There appear to be two separate arms originating from the same general location at the crust-mantle boundary. One branch slopes northeast to feed the Yellowstone caldera, while a second branches off toward the Snake River Plain. The branches are divided in such a way that the zone is free of volcanoes between the two formations.
The researchers reasoned that whatever was happening to provide molten material, the paths to the surface were likely enabled by stresses in the crust. And that was going to depend both on existing features in the crust (obtained largely through seismic data) and on larger-scale processes taking place in the underlying mantle. So the model included basic geological details, known physical processes, and a little bit of history in the sense of what we know about how that section of crust arose.
And that is where we return to the Farallon plate. Its remains, after being swept beneath the North American plate, continue to sink and move through the mantle. This, the researchers hypothesize, is driving a general eastward flow of material through the viscous mantle. However, just east of Yellowstone, that flow empties into the older edge of the North American plate, where the crust is thicker and denser than the portion of the continent formed by the Farallon plate.
New paths
This thick crust causes the mantle flow to descend. And that change in flow causes a series of stresses in the crust, particularly a compressive force between the older and newer sections of the North American plate, as well as downward drag on the older section. Adding to the local stresses is the fact that all of the material that erupted to form the Snake River Plain is denser than much of the surrounding rock, putting stress on nearby rocks as they try to sink.
