to 0.8 million years ago. After some time, a cluster of mostly rhyolitic volcanoes formed in the area. All told, about were covered by lava. All but one of these volcanoes, 1–2 million year old Glass Mountain (made of obsidian), were destroyed by the major (volcanic explosivity index (VEI) 7) eruption of the area 760,000 years ago, which released of material from vents just inside the margin of the caldera. (The 1980 Mount St. Helens eruption was a VEI-5 eruption releasing .) About half of this material was ejected in a series of pyroclastic flows of a very hot () 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. Another lobe moved west over the crest of the Sierra Nevada and into the drainage of the San Joaquin River. The rest of the pyroclastic material, along with of other matter, was blown as far as into the air where winds distributed it as far away as eastern Nebraska and Kansas. The eruption initially produced a caldera deep. However, much of the ejecta went straight up, fell down, and filled the initial caldera about two-thirds full. Eruptions Subsequent eruptions from the Long Valley magma chamber were confined within the caldera with extrusions of relatively hot (crystal-free) rhyolite 700,000 to 600,000 years ago as the caldera floor was uplifted to form the resurgent dome followed by extrusions of cooler, crystal-rich moat rhyolite at