Two separate arms appear to arise from the same general location at the crust-mantle boundary. One branch curves northeast to feed the Yellowstone Caldera, while the other branch heads toward the Snake River Plain. The branches split in such a way that a volcano-free zone is formed between the two features.
The researchers reasoned that, whatever was going on to provide the molten material, its pathways to the surface were likely enabled by stresses in the crust. And this was going to depend on both existing features in the crust (obtained largely through seismic data) as well as large-scale processes going on in the mantle below. So, the model included basic geologic details, known physical processes, and a little bit of the history we know about how that part of the crust formed.
And this is where we come back to the Farallon plate. Its remains, as the North American plate moves beneath, are continually sinking and moving through the mantle. The researchers estimate that there is a general eastward flow of material through the viscous mantle. However, just east of Yellowstone, that flow runs into the old boundary of the North American Plate, where the crust is thicker and denser than the part of the continent that was emplaced by the Farallon Plate.
new paths
Due to this thick layer, the flow of the mantle bends downwards. And that change in flow causes a series of stresses in the crust, specifically a compression force between the older and newer segments of the North American plate, as well as downward pull on the older segment. Adding to the local stresses is the fact that all the material erupted to form the Snake River Plain is denser than most of the surrounding rocks, which exerts stress on the surrounding rocks as they try to sink.
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