Yellowstone Volcanic Plateau
One million years ago the hot spot was located beneath the Yellowstone Volcanic Plateau. Both the Huckleberry Ridge and the Henry's Fork calderas had formed, the latter by the eruption of the Mesa Falls Tuff. The high terrain of the northern Rocky Mountains spread out like an eastward-pointed crescent from the hot spot. The map shows a Pleistocene glaciation at its maximum. Lakes on the Snake River Plain were, from west to east, Lake Idaho, Raft Lake, and Lake Terreton. Continuation of Basin and Range extension had thinned the crust so that basalt magma, melted from mantle lithosphere by the hot spot, reached the surface and produced extensive basalt lava fields across the Snake River Plain. The Snake River had begun to cut Hells Canyon, its entry into the Salmon-Columbia River system. The Salmon River had begun to cut its steep canyons westward across the Idaho Batholith. Topographic subsidence of the eastern Plain had captured the headwaters of the Snake River in Jackson Hole, thus moving the continental divide northeastward into Montana and Wyoming. The Big Lost River drained southeastward, its water sinking into the basalt and joining the Snake River Aquifer. The Bear River near Montpelier likely flowed northward into the Snake, reversing its former southeastward course.
Yellowstone Volcanic Plateau Cross-section of the plateau.

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