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Geologic Map
Shaded Relief
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Generalized Geologic Map of the Conterminous United States
North America Shaded Relief


  The North American Tapestry of Time and Terrain

The Two Maps
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Legend and Rock Ages
Rock Types
Political Boundaries


Snake River Plain

  the Snake River Valley



Large volcanoes appear as bumps on the otherwise smooth surface of the Snake River Plain, a kidney-shaped expanse in southern Idaho. These volcanoes are made of a silica-rich lava which produces very explosive eruptions, cone-shaped volcanoes, and forms a type of igneous rock called rhyolite.

The oldest of these volcanoes is about 17 million years old and are in the western and southern parts of the Plain. The age of the rhyolite volcanoes in the Snake River Plain decreases from the southwest to the northeast. As recently as 2,000 years ago, a different type of lava known as basalt flowed onto the surface and covered the rhyolitic flows. Basalt is a very fluid type of lava which produces low, smooth volcanoes such as those in Hawaii. The age trend of the volcanoes from west to east over the last 17 million years indicates that the line of active volcanoes moved in that direction. This marks the movement of the North American Plate westward across a relatively stationary source of magma. These are called hot spots, or plumes, and they rise into the Earth's crust from the underlying mantle. Yellowstone National Park in northwest Wyoming currently lies above the hot spot. A similar hot spot beneath the Pacific Plate has formed the Hawaiian Islands.