Volcanic activity in the Santiam Pass and McKenzie Pass has included eruptions of basalt and basaltic andesite lava flows from cinder cones and shield volcanoes. Most of these events occurred within the past 7,650 years after the deposition of Mazama Ash, yielding lava flows with rugged surfaces, unlike the more ancient, glaciated lava deposits that preceded them. Ash from these eruptions is also present. Three regional pulses of eruptive activity took place: the first from 4,000 to 3,000 years ago, the second from 2,900 to 2,500 years ago, and the third from 2,000 to 1,300 years ago. Age progression is not clearly related to geographic distribution, though Blue Lake Crater and the spatter cone chain nearby mark two small eruptive centers in the eastern Santiam Pass area.