Large volcanoes appear as bumps on the otherwise smooth surface of Snake River Plain, a kidney-shaped expanse in southern Idaho. These volcanoes are made of a lava known as rhyolite, which produces very explosive eruptions and cone-shaped volcanoes. The oldest of these volcanoes, about 17 million years old, 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.
The apparent movement of the volcanoes from west to east over
the last 17 million years marks the movement of the North American
Plate westward across a source of magma. Such hot spots or plumes
rise into the Earth's crust from the underlying mantle. Yellowstone
National Park in northwest Wyoming lies above the current
location of the hot spot. A similar hot spot formed the Hawaiian