The impact of a large body in the oceans would inject large quantities of water through the tropopause cold trap into the stratosphere and lower mesosophere. We consider the consequences of enhanced water vapor concentrations on the middle atmosphere (50–100 km) chemistry and heat budget. The increased mixing ratio of hydrogen dramatically decreases the ozone concentration above 60 km. Catalytic reactions with odd hydrogen are the main sink of ozone in this region. The ozone reduction causes a lowering of the average height of the mesopause, as well as a lowering of the average temperature. The lower colder mesopause and the creation of saturation conditions over much of the upper mesosphere would have resulted in a permanent layer of mesospheric ice clouds of nearly world-wide extent. (At present, these exist only at high latitudes and are observed in summer as “noctilucent clouds.”) The globally-averaged albedo resulting from these clouds is dependent on the particulate size and shape, and can be as high as several percent, preferentially covering the summer hemisphere. This could have important implications for the short-term climate following a large-body impact. Similar effects would also result from an encounter with a more extended object such as a swarm of cosmic debris or a dense interstellar cloud.

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.

Plume tracks at the Earth's surface probably have various origins, such as wet spots, simple rifts, and shear heating. Because plate boundaries move relative to one another and relative to the mantle, plumes located on or close to them cannot be considered as reliable for establishing a reference frame. Using only relatively fixed intraplate Pacific hotspots, plate motions with respect to the mantle in two different reference frames, one fed from below the asthenosphere, and one fed by the asthenosphere itself, provide different kinematic results, stimulating opposite dynamic speculations. Plates move faster relative to the mantle if the source of hotspots is taken to be the middle-upper asthenosphere, because hotspot tracks would then not record the entire decoupling occurring in the low-velocity zone. A shallow intra-asthenospheric origin for hotspots would raise the Pacific deep-fed velocity from a value of 10 cm/year to a faster hypothetical velocity of ∼20 cm/year. In this setting, the net rotation of the lithosphere relative to the mesosphere would increase from a value of 0.4359°/m.y. (deep-fed hotspots) to 1.4901°/m.y. (shallow-fed hotspots). In this framework, all plates move westward along an undulated sinusoidal stream, and plate rotation poles are largely located in a restricted area at a mean latitude of 58°S. This reference frame seems more consistent with the persistent geological asymmetry that suggests a global tuning of plate motions related to Earth's rotation. Another significant result is that along east- or northeast-directed subduction zones, slabs move relative to the mantle in the direction opposed to the subduction, casting doubts on slab pull as the first-order driving mechanism of plate dynamics.