When the Columbia Plateau exposes dark rock that spans the states of Washington Oregon and Idaho. What made this vast plateau?

The Columbia Plateau was formed primarily by volcanic activity, particularly from the Columbia River Basalts, which erupted millions of years ago. These basalt flows created extensive layers of dark rock that now characterize the region. Additionally, the plateau has been shaped by erosion and geological processes, including the carving effects of the Missoula Floods during the last Ice Age, which further sculpted the landscape. This combination of volcanic activity and erosion has resulted in the vast expanse of dark rock found across Washington, Oregon, and Idaho.