Evolution of the Earth: Plate Tectonics Through Time

Abstract

The tectonic mechanisms of heat escape have evolved over time as the Earth's interior cooled. The Earth condensed from rock vapor over liquid magma immediately following the Moon-forming impact, ~ 4.5 billion years ago. The liquid magma convected vigorously and cooled rapidly until solids formed in the deep mantle. Multiple layers of clouds made the atmosphere opaque, so heat escaped slowly. Tidal dissipation maintained a thin solid layer in the deep mantle over a few million years until the Moon moved far enough away that dissipation no longer balance the heat lost to space. Over a few more million years, the Earth cooled to mostly solid mush capped solid rock. This ‘mush ocean’ eventually evolved into modern plate tectonics. About a billion years in the future, the Earth's interior will become too cold for plate tectonics. Then, the Earth will become a one-plate planet like Mars. Current geodynamic understanding of the modern Earth provides only shopping lists of and qualitative inferences on plausible tectonic mechanisms within the early Earth. Plate processes are complex and involve poorly understood mechanical properties of partly molten rock and faults. Poor geologic preservation cloaks tectonic processes on the early Earth. A few detrital zircons indicate chemically isolated continent-like reservoirs by ~ 4.5 Ga shortly after the Moon-forming impact and survival of some zircons by ~ 4.4 Ga. Periods of non-zircon-forming local stability occur after ~ 4.36 Ga, as on the modern Earth. Plate-like processes became evident as soon as there was a geologic record that might have preserved them. Transform faults, oceanic crust, rifting, continents with thick chemically buoyant lithosphere, and subduction were all present in the Archean. As on the modern Earth, vertical tectonic processes dominated in many Archean regions. The high temperature of the interior allowed for broad regions of hot mobile continental crust. Other differences between modern and Archean plate processes are less clear. The thermal history of the interior may not even have been monotonic. That is, plate tectonics may have been unable to vent the heat supplied by radioactive decay. Physically, thick oceanic crust produced above the hot mantle is difficult to subduct into the mantle asthenosphere, like with modern oceanic plateaus. The pileup of thick hot oceanic plateaus likely produced regional conditions analogous to the earlier mush ocean throughout the Archean. Hydrated basaltic crust subducted into gabbroic mush, producing low-Ni granitic rocks. The geologic record after 3.0 Ga is well enough preserved to relate to modern processes. Still, the cooling rate of the Earth's interior and the rate of plate motions are only modestly constrained. Overall, the life span of oceanic crust after 3.0 Ga was comparable to that of the present. However, limited data indicate short-lived episodes of rapid plate motion in the Archean.