The hotspot first arose farther west, in Washington and Oregon, during the Miocene Epoch some 20 million years ago. The first thing it did was to produce a gigantic volume of highly fluid lava, the Columbia River basalt, some of which is present in western Idaho (blue). As time went on the hotspot moved east, pouring more lava upon the Snake River plain (yellow), and now lies just over the eastern border in Wyoming beneath Yellowstone National Park.
To the south of the Snake River plain is part of the extensional Great Basin, broken like nearby Nevada into downdropped basins and tilted ranges. This region is also profusely volcanic (brown and dark gray).
The southwestern corner of Idaho is highly productive farmland where fine volcanic sediment, ground into dust by the Ice Age glaciers, was blown into Idaho by the wind. The resulting thick beds of loess support deep and fertile soils.
See a version of this map with a key to the different colors here.
More Idaho resources on About.com:
Idaho Geography, State Symbols & Facts
Idaho National Parks
Idaho Parks & Forests
Idaho Scenic Roads