Plate tectonics: A deep time and planetary perspective

John Weber, Geology Department, Grand Valley State University

James Lawrence Powell, in his poignant and insightful popular science book, Mysteries of Terra Firma, lists and discusses the three big ideas (paradigms) in Earth science: time (age of the Earth), drift (plate tectonics), and chance (impact). Perhaps a fourth, ice ages and global climate change, could be added.

Much to our surprise, and after much exploration and expectation – e.g., we had expected to find evidence for plate tectonics on Venus, – it turns out that Earth is the only planet in the solar system on which plate tectonics occurs, today. Some interesting questions arise: When did plate tectonics get started on planet Earth, and how long has it been operating? If not always plate tectonics, what other kinds of tectonics operated on Earth? Why do all of the other terrestrial planets have single-plates and what has been called "stagnant lid" tectonics? Could these neighbors have had plate tectonics in the past? What about geology and planetary history might help us address these questions?

Small objects (< 200 km in diameter) in the solar system (e.g., Phobos, Gaspra) are lumpy and have irregular shapes. The nearly perfectly spherical shapes of planets and other objects in the solar system that are larger than ~200 km indicates that they must have initially formed as completely molten drops of liquid.

An early Earth (Hadean; 4.6-3.8 billion years ago; no record preserved) with a magma ocean is thus inferred, on which the presence of plates and plate tectonics would not have been possible. The ubiquitous record of sunken greenstone belts and diapirically emplaced TTG (tonalite-trondhjemite-granite intrusive igneous rocks) suites in rocks 3.8-2.5 billion years old can be taken to infer that the Archean Earth may have supported a largely vertical style of tectonics. Mesoproterozoic (1.6 billion years ago) rocks, including passive margin sequences, blueschists, UHP (ultra-high pressure) rocks, and ophiolites, provide the first convincing record that can easily be related to modern plate tectonic-like processes. Thus, Proterozoic and Phanerozoic (< 1.6 billion years ago) Earth tectonics were probably much like modern plate tectonics.

As originally formulated in 1968, Earth's rigid plate motion is measured using transform fault azimuths, seafloor magnetic "zebra" stripes, and earthquake slip vectors. These features average relative plate motion across plate boundaries over the past 1-2 million years. With few exceptions, plate motions measured using space geodetic techniques, like GPS (the Global Positioning System), average plate motion over just the past several decades, and match those (at > 95% confidence) measured using the longer-term geological data.

Earth's plates are probably driven by subducted oceanic slabs that cool, sink, and pull the plates around. The presence of water on Earth may be the key ingredient that caused plate tectonics here.

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