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.