Figure 5. Forced mantle flow and decompression melting resulting from local plate motions. Far from subduction zone, a northeast-directed forced shear across upper mantle (right red arrow) results from northeast motion of stable lower mantle relative to southwest-moving North America (NA). Near the subduction zone upper mantle is forced to flow northwest (left red arrows) because of corner flow driven by subducting plate. Yellowstone and Newberry magmatism follows these trends as fertile mantle flows past residuum and ascends (red-to-white arrows). Decompression melting causes convection (white arrows) and magmatism, creating new residuum at ends of residuum body (Fig. 4 shows details of process). Diverging upper mantle flow evacuates asthenosphere from central area, forcing mantle ascent.