The persistent mantle plume myth
Seismology, thermodynamics and classical physics—the physics associated with the names of Fourier, Debye, Born, Grüneisen, Kelvin, Rayleigh, Rutherford, Ramberg and Birch—show that ambient shallow mantle under large long-lived plates is hundreds of degrees hotter than in the passive upwellings that fuel the global spreading ridge system, that potential temperatures in mantle below about 200 km generally decrease with depth and that deep mantle low shear wave-speed features are broad, sluggish and dome-like rather than narrow and mantle-plume-like. The surface boundary layer of the mantle is more voluminous and potentially hotter than regions usually considered as sources for intraplate volcanoes. This means that the ‘mantle plume’ explanation for Hawaii and large igneous provinces is unnecessary. In isolated systems, heated from within and cooled from above, upwellings are passive and large, which suggests that tomographic features, and upwellings, are responses to plate tectonics, spreading and subduction, at least until melting introduces a small intrinsic buoyancy at shallow depths. Melting anomalies, or ‘hotspots,’ are side-effects of plate tectonics and are fed primarily by shear-driven processes in the boundary layer (BL), not by deep buoyant upwellings. A dense basal melange (BAM) component further stabilises the lower boundary layer of the mantle. Mid-ocean ridges and associated broad passive depleted mantle (DM) upwellings probably originate in the transition region. Deeper mantle upwellings are broad domes that stay in the lower mantle.
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