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Plate tectonics and convection of the solid, rocky mantle are responsible for transporting heat out of Earth. However, the physics of plate tectonics is poorly understood; other planets do not exhibit it. Recent seismic evidence for convection and mixing throughout the mantle seems at odds with the chemical composition of erupted magmas requiring the presence of several chemically distinct reservoirs within the mantle. There has been rapid progress on these two problems, with the emergence of the first self-consistent models of plate tectonics and mantle convection, along with new geochemical models that may be consistent with seismic and dynamical constraints on mantle structure.
Subsolidus (1) convection of the rocky, 2900-km-thick mantle of Earth (2) is the driving mechanism for plate tectonics and associated geological activity on the surface of our planet, including continental drift, earthquakes, volcanoes, and mountain building (3). Mantle convection and plate tectonics are one system, because oceanic plates are the cold upper thermal boundary layer of the convection. The slow motion of plates and the mantle is powered by radiogenic heating and by the slow cooling of our planet over its 4.5-billion-year history (4). Mantle convection and plate tectonics provide the central framework linking the subdisciplines of solid Earth science, including geochemistry, seismology, mineral physics, geodesy, tectonics, and geology. A successful model must thus satisfy constraints from all of these fields. For example, seismic waves provide a direct probe of structures inside Earth, chemical analyses of erupted lavas and other volcanic products give information about different compositions that exist in the mantle, and laboratory experiments determine the properties and deformation mechanisms of rocks at the high pressures (<= 136 GPa) and temperatures (<=4000 K) of Earth's mantle.
Despite three decades of research, some first-order questions regarding the plate-- mantle system remain largely unresolved.
The first of these is why Earth developed plate tectonics at all, given that other terrestrial planets such as Mars and Venus do not currently exhibit this behavior (3). The problem is complex because rocks exhibit many different deformation mechanisms, ranging from brittle failure to viscous creep, depending on the pressure, temperature, differential stress, and past history of deformation (5, 6), and many of these mechanisms are not well quantified. The second major question is how the differing...