Editing Plate tectonics (section)

== Key principles ==
{{More citations needed section|date=July 2021}}

The [[Structure of Earth|outer layers of Earth]] are divided into the [[lithosphere]] and [[asthenosphere]]. The division is based on differences in [[mechanical properties]] and in the method for [[heat transfer|the transfer of heat]]. The lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by [[heat conduction|conduction]], whereas the asthenosphere also transfers heat by [[convection]] and has a nearly [[Adiabatic process|adiabatic]] temperature gradient. This division should not be confused with the ''chemical'' subdivision of these same layers into the mantle (comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure.

The key principle of plate tectonics is that the lithosphere exists as separate and distinct [[list of tectonic plates|tectonic plates]], which ride on the [[Viscoelasticity|viscoelastic]] asthenosphere. Plate motions range from {{convert|10|to|40|mm/year|in/year|1}} at the [[Mid-Atlantic Ridge]] (about as fast as [[fingernail]]s grow), to about {{convert|160|mm/year|in/year}} for the [[Nazca plate]] (about as fast as [[hair]] grows).{{sfn|Zhen Shao|1997}}{{sfn|Hancock|Skinner|Dineley|2000}}

Tectonic lithospheric plates consist of lithospheric mantle overlain by one or two types of crustal material: [[oceanic crust]] (in older texts called ''[[sima (geology)|sima]]'' from [[silicon]] and [[magnesium]]) and [[continental crust]] (''[[sial]]'' from silicon and [[aluminium]]). The distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers. Continental crust is formed through [[volcanic arc|arc volcanism]] and [[accretion (geology)|accretion]] of [[terrane]]s through plate tectonic processes. Oceanic crust is denser than continental crust because it has less silicon and more of the [[mafic|heavier elements]] than [[felsic|continental crust]].{{sfn|Schmidt|Harbert|1998}}<ref>{{Cite book |last=McGuire |first=Thomas |title=Earth Science: The Physical Setting |date=2005 |publisher=AMSCO School Publications Inc. |isbn=978-0-87720-196-0 |pages=182–184 |chapter=Earthquakes and Earth's Interior}}</ref> As a result of this density difference, oceanic crust generally lies below [[sea level]], while continental crust [[isostasy|buoyantly projects]] above sea level.

Average oceanic lithosphere is typically {{convert|100|km|0|abbr=on}} thick.{{sfn|Turcotte|Schubert|2002|p=5}} Its thickness is a function of its age. As time passes, it cools by conducting heat from below, and releasing it radiatively into space. The adjacent mantle below is cooled by this process and added to its base. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed. For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about {{convert|6|km|0|abbr=on}} thick at mid-ocean ridges to greater than {{convert|100|km|0|abbr=on}} at [[subduction]] zones. For shorter or longer distances, the subduction zone, and therefore also the mean, thickness becomes smaller or larger, respectively.{{sfn|Turcotte|Schubert|2002}} Continental lithosphere is typically about {{convert|200|km|abbr=on}} thick, though this varies considerably between basins, mountain ranges, and stable [[craton]]ic interiors of continents.

The location where two plates meet is called a ''plate boundary''. Plate boundaries are where geological events occur, such as [[earthquake]]s and the creation of topographic features such as [[mountain]]s, [[volcano]]es, [[mid-ocean ridge]]s, and [[oceanic trench]]es. The vast majority of the world's active volcanoes occur along plate boundaries, with the Pacific plate's [[Pacific Ring of Fire|Ring of Fire]] being the most active and widely known. Some volcanoes occur in the interiors of plates, and these have been variously attributed to internal plate deformation{{sfn|Foulger|2010}} and to mantle plumes.

Tectonic plates may include continental crust or oceanic crust, or both. For example, the [[African plate]] includes the continent and parts of the floor of the [[Atlantic Ocean|Atlantic]] and [[Indian Ocean|Indian]] Oceans.

Some pieces of oceanic crust, known as [[ophiolite]]s, failed to be subducted under continental crust at destructive plate boundaries; instead, these oceanic crustal fragments were pushed upward and were preserved within continental crust.