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 at 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, leading to 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.

Catastrophic and unusual events on Earth such as bolide impacts, megafloods, supereruptions, flood volcanism, and subice volcanism may have devastating effects when they occur. Although these processes have unique characteristics and form distinctive features and deposits, we have difficulties identifying them and measuring the magnitude of their effects. Our difficulties with interpreting these processes and identifying their consequences are understandable considering their infrequency on Earth, combined with the low preservation potential of their deposits in the terrestrial rock record. Although we know these events do happen, they are infrequent enough that the deposits are poorly preserved on the geologically active face of the Earth, where erosion, volcanism, and tectonism constantly change the surface. Unlike the Earth, on Mars catastrophic and unusual features are well preserved because of the slow modification of the surface. Significant precipitation has not occurred on Mars for billions of years and there appears to be no discrete crustal plates to have undergone subduction and destruction. Therefore the ancient surface of Mars preserves geologic features and deposits that result from these extraordinary events. Also, unlike the other planets, Mars is the most similar to our own, having an atmosphere, surface ice, volcanism, and evidence of onceflowing water. So although our understanding of precursors, processes, and possible biological effects of catastrophic and unusual processes is limited on Earth, some of these mysteries may be better understood through investigating the surface of Mars.