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   General Theory of Plate Tectonics
The General Theory of Plate Tectonics

Don L. Anderson

Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125

Plate tectonics introduces chemical, thermal, viscosity, melting and density inhomogeneities into the mantle, and stress inhomogeneity into the plates. Idealized models often assume uniform mantle and rigid, homogeneous plates, and require separate, ad hoc explanations for island chains, melting anomalies and continental breakup. Plates, however, drive and break themselves and organize the underlying mantle, in common with other cooled-from-above systems.

Pressure, often ignored or approximated in simulations, suppresses thermal expansion and the Rayleigh number. This makes the deep mantle a sluggish system with gigantic features, consistent with tomography, and isolated from the upper mantle and plate tectonics except by conduction and gravity. Chemical stratification is thus likely.

Plate tectonics, with adjectives such as rigid, homogeneous, isothermal, fixed, subsolidus, reservoir, steady-state etc. dropped, is a much more powerful concept than generally believed. Cracks, rifts, dikes, incipient plate boundaries, melting anomalies and variations in melt volume and chemistry are natural parts of the General Theory of Plate Tectonics just as plate boundaries and mountain belts are. The long-sought-after alternative theory to plumes may therefore just be a less restricted view of plate tectonics. It appears to be the adjectives, assumptions and other baggage that are the problem.

Many of the geochemical paradoxes associated with the plume and primordial views of the mantle can be traced to the reservoir concept where seismic boundaries are assumed to delineate reservoirs. The mantle is heterogeneous, and should be from plate tectonic considerations, which involve recycling, inefficient melt and gas extraction, and the history of subduction. This suggests that sampling theory and the dispersed-component approach may be responsible for the diversity of basalts. The Central Limit Theorem predicts that large-scale averagers such as ridges should have less variance and less extreme values than xenoliths, inclusions, seamounts or ocean island basalts (OIB), as observed. Homogeneity is achieved by sampling, not by large scale convection. This idea has been tested with Os and He isotopes, which are as different from each other and from the standard isotopes as is possible.

Much of the buoyancy and resistance of plate tectonics is in the plate-slab system, even though most of the energy and mass is in the mantle.  Without plates, mantle convection could be described as a far-from-equilibrium self-organized system (SOFFE). With plates, the mantle feeds energy to the plates, which then  become the SOFFE system, and they organize mantle convection. This is the reverse of what is usually assumed.

Different volcanoes average over different  volumes of the mantle and involve different degrees of melting, and magma chamber processes differ. The Central Limit Theorem, combined with mass balance calculations, obviates the need for lower mantle or undegassed reservoirs or contributions from these. Extreme values of isotopic ratios including high 3He/4He ratios are predicted to occur in tectonic environments that sample small volumes of the mantle or involve small degrees of melting, such as oceanic islandss and seamounts. The Central Limit Theorem means that large volume averagers such as mid-ocean ridges will not have such extreme values.
The  conclusion is that both mid-ocean ridge basalts (MORB) and OIB are products of a heterogeneous upper mantle, sampled in different ways.