Bachelor last erupted between 10,000 and 8,000 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.