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 split in a way that the volcano-free zone between the two features results.
The researchers reasoned that, whatever else was going on to provide molten material, the paths to the surface were likely to be enabled by stresses in the crust. And that was going to depend on both the existing features in the crust (obtained largely through seismic data) as well as larger-scale processes going on in the mantle underneath.
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