TECTONICS: 50 YEARS AFTER THE REVOLUTION

The Plate Tectonic Revolution that transformed Earth science has occurred together with revolutions in imagery and planetary studies. Earth's outer layer (lithosphere: upper mantle and crust) comprises relatively rigid plates ranging in size from near-global to kilometer-scale; boundaries can be sharp (a few kilometers wide) to diffuse (hundreds of kilometers), and are reflected in earthquake distribution. Divergent, transform fault, and convergent (subduction) margins are present at all scales. Collisions can occur between several crustal types and subduction zones of varying polarity. Modern plate processes and their geologic products permit inference of Earth's plate tectonic history in times before extant oceanic crust. Ophiolites provide an insight into the products and processes of oceanic crust formation. Ophiolite emplacement involves a tectonic process related to collision of crustal margins with subduction zones. The Earth's mantle comprises from top to bottom, the lithosphere, asthenosphere, mesosphere, and a "hot boundary layer". Plume-related magmatism may arise from bulges in the latter, which in turn may alternate with depressions caused by pronounced subduction occasioned by assembly of supercontinents. Plate tectonic activity probably occurred on an early Archean, or even Hadean, Earth. Earth-like plate tectonic activity seems not to be present on other terrestrial planets, although strike-slip faulting is present in Mars's Valles Marineris. Possible extensional and compressional tectonics on Venus and an inferred unimodal hypsographic curve for early Earth suggest that Venus may be a modern analogue for a young Earth.