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Abstract Plate tectonics distinguishes Earth from the other terrestrial planets but its initiation mechanism and onset time are debated. We propose plate tectonics was initiated by the deposition of magnetite‐rich banded iron formations (BIFs) through biogeochemical iron cycling in Neoarchean oceans. In the photic zone of proto‐continental margins, photoferrotrophic bacteria efficiently oxidized the dissolved Fe(II) and induced massive precipitation of ferric oxyhydroxide, which would rapidly react with Fe(II)‐rich hydrothermal fluids from coeval vigorous volcanism in Neoarchean oceans to produce magnetite‐rich BIFs. Mechanical models demonstrate that the localization of high‐density BIF deposition near proto‐continents induces collapse of the lithosphere and can initiate the earliest subduction. The peak deposition of BIFs in 2.75–2.40 Ga provides a time constraint on the inception of plate tectonics. 

Abstract The radial viscosity of the mantle is generally thought to increase by ∼10–100 times from the upper to lower mantle with a putative, abrupt increase at 660 km depth. Recently, a low viscosity channel (LVC) between 660 and 1,000 km has been suggested. We conduct a series of time‐dependent flow models with viscosity either increasing or decreasing at 660 km depth while tracking slab structure, state‐of‐stress, and geoid. We find that a LVC will lower the amplitude of long wavelength (>5,000 km) geoid highs over slabs, with amplitudes <10 m in height, while increasing the slab dip angle and downdip tension in the upper 300 km of slabs. A viscosity increase at 660 km gives rise to strong downdip compression throughout a slab and this pattern will largely go away with the introduction of the LVC. In addition, the endothermic phase change at 660 km depth can substantially affect the stress distribution within slabs but has a minor influence on the geoid. Models that fit the observed long wavelength geoid and observed focal mechanism in the western Pacific favor models without the presence of the LVC between 660 km and 1,000 km depths.