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Eruption History of Mount Bachelor Volcanic Chain, Oregon

Eruptive activity along the Mount Bachelor volcanic chain was not continuous—four discrete eruptive episodes are recognized. Although the earliest eruptions were widespread and occurred along the entire length of the chain, activity was primarily focused near its center and built the Sheridan mountain shield. Lava flows from Sheridan Mountain overlie glacial outwash and include some of the earliest recognized deposits from the Mount Bachelor volcanic chain.

During the second eruptive episode, activity shifted to the south to form a chain of scoria cones and lava flows, the Siah Butte vents. Either during the end of episode two or after a short hiatus of no eruption, activity shifted north to build the underlying shield and main cone of Mount Bachelor. Most of Mount Bachelor was constructed before 9.5 ka and perhaps as early as 12.5 ka, although clearly some lava flows post-date 12.5 ka. After construction of the Mount Bachelor summit cone was complete (about 10ka), a hiatus of at least 2,000 years occurred before the most recent eruption along the chain, which produced Egan cone, a scoria cone and lava flows on the lower north flank of Mount Bachelor.

Ash from the climactic eruption of Mount Mazama, which formed Crater Lake, overlies all of the products of the Mount Bachelor volcanic chain, and indicates that all eruptive activity ended before 7.7 ka. There is no soil between the Egan scoria and Mazama ash, suggesting that Egan cone erupted shortly before Mazama ash fell in the area.