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Jefferson volcanoMt Jefferson volcano, one of the lesser known Cascade volcanoes, is the second highest mountain in Oregon. Although it is deeply eroded and has probably not erupted for at least about 1000 years, it is still considered active.
Background:Jefferson was constructed in 2 major episodes interrupted by extensive glacial erosion. The first eruptive period began about 290,000 years ago and built an andesitic-to-dacitic volcano which might have even been higher than the current summit.
The second eruptive phase, including Jefferson volcano's largest known eruptions, took place between ca. 70,000 and 35,000 years ago and their products for most of the upper part of the stratovolcano. Pyroclastic flow deposits are found as far as 15 km to the east and ash fall deposits can still found as far as Arco in southeast Idaho. In the past 12,000 years, several eruptions took place on vents near the Cascade crest south of Jefferson, building cinder cones and lava flows that traveled down glacial valleys, including Forked Butte and North Cinder Peak. The most recent eruption from Jefferson volcano was also from a cinder cone on the flank of South Cinder Peak, with a lava flow reaching Lake Marion to the west.
Jefferson had been named by Lewis and Clark in order to honor the president who had sponsored their expedition.