The thermal structure of the solid Earth was estimated from the phase diagrams of MORB and pyrolite combined with seismic discontinuity planes at 410-660 km, thickness of the D″ layers, and distribution of the ultra-low velocity zone (ULVZ). The result clearly shows the presence of two major superplumes and one cold super-downwelling. A temperature difference at the core-mantle boundary layer reaches an average of 1500 K, strongly indicating that the driving force of mantle convection is core heat that generates superplumes as a main engine to drive the mantle dynamics. Superplumes cause the breakup of supercontinents and control plate tectonics, until subduction zones develop at the ocean margins. The major driving force of whole Earth dynamics is derived from a superplume that transports core heat to planetary space. Plume tectonics dominates in the mantle, except for the uppermost part where plate tectonics operates, but only independently if a slab-pull force is generated. Comparison of the thermal structure combined with seismic tomography under the western Pacific, Asia, Africa, and the Pacific leads us to speculate on the mechanism of the formation of a superplume. A mantle made extremely cold by extensive subduction during the formation of a supercontinent is transformed into the hottest mantle triggered by the exothermic reaction of perovskite to post-perovskite (pPV) underneath the supercontinent over several 100 million years. The fuel to develop a superplume is recycled MORB at the CMB that partially melts to sink down to form an anti-crust and a buoyant andesitic restite to promote a rising plume. When all recycled MORB slabs are spent, it is the time of the demise of a superplume. We propose here that mantle dynamics evolved from double-layer convection in the Archean to episodic whole mantle convection thereafter. This change was due to the widespread occurrence of majorite (Mj) instead of ringwoodite in the MBL in the Archean and vice versa thereafter. During the transition period mantle overturn occurred at 2.8-2.7 Ga, and probably at 2.3 Ga, when pPV was born at the CMB to initiate a new superplume-supercontinent cycle that lasted until now. The return-flow by the subduction of seawater into the mantle began at 0.75 Ga causing the sea-level to drop. Through the extensive surface erosion of the continental landmass, the surface environment of the Earth drastically changed making it possible that the large multi-cellular animals appeared to evolve in the Phanerozoic.
ASJC Scopus subject areas
- Environmental Science(all)
- Earth and Planetary Sciences(all)