There are signs of it all over Oregon and Washington: the dramatic cliffs of the Columbia River Gorge, the layered rock along the Palouse River, the ash deposits around the Zumwalt Prairie. A series of volcanic eruptions, starting 17.5 million years ago, formed the Columbia River Basalt Group, a complex of rock formations that was created over a few million years as lava erupted from fissures in the ground and seeped over the landscape.
The eruptions deposited about 10,000 cubic miles of rock and, according to new research, probably released enough sulfur gas to cool the whole planet down.
“The climate was already warming up rapidly before the whole eruption period started,” says John Wolff, a geologist at Washington State University and coauthor of the study. “Right at the peak of the [Miocene] Climatic Optimum, when these eruptions happened, there’s a little downturn in temperature. It’s actually two peaks of warming, separated by this cooling period.”