<P> Lassen Peak, commonly referred to as Mount Lassen, is the southernmost active volcano in the Cascade Range of the Western United States . Located in the Shasta Cascade region of Northern California, it is part of the Cascade Volcanic Arc, which stretches from southwestern British Columbia to northern California . Lassen Peak reaches an elevation of 10,457 feet (3,187 m), standing above the northern Sacramento Valley . It supports many flora and fauna among its diverse habitats, which are subject to frequent snowfall and reach high elevations . </P> <P> A lava dome, Lassen Peak has a volume of 0.6 cubic miles (2.5 km) making it one of the largest lava domes on Earth . The volcano arose from the destroyed northern flank of now - collapsed Mount Tehama about 27,000 years ago, from a series of eruptions over the course of a few years . Since 25,000 years ago, the mountain has been significantly eroded by glaciers, and it is now covered in talus deposits . </P> <P> On May 22, 1915, a powerful explosive eruption at Lassen Peak devastated nearby areas, and spread volcanic ash as far as 280 miles (450 km) to the east . This explosion was the most powerful in a series of eruptions from 1914 through 1917 . Lassen Peak and Mount St. Helens were the only two volcanoes in the contiguous United States to erupt during the 20th century . Lassen Volcanic National Park, which encompasses an area of 106,372 acres (430.47 km), was created to preserve the areas affected by the eruption, for future observation and study, and to protect the nearby volcanic features . The park, along with the nearby Lassen National Forest and Lassen Peak itself, represent popular destinations for recreational activities including climbing, hiking, backpacking, snowshoeing, kayaking, and backcountry skiing . Lassen Peak is still considered active, and it continues to pose a threat to the nearby area through lava flows, pyroclastic flows, lahars (volcanically induced mudslides, landslides, and debris flows), ash, avalanches, and floods . To monitor this threat, Lassen Peak and the surrounding vicinity are closely monitored with sensors by the California Volcano Observatory . </P> <P> Located in Lassen Volcanic National Park, Lassen Peak lies in Shasta County, 55 miles (89 km) east of the city of Redding, in the U.S. state of California . Lassen Peak and the rest of the National Park area are surrounded by the Lassen National Forest, which has an area of 1,200,000 acres (4,900 km). Nearby towns include Mineral in Tehama County and Viola in Shasta County . </P>

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