If you look at a topographic map of Idaho you can’t help but notice the swoop of the Snake River as it moves east to west through the southern part of the state. That’s called the Snake River Plain. It would be natural to assume that the Snake River created it, perhaps as it meandered back and forth through geologic time. That assumption would be wrong. The Snake River Plain was created by a lot of geological forces, including water erosion, but it’s primarily a signature left behind by a hot spot beneath the earth’s crust.
The crust of the earth is moving west in relation to this hot spot, so its impact is moving east. Think of the hot spot as being stationary while the crust moves west. In the illustration you can see where the hot spot was 16 million years ago along the Oregon/Nevada border. As the crust continued west, the stationary hot spot effectively move east. It “entered” the state some 15 million years ago, was somewhere around Bruneau Dunes about 12 million years ago, and was parked under Craters of the Moon National Monument about 7 to 10 million years ago.
As the hot spot moved, it produced a lot of volcanic activity on the surface. The Craters area is one visible indication of this. The area known as Island Park is where the hot spot made a big showing about 5 million years ago. It left behind a couple of calderas, or collapsed volcanoes. The Henry’s Fork Caldera is between 18 and 23 miles across. That’s huge. Still, it comfortably fits INSIDE the Island Park Caldera, which is 50 to 65 miles wide. Upper Mesa Falls is where the water of the Henrys Fork splashes over the edge of the old volcano.
So, has the hot spot gone away? Nope. You can find it today under Yellowstone National Park heating up the geysers and hot pools and causing social media to come unglued whenever there is swarm of earthquakes.
By the way, this is the same thing that’s going on with the Hawaiian Islands. That hot spot acts of as the earth’s crust moves over the top of it, too. Since there is no land mass the resulting volcanic activity produces islands. One of the oldest islands, Kauai, is to the furthest west. As that hot spot moved—in relation to the crust—east, it created Oahu, Maui, and then the Big Island, Hawaii. That’s where the volcanic activity is today.