Geology
Mount Bachelor was formed between 11,000 and 15,000 years ago as a shield volcano but was later capped with a stratovolcano as the eruptions apparently became more explosive over time. It is the youngest prominent volcano in the Three Sisters area of Oregon. Bachelor is composed mainly of basalt and basaltic andesite.
It last erupted between 8,000 and 10,000 years ago and is entirely covered with Mazama Ash from the catastrophic eruption of Mount Mazama about 6,845 years ago. There is no geothermal activity at present, though some areas popularly thought to be fumaroles are caused by air movement through the porous structure. Several are near the top of the Pine Marten chairlift and occasionally present a hazard to skiers and snowboarders where the snow is undermined.
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