When did plate tectonics begin?
Submitted by Julie Baldwin, The University of Montana
Plate tectonics is a feature that makes planet Earth unique in our solar system. However, there is controversy over how and when plate tectonics begin. Most workers agree that plate tectonics did not exist in the earliest stages of Earth's history. Therefore there was likely a period of time over which the process of plate tectonics was conceived and evolved. Some workers argue that plate tectonics begin in the Archean whereas others argue for a much later onset of plate-like behavior of the lithosphere. "Modern-style" plate tectonics may have evolved form some earlier proto-plate tectonic regime. This is evidenced in the nature of metamorphic rocks that suggest a hotter Earth in the Archean where plates did not subduct to the depths required to form rocks such as coesite-bearing eclogites. Subduction and seafloor spreading are primary drivers of plate motions and the warmer early Earth may have had a weaker, less dense lithosphere and thus mantle convection processes may have been different in the early Earth. Zircons as old as 4.4 Ga have recently been identified and indicate that continental crust was formed very early in Earth's history, but this is not necessarily clear evidence that plate tectonics and crustal recycling was taking place.
Arguments for a later onset of plate tectonics are based on the logic that plate tectonics could not begin until the crust was cool enough, and that and that very few ophiolites are preserved that are older than 1 Ga, indicating that modern style subduction may have started relatively late in Earth history. Blueschist rocks characteristic of subduction zones are generally less than about 800 Ma and the oldest UHP metamorphic rocks are around 620 Ma.
Key questions to resolve this debate include
- understanding the thermal history of the earth through time,
- understanding the important "plate drivers", and
- examining the secular distribution of distinctive rock types characteristic of plate tectonic processes.
The following answer is regarding Phanerozoic-style plate tectonicssubmitted by Staci Loewy, California State University at Bakersfield
This appears to be an actively debated question and was the subject of a recent Penrose Conference (June 2006). Meso- and Neoproterozoic orogens, such as the Grenville (and worldwide temporal equivalents, ca. 1 Ga) and Pan African/Brazilide (ca. 0.6 Ga) orogenies, contain strong evidence for subduction and collision, and thus lateral motion, of continents. The Grenville Orogen is commonly cited as a classic example of an ancient continent-continent collision. Paleomagnetic data from Meso- and Neoproterozoic rocks demonstrate that cratons have moved across the surface of the earth relative to one another. Paleoproterozoic (ca. 2.0 Ga) orogens, such as the Trans Hudson Orogen, preserve ophiolites, island arc assemblages and accretionary structures similar to those of later orogens. From as early as the Mesoarchean (ca. 3.0 Ga), the rock record contains features that resemble those formed during Phanerozoic plate tectonics, such as accretionary wedges.
That said, there are clearly significant differences between the rocks that formed during the Archean and those that form today. For example, granite and greenstone belts are a prominent feature of Archean terranes and are absent in younger crustal provinces. Certain rock types and/or lithologic associations appear to form more commonly within particular periods of geologic time. Presumably this temporal variation is related to the continuous cooling of the earth and consequent thickening of the lithosphere. If compositions of rocks have varied with time, then the processes, by which they form, namely plate tectonics, may also have evolved through time.
A corollary to the original question is "What processes existed prior to the establishment of lateral motion of rigid plates?"
References and other Resources
Brown M. 2006 Duality of thermal regimes is the distinctive characteristic of plate tectonics since the Neoarchean Geology 34 961-964.
Cawood P.A. Krner A. and Pisarevsky S. 2006 Precambrian plate tectonics: Criteria and evidence: GSA Today 16 no. 7 4-11.
Condie K.C. Krner A. and Stern R.J. 2006 When Did Plate Tectonics Begin: GSA Today 16 no. 10 40-41.
Davies G.F. 2006 Gravitational depletion of the early Earths upper mantle and the viability of early plate tectonics EPSL 376-382.
Harrison T. M. et al. Science 310 19471950 2005.
Witze A. 2006 The start of the world as we know it: Nature 442 128-131.
Stern R. J. Geology 33 557560 2005.