CENTRAL IDAHO — The Yellowstone caldera isn’t the only major player in the volcanic history of the region; Idaho also hosts a colorful volcanic history. In this week’s Caldera Chronicles, geologist Zach Lifton of the Idaho Geological Survey shares the history of the geology of central Idaho.
Idaho is home to a diverse range of volcanic rock. Some of the rocks are 200-million-year-old basalts from the ocean floor, and others include the younger, 2,000-year-old lava flows found at the Craters of the Moon. The volcanic rocks form the Challis Volcanic Group, or “Challis volcanics.” According to Lifton, this name refers to “a large volume of Eocene (a geologic epoch that lasted from 56 to 34 million years ago) volcanic rocks.”
“Challis volcanism” occurred about 45 to 52 million years ago in central Idaho as part of a larger flare up of volcanic activity in British Columbia, Washington, Montana and Wyoming. The Absaroka volcanic field east of Yellowstone is one well-known part of this activity.
Geologically, the Challis volcanics occurred over a relatively short time period and are related to subducting tectonic plate movement. Subduction is when two tectonic plates collide, and the heavier, denser plate gets forced below the less dense one.
Lifton writes that part of the subducted slab broke off and tilted down steeply, which allowed hot material to well up under the Earth’s crust, which caused melting and then widespread volcanism as a result.
So, while the Yellowstone caldera has been “the driving force for volcanism for the last 17 million years,” the Challis volcanics of Idaho precede that activity by millions of years and are much more diverse. Those volcanics show the importance of continental-scale events having both regional and local impacts.
