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Hot spots are caused by plumes of hot material that rise from the core/mantle boundary all the way up through the mantle and crust to the surface. The islands of Hawaii and Iceland are both located over hotspots. There is also a hotspot under Yellowstone Park. Hotspots are relatively stationary, compared with the motions of the plates. Thus, the apparent motion of a hot spot on a moving plate can be easily interpreted in terms of the motion of the plate with respect to the fixed hot spot.
The apparent motion of the Yellowstone hot spot puts in under Boise, Idaho about 18 million
years ago with migration to the ENE, forming the Snake River Plain. However, what is actually
shown is the WSW motion of the North American plate with respect to the hot spot underneath.
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Plate Tectonics of Montana (Page 14 of 14)