Heise Eruptive Center
About 4 million years ago the hot spot was located in eastern Idaho, near Rexburg. Lake Idaho, in the subsiding central and western Snake River Plain, was at its maximum extent. The Centennial and Teton Ranges were uplifted, with normal faults on their northern and eastern margins. Teewinot Lake occupied present-day Jackson Hole, and the headwaters of the Snake River drained eastward up the Hoback River Canyon to the Green River. In general, drainage was centripetal, away from the hot spot, south to the Bonneville Basin and north to Montana. Near Hagerman, at the southeast corner of Lake Idaho, streams and marshes were home to a diverse set of Pliocene animals, including the Hagerman horse (the Idaho state fossil). We show Lake Idaho as draining to the south through northern Nevada to the Sacramento River, as suggested by ages of distinctive sedimentary minerals (zircons) and the distribution of giant beaver and pygmy muskrat fossils.
Heise Eruptive Center Cross-section of the plateau.

Images courtesy of the U.S. Forest Service, Department of Agriculture.