Yellowstone hot spot first appeared about 17 million years ago with rhyolite eruptions
on the Owyhee Plateau of southeast Oregon and northern Nevada. At about the same
time, the Columbia River Basalts, containing over 150,000 cubic miles of lava,
were erupting in eastern Oregon and Washington from north-northwest-trending fissures.
Some of the more extensive lava flows moved westward down the ancestral Columbia
River to the Pacific Ocean. The continental divide was likely located north of
Boise in the Salmon River Mountains where the Idaho Batholith formed a resistant
topographic high area. The Albion Range was uplifted near the Nevada-Utah border
and drainage in eastern Idaho was eastward across a broad alluvial surface, above
the eroded Idaho-Wyoming fold-thrust belt.
Cross-section of the
Images courtesy of the U.S.
Forest Service, Department of Agriculture.