Broken Top joins several other volcanoes in the eastern segment of the Cascade Range known as the High Cascades, which trends from north-south. This includes the Three Sisters complex, Belknap Crater, Mount Washington, Black Butte, and Three Fingered Jack, and Mount Bachelor. Constructed towards the end of the Pleistocene epoch, these mountains are underlain by more ancient volcanoes that sank within parallel north-south trending faults in the surrounding region. The Three Sisters form the centerpiece of a region of closely grouped volcanic peaks, an exception to the typical spacing between volcanoes elsewhere in the Cascades. This vicinity is among the most active volcanic areas in the Cascades and one of the most densely populated volcanic centers in the world, as well as the second largest volcanic field of silicic rock within the Quaternary Cascades. The area from the Three Sisters to Broken Top and Mount Bachelor features at least 50 eruptive vents for rhyolitic and rhyodacitic lava.