Recent studies show that the Yellowstone hotspot is associated with a plume-like low-velocity pipe that ascends from the transition zone to the lithosphere-asthenosphere boundary, where the plume is sheared to the southwest by North American plate motion. Rayleigh wave tomography shows this plate-sheared plume layer has an extremely low S wave velocity of 3.8 ± 0.1 km/s at 80 km depth, ~0.15–0.3 km/s lower than the velocity observed beneath normal mid-ocean ridges. To constrain the temperature of the plume layer, a grid search with respect to grain size and temperature is performed to fit the observed Rayleigh wave phase velocities. This search finds that the excess temperature of the plume layer is >55–80 °C at 95% confidence for two different temperature-velocity and two different melt-velocity models, confirming that a thermal mantle plume exists.

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