6.02 – Plate Tectonics

Plate tectonics, our major paradigm for how the Earth works, was established in the 1960s following decades of observational research that culminated in key discoveries such as geomagnetic reversals, mid-ocean ridges, transform faults, and seafloor spreading; collectively these insights gave rise to the ‘new global tectonics’ or theory of plate tectonics. The motions of plates relative to each other and the mantle beneath have been the subjects of numerous investigations. The history of relative plate motions is inferred from the mapping of marine magnetic anomalies and seafloor morphology using implementations of Euler’s theorem for rotations on a sphere. The procedures for determining optimal plate rotation parameters were developed in the 1980s when the criterion for fitting of conjugate features was established and it was realized that the uncertainties in plate rotations were best expressed as small perturbations to the optimal rotation. Using improved observations of seafloor morphology derived from multibeam bathymetry and satellite gravity combined with better coverage of magnetic anomalies have allowed researchers to improve the resolution of relative plate motion models, revealing departures from the first-order rigid plate approximation. Such departures are now routinely interpreted as diffuse plate boundaries. Absolute plate motions typically refer to motion relative to a fixed mantle, and the most prominent mantle reference point has been the proposed fixed hot spot reference frame. This reference frame is derived from the geometries of dated hot spot islands and seamount chains. The distribution of seamounts has been used to assess the origins of intraplate volcanism, and it seems clear this distribution consists of contributions from small seamounts produced in a near- or on-ridge environment, larger seamounts associated with island arcs, and a range of sizes associated with numerous hot spots. The methodologies for determining absolute plate motions have long lacked the rigor of the corresponding techniques employed in studies of relative plate motions, but recently proposed methods are rapidly closing the gap and allowing results from relative and absolute plate motion studies to be combined with rigorous error propagation. The foundation of this absolute reference frame, that is, the fixed hot spots, has been the focus of much research during the last years. Paleolatitudes of the Emperor seamount chain have been inferred from the frozen paleomagnetic field and these imply a birthplace significantly further north than the present location of the Hawaiian hot spot. The most logical conclusion is that the Hawaiian plume must have been further north in the past and subsequently drifted south. The extent to which such drift contributed to the prominent Hawaii–Emperor bend, now believed to have formed around 47–50 Ma (Chron 21–22), is the topic of ongoing research. Plate tectonics is believed to be the surface manifestation of mantle convection, but the large contrast in viscosities between the lithosphere and the deeper mantle leads to departures from a simple boiling pot analogy. The forces that act on the plates continue to be vigorously examined, with both observational and theoretical inferences pointing to plate boundary forces as the most significant, such as slab pull and ridge push. Despite the progress over the last several decades, the field of plate tectonics continues to be relatively data starved. Significant improvements in the quality and quantity of dating (both of seamounts and seafloor) and geometrical imaging (both with increased multibeam coverage as well as higher-resolution altimetry), in conjunction with how scientists employ modern information technology principles to organize and distribute both their data and models, will be instrumental in the quest for new breakthroughs in the future.