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Geology of the Lassen volcanic area
The geology of the Lassen volcanic area presents a record of sedimentation and volcanic activity in the area in and around Lassen Volcanic National Park in Northern California, U.S. The park is located in the southernmost part of the Cascade Mountain Range in the Pacific Northwest region of the United States. Pacific Oceanic tectonic plates have plunged below the North American Plate in this part of North America for hundreds of millions of years. Heat from these subducting plates has fed scores of volcanoes in California, Oregon, Washington and British Columbia over at least the past 30 million years (see Geology of the Pacific Northwest) and is also responsible for activities in the Lassen volcanic area.
Between 3 and 4 million years ago,〔 volcanic-derived mud flows called lahars streamed down several major mountains that included nearby but now extinct Mount Yana and Mount Maidu to become the Tuscan Formation. Basaltic and later andesitic to dacitic flows of lava covered increasingly larger areas of this formation to eventually form the lava plateau that the park sits on. About 600,000 years ago, Mount Tehama started to rise as a stratovolcano in the southwestern corner of the park, eventually reaching an estimated in height. Following a series of eruptions approximately 350,000 years ago, its cone collapsed into itself to form a wide caldera.
Roughly 27,000 years ago,〔older sources give an age of 18,000 years〕 a dacite lava dome quickly pushed its way through Tehama's destroyed north-eastern flank, becoming the approximately shorter Lassen Peak. Lassen's shape was significantly altered by glacial erosion from 25,000 to 18,000 years ago during the Wisconsin glaciation. Since then, smaller dacite domes such as the 1,100-year-old Chaos Crags have formed around Lassen. Phreatic (steam explosion) eruptions, dacite and andesite lava flows along with cinder cone formation have persisted into modern times. Most notable of these is the mid to late 17th century eruption and formation (Tree Ring dates) of Cinder Cone and the early 20th century eruption of Lassen Peak. The only activity since then has been the constant bubbling of mud pots and steaming of fumaroles from the various geothermal areas in Lassen Volcanic National Park. However, a potential exists for renewed vigorous volcanic activity that could threaten life and property in the area.
== Regional geologic setting ==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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