When Did Plate Tectonics Begin?

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When did plate tectonics begin?
The plate tectonic paradigm has been the dominant model for understanding the solid Earth for over 40 years. However, although the model is hugely successful, there is still great uncertainty as to when the plate tectonic process began. Two recent papers have highlighted this difficulty by proposing two very different start times for platetectonics. One model argues that plate tectonic processes took over from an earlier (unspecified) tectonic regime in a hotter, younger Earth at 1.0 Ga, whereas the other proposes a much earlier start at 4.4–4.5 Ga, and within a 100 Ma of planetary accretion. This feature discusses the evidence for the early and late start hypotheses and argues for a middle position in which plate tectonic processesbegan during the Archaean (> 2.5 Ga ago).
Plate tectonics has been the dominant paradigm for the solid Earth since the early 1960s. Over the intervening years many of the details of this way of understanding our Earth have been clarified, with the exception of one important parameter—when the whole process started. This is not a trivial question, for we do not find evidence for plate tectonics onother terrestrial planets—indicating that a silicate planet does not automatically generate plate tectonics. In its early days, the plate tectonic model rapidly gained acceptance because of its ability to provide a unifying model for the workings of the modern Earth. In a short period of time, it also began to provide explanations for assemblages of rocks in the geological record, indicating thatmodern-style processes had operated for a significant time on our planet. But how far back can we go? Was there a time before plate tectonics when a different tectonic regime operated on Earth? The answer to this question is almost certainly ‘yes’, for there is evidence that the Earth’s mantle was much hotter when the planet was young. One of the consequences of a hot mantle in the young Earthwould be that the ocean crust was probably thicker. If thicker, then more buoyant and so less likely to subduct—implying a style of global tectonics that was different from the modern era. So when exactly modern-style plate tectonics took over from an earlier style of tectonics is a serious scientific question, and one for which there should be answers in the rock record. This question recentlyreemerged as a result of two provocative papers representing, as it happens, completely contrasting points of view. Bob Stern of the University of Dallas has recently suggested that plate tectonics began late in Earth history during the Neoproterozoic—i.e. within the last 1000 million years. In contrast, Mark Harrison of the University of California has argued that some sort of plate tectonics wasworking as early as 4.4–4.5 billion years ago. Clearly, both authors cannot be right, but, as is the way with science, their conflicting viewpoints have set the stage for an important debate.

Hugh Rollinson
Department of Earth Sciences, Sultan Qaboos University, Box 36, Muscat, Postal Code Al-Khodh-123, Sultanate of Oman. hrollin@squ.edu.om

The evidence for a late start to plate tectonicsCentral to this discussion is how we recognize plate tectonics in the geological record. Bob Stern argued that the key indicators are those that are diagnostic of the process of subduction. Thus he identified three key rock assemblages preserved in the geological record—ophiolites, blueschists (metamorphic rocks containing a diagnostic blue amphibole) and ultrahigh pressure metamorphic terrains—asessential indicators of former plate tectonics. Ophiolites are fragments of oceanic crust that have been preserved within the continental crust through collision tectonics (Fig. 1). Whereas they used to be regarded as fragments of an ocean ridge, many are now regarded as having formed in back-arc basins in a subduction setting. Either way, they indicate plate tectonic processes. Ophiolites are...