Mount Bachelor

Geology and History Summary for Mount Bachelor

Mount Bachelor seen from South Sister across Sparks Lake. One of th...

Mount Bachelor seen from South Sister across Sparks Lake. One of the "Devils Chain" domes visible in lower image foreground. Oregon (Credit: Topinka, Lyn. Public domain.)

Mount Bachelor is a basalt to basaltic andesite summit cone that sits atop a shield volcano 15 km southeast of South Sister. It is the largest feature of the many eruptive vents that make up the Mount Bachelor volcanic chain, a 25-km-long, north to south aligned chain of about 50 vents. The majority of the eruptive activity along the volcanic chain occurred in 4 discrete episodes, which are defined on the basis of paleomagnetic data, between 18 and 8 ka. The total volume of the chain is estimated at about 40 km3, with the volume of Mount Bachelor estimated at about 25km3 or more than 50% the volume of the entire chain. Glacial striations about 1 km (0.6 mi) north of the Mount Bachelor suggests that the flow of ice was not obstructed by the mountain during the latest glacial maximum (22-18 ka), the Suttle Lake stade; however, the cone was almost completely built by the time a subsequent glacial advance occurred, between 12.5 and 11 ka as evidenced by glacial deposits on the volcano edifice.