The
Snake River Plain can be divided into three sections: western, central, and eastern. The western Snake River Plain is a large tectonic
graben or rift valley filled with several kilometers of
Lake Idaho sediments; the sediments are underlain by
rhyolite and
basalt, and overlain by basalt. The western plain began to form around 11–12
Ma (million years ago) with the eruption of rhyolite lavas and
ignimbrites. The western plain is not parallel to
North American Plate motion and lies at a high angle to the central and eastern Snake River Plain. Its
morphology is similar to other
volcanic plateaus such as the
Chilcotin Group in south-central
British Columbia, Canada. The eastern Snake River Plain traces the path of the North American Plate over the
Yellowstone hotspot, now centered in
Yellowstone National Park. The eastern plain is a topographic depression that cuts across
Basin and Range mountain structures, more or less parallel to North American Plate motion. It is underlain almost entirely by basalt erupted from large
shield volcanoes. Beneath the basalts are rhyolite lavas and ignimbrites that erupted as the
lithosphere passed over the hotspot. The central Snake River plain is similar to the eastern plain but differs by having thick sections of
interbedded lacustrine (lake) and fluvial (stream) sediments, including the
Hagerman fossil beds.
Island Park and
Yellowstone Calderas formed as the result of enormous rhyolite ignimbrite eruptions, with single eruptions producing up to of ash.
Henry's Fork Caldera, measuring by , may be the largest symmetrical
caldera in the world. The caldera formed when a dome of magma built up and then drained away. The center of the dome collapsed, leaving a caldera. Henry's Fork Caldera lies within the older and larger
Island Park Caldera, which is by . Younger volcanoes that erupted after passing over the hotspot covered the plain with young basalt lava flows in places, including
Craters of the Moon National Monument. ==Effects on climate==