A fundamental question regarding the dynamics of mantle convection is whether some intraplate volcanic centers, known as "hotspots," are the surface manifestations of hot, narrow, thermally driven upwellings, or plumes, rising from the lower mantle. Shown here is a global negative correlation between the thickness of the mantle transition zone (near 410-660 km depth) and petrologically determined potential temperatures of mid-ocean ridge and hotspot magmas. Hotspot potential temperatures are systematically higher than those for mid-ocean ridges, and the transition zone thicknesses beneath these hotspots are thinner. Thus, the majority of oceanic intraplate magmatic centers are associated with deep-seated thermal anomalies, suggesting that such magmatism is probably associated with thermal plumes.

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We examined mantle structure beneath the southeast Hawaiian Islands using multiple ScS reverberations from four earthquakes from the island of Hawaii and recorded at station KIP on the island of Oahu. We find an unusually deep 410‐km discontinuity and a transition zone thickness of 227 km, corresponding to a temperature increase of 87 K above the global average. Other reflectors include a lid‐low‐velocity zone boundary, a weak 520‐km discontinuity, and smaller discontinuities at 224 km, 288 km, and 1000 km. Whole mantle travel time is near the global average, which we attribute to an inclined or branching plume, lowermost mantle anisotropy, and estimate bias due to a possible ultra‐low velocity zone atop the core.

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We report a new technique to describe seismic velocity and impedance anomalies atop a seismic low‐velocity layer (LVL) at 350 km depth. We model shear wave speed reductions detected with Ps conversions beneath the Hawaiian Islands and negative impedance contrasts detected with ScS reverberations beneath the Coral Sea in the South Pacific, by varying the bulk solid composition, reference potential temperature, dihedral angle of melt, and melt composition. For a given bulk solid composition, the effects of elevated temperature and melt volume fraction on the seismic properties trade off with one another. At a given temperature, the calculated melt volume fraction is nearly insensitive to variations in the bulk solid composition. A low volume fraction of low dihedral angle melts mimics the seismic signature of a higher volume fraction of high dihedral angle melts. Despite stronger lateral variations in the LVL structure beneath Hawaii compared to the Coral Sea, both regional averages are similar. For a basalt volume fraction of 0.2 and a dihedral angle of 10°, we estimate regional averages of 1.1 ± 0.8 vol % melt at a depth of 350 km beneath the Hawaiian Islands for a reference potential temperature of 1800 K and 1.2 ± 0.005 vol % melt at a depth of 350 km beneath the Coral Sea region for a reference potential temperature of 1500 K. Our model of the seismic signal is unable to distinguish between melt compositions of mid‐ocean ridge basalt and carbonated peridotite melts at such small melt volume fractions.

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