9.06 - 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 to liquid magma in a few t-housand years following the Moon-forming impact, ∼4.5 billion years ago. The liquid magma convected vigorously and cooled rapidly, cooling the Earth in a few million years 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 geodynamical 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 geological preservation cloaks tectonic processes on the early Earth. A few detrital zircons indicate chemically isolated continent-like reservoirs by ∼4.5Ga shortly after the Moon-forming impact and survival of some zircons by ∼4.4Ga. Clusters and gaps in detrital zircon age populations indicate periods of activity and stability, like on the modern Earth. The gaps are comparable ∼108 years to those in modern active regions. Plate-like processes became evident as soon as there was a geological 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 hot mantle is difficult to subduct into mantle asthenosphere, like with modern oceanic plateaus. Periods of sluggish plate tectonics with the interior heating up may have alternated with vigorous cooling by short-lived mush oceans. Alternatively, the tectonics evolved gradually from basaltic plates above mush oceans to standard plates with mantle lithosphere throughout the Archean. The geological record after 3.0Ga 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 3.0Ga was comparable to that of the present. However, limited data indicate short-lived episodes of rapid plate motion in the Archean.