How Volcanoes Work

 Image Described in Caption
Columbia River Basalt Dike -- Dike exposed in the Snake River Canyon of west-central Idaho. The dike, composed of Grande Ronde basalt, cuts discordantly across several Grande Ronde flows of the Columbia River Basalt Group. Numerous dikes in the tri-state area of Oregon, Washington, and Idaho mark the sites of fissure eruptions that generated the Columbia River Flood basalt province. The great bulk of these eruptions occurred between 17 and 15 million years ago. Photo by Vic Camp.