Three Sisters

Eruption History for Broken Top

Broken Top is a long extinct volcano that was active in the middle Pleistocene between about 300 and 150 ka.

Eruptive products from Broken Top range from basalt to rhyodacite. Its ancient cone has been deeply eroded leaving isolated peaks with little of its original surface intact. There are deep erosional embayments in addition to the south cirque, an amphitheater-like opening with steep ridges at the summit. The basalt of Cayuse Crater forms an unrelated area of lava flows and scoria cones that erupted about 11 ka on the southwest apron of the long-inactive Broken Top edifice.

Broken Top volcano, inactive since 150 ka or longer, viewed to the ...

Broken Top volcano, inactive since 150 ka or longer, viewed to the northwest. Three Sisters Wilderness, Oregon. (Credit: Hildreth, Wes. Public domain.)