THE PLATE AND PLUME PARADIGMS

“…it is a privilege to see so much confusion.” Marianne Moore, The steeple-jack It is widely believed that plate tectonics cannot explain so-called hotspots, melting anomalies, hotspot tracks and large igneous provinces. It is also widely believed that the results of seismology and geochemistry for mantle structure are discordant, with the former favoring whole-mantle convection and the later favoring layered convection with a boundary near 650-km. Neither scenerio is physically plausible; neither is consistent with the data. Plate tectonics, in its most general sense, involves recycling, crustal thickening and delamination, and produces a heterogeneous variably fertile upper mantle with a wide range of melting points. Subduction zones and arcs migrate about the surface of the Earth. Subducted and delaminated material equilibrates at various depths and warms up by conduction from ambient mantle. Migrating ridges and rifts sample this heterogeneous mantle, which moves at rates much slower than plates or plate boundaries; fertile blobs in the asthenosphere and transition zone act as relatively stationary melting anomalies, requiring only lithospheric extension to form incipient ridges and volcanic chains. Large chunks of basalt/eclogite–including seamount chains, and delaminated crust–are introduced into the mantle and become melting anomalies in spite of their low temperatures. Midocean ridges are enormous averaging machines, turning heterogeneous mantle into uniform MORB while off-ridge volcanoes sense the true heterogeneity of the upper mantle. Chaotic mantle convection, stirring and mixing is the usual explanation but this is unnecessary and unlikely. Global mantle tomography averages and smooths out the stucture but high-frequency high-resolution techniques yield a mantle structure that is laminated and turbid, and is unlike either the two-layered geochemical model or the one-layer model based on visual inspection of tomographic cross-sections. A mantle with internal heating and cooling from above is driven by the top boundary layer and dense sinkers. Plates and slabs drive themselves, they are not driven by something independent called convection cells or plumes. Plate tectonics is a far-from-equilibrium self-organized system that organizes and drives both itself, and passive mantle convection.