New paradigm for the early Earth: Did plate tectonics as we know it not operate until the end of the Archean?

  • McCall G
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Abstract

Here, I question the apparent absence of 'oceanic' crust from the Archean and whether plate tectonics as we know it, involving ridges, deep oceans, subduction and continent-sized plates, operated during the Archean. The suggestion is advanced that the early seas were formed after the initiation of continental crust at 4 Ga; that this crust was globe enveloping; and that the first seas were shallow and above this crust, thus the submarine greenstone lavas, extruded through them, overlie sial. The Earth is the only non-uniplate planet: its uniqueness and difference from the probably similar Venus in possessing plates is attributed to the greater amount of water on the Earth, as the book 'Planetary Crusts' concludes. The fundamental uniqueness is in having a 'tertiary' continental or sialic crust (no other planet has one). The greenstone belts represent eruption of mainly mafic volcanics into mostly shallow seas, occupying rifted zones in this crust, which was probably up to 50km thick. The recognition of analogues of large igneous provinces in the greenstone belts of the Pilbara is a complication, but these also appear to have been erupted mainly from plumes in scattered rifts in the sialic envelope, not as continuous systems analogous to the later ridges. The uniqueness of the Earth in developing plate tectonics is secondary to the early development of the sialic crust. Large, deep oceans, mid-ocean ridges, spreading, continental-sized plates and subduction may have been only initiated when the crust became cool enough to allow deep fracturing and spreading to occur, and extensive and continuous ridge eruptivity developed: possibly due to a change in the pattern of convection in the mantle? Sialic crust may well have existed well before 4.0 Ga, and possibly it originated in a magma ocean, such as is widely accepted for the origin of the early anorthositic crust of the Moon. The initial sialic crust will have been lost due to later reworking though the long time span of the Archean. The Archean seas are likely to have existed under a thin carbon dioxide and nitrogen atmosphere like the present atmosphere of Mars. It is also suggested that, though drilling for evidence of early global oxygenation has been successful, and some early bacterial life existed then, it probably existed not in any way on a scale able to supply global oxygenation, as did the prolific later Phanerozoic life.

Author-supplied keywords

  • Archean
  • Continental crust
  • Hadean
  • Oceanic crust
  • Oxygenation
  • Planetary crusts
  • Plate tectonics
  • Plumes

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Authors

  • G. J.H. McCall

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