The thermal state of the upper mantle; no role for mantle plumes

A variety of geophysical data indicates that long wavelength temperature variations of the asthenosphere depart from the mean by +/-200 degreesC, not the +/- 20 degreesC adopted by plume theoreticians. The 'normal' variation, caused by plate tectonic processes (subduction cooling, continental insulation, small-scale convection) encompasses the temperature excesses that have been attributed to hot jets and thermal plumes. Geophysical estimates of the average potential temperature of the upper mantle are about 1400 degreesC. Asthenospheric convection at ridges, rifts and fracture zones and at the onset of continental breakup is intrinsically 3D, giving rise to shallow pseudo-plume-like structures without deep thermal instabilities. Deep narrow thermal plumes are unnecessary and are precluded by uplift and subsidence data. The locations and volumes of 'midplate' volcanism appear to be controlled by lithospheric architecture, stress and cracks.