Mount Bachelor


Map showing one-year probability of accumulation of 1 centimeter

Map showing one-year probability of accumulation of 1 centimeter (0.4 inch) or more of tephra from eruptions of volcanoes in the Cascade Range. (Credit: Nathensen, Manny. Public domain.)

Although Mount Bachelor looks similar to many of the more long-lived and potentially dangerous volcanic peaks of the Cascade Range, the mountain is part of a mafic volcanic chain that poses little danger for future eruptive activity. Mount Bachelor is just south of the Three Sisters, and tephra hazards from Cascades volcanoes are similar in both areas.

The Mount Bachelor volcanic chain sits within a regional lava-flow hazard zone in central Oregon. This zone is defined by the distribution of mafic volcanoes that have formed in central Oregon during roughly the past one million years. This type of area is considered a "mafic field," which poses some general hazards that are not specific to the Mount Bachelor area. Similar mafic fields are common the Cascades, and typically erupt for brief intervals (weeks to perhaps centuries), but some of the eruptive centers can grow to almost as large as composite volcano peaks, such as Three Sisters to the north. Mafic volcanoes typically erupt less explosively than do composite volcanoes, so their eruption impacts are less widespread.

Eruptions from mafic fields produce limited tephraand lava flows that typically travel 5 to 15 km (3 to 9 mi) and rarely 15 to 20 km (9 to 12 mi) from vents. Tephra deposits may produce deposits up to several meters (about 10 ft) thick within 2 km (1.2 mi) of the vent, but seldom exceed 10 cm (4 in) at distances 10 km (6 mi) away from vents. Therefore, tephra may disrupt recreation activities around the vent, but especially in the case of Mount Bachelor it is unlikely to heavily impact existing residential areas. Lava flows from mafic fields can occur, but are slow moving and can be outrun. They may dam or divert streams and rivers, which can generate flooding hazards.