EarthRef.org Reference Database (ERR) -- Steinberger & O'Connell 1998

Because of their slow relative motion, hotspots, mainly in the Pacific, are often used as a reference frame for defining plate motions. A coherent motion of all Pacific hotspots relative to the deep mantle may, however, bias the hotspot reference frame. Numerical results on the advection of plumes, which are thought to cause the hotspots on the Earth's surface, in a large-scale mantle flow field are therefore presented. Bringing the results into agreement with observations also leads to conclusions regarding the viscosity structure of the Earth's mantle, as well as the sources and distribution of plumes. The abrupt change in direction of the Hawaiian-Emperor chain implies an upper-mantle viscosity under the Pacific of similar to 1.5 x 10(20) Pa s or less. Slow relative motion of hotspots requires high lower-mantle viscosity, unless hotspots are located at large-scale stationary upwellings that are currently unresolved by seismic tomography. For our preferred model, we obtain coherent motion of Pacific hotspots in a reference frame of no net rotation, as well as coherent motion relative to African hotspots, caused by return flow antiparallel to plate motion. Advection and regional differences in life expectancy can to a large part explain the distribution of plumes in relation to ridges, subduction zones (present and past) and seismic anomalies. Plume conduits are substantially tilted in the lower mantle. The surface motion of hotspots is often smaller than the advection rate of plume conduits in the lowermost mantle.