Bedrock Geologic Map of Idaho
(c) 2006 Andrew Alden, licensed to About.com, Inc. Modified from Idaho Geological Survey image. (fair use policy)
Idaho is an igneous state, built from many different episodes of volcanism and intrusion, plus vigorous uplift and erosion by ice and water. Its two largest features are the great Idaho batholith, a huge emplacement of plutonic rock of Mesozoic age, and the swath of lava beds across the south that marks the path of the Yellowstone hotspot.
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 Idaho as well. As the North American plate drifted over it, the hotspot poured more lava upon the Snake River plain and now lies just over the border in Wyoming beneath Yellowstone National Park.
To the south of 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.
The southwestern corner of the state 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.
More Idaho resources on About.com:
Idaho Maps
Idaho Geography, State Symbols & Facts
Idaho Travel
Idaho National Parks
Idaho Parks & Forests
Idaho Campgrounds
Idaho Scenic Roads
Idaho Skiing
Idaho Archaeology
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