How Volcanoes Work

 Image Described in Caption Clastic dike -- This Imnaha basalt flow is one of the early flows of the Columbia River Basalt Group in southeastern Washington. As the flow advanced over wet sediment, steaming action shot a slurry of steam and sediment through fissures into the flow, thus producing a several clastic dikes. The flow is ~16 million years old. Photo by Vic Camp.