Editing Plate tectonics (section)

== Driving forces of plate motion ==
[[File:Global plate motion 2008-04-17.jpg|thumb|upright=1.35|Plate motion based on [[Global Positioning System]] (GPS) satellite data from NASA [http://sideshow.jpl.nasa.gov/mbh/series.html JPL]. Each red dot is a measuring point and vectors show direction and magnitude of motion.]]

Tectonic plates are able to move because of the relative density of [[oceanic lithosphere]] and the relative weakness of the [[asthenosphere]]. [[Earth's internal heat budget|Dissipation of heat from the mantle]] is the original source of the energy required to drive plate tectonics through convection or large scale upwelling and doming. As a consequence, a powerful source generating plate motion is the excess density of the oceanic lithosphere sinking in subduction zones. When the new crust forms at mid-ocean ridges, this oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes denser with age as it conductively cools and thickens. The greater [[density]] of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate movement. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.<ref>{{Cite web |last=Mendia-Landa |first=Pedro |title=Myths and Legends on Natural Disasters: Making Sense of Our World |url=http://www.yale.edu/ynhti/curriculum/units/2007/4/07.04.13.x.html |url-status=live |archive-url=https://web.archive.org/web/20160721160510/http://www.yale.edu/ynhti/curriculum/units/2007/4/07.04.13.x.html |archive-date=2016-07-21 |access-date=2008-02-05}}</ref>

=== Driving forces related to mantle dynamics ===
{{Main|Mantle convection}}
For much of the first quarter of the 20th century, the leading theory of the driving force behind tectonic plate motions envisaged large scale convection currents in the upper mantle, which can be transmitted through the asthenosphere. This theory was launched by [[Arthur Holmes]] and some forerunners in the 1930s<ref>{{Cite journal |last=Holmes |first=Arthur |author-link=Arthur Holmes |year=1931 |title=Radioactivity and Earth Movements |url=http://www.mantleplumes.org/WebDocuments/Holmes1931.pdf |url-status=live |journal=[[Transactions of the Geological Society of Glasgow]] |volume=18 |issue=3 |pages=559–606 |doi=10.1144/transglas.18.3.559 |s2cid=122872384 |archive-url=https://web.archive.org/web/20191009101823/http://www.mantleplumes.org/WebDocuments/Holmes1931.pdf |archive-date=2019-10-09 |access-date=2014-01-15}}</ref> and was immediately recognized as the solution for the acceptance of the theory as originally discussed in the papers of [[Alfred Wegener]] in the early years of the 20th century. However, despite its acceptance, it was long debated in the scientific community because the leading theory still envisaged a static Earth without moving continents up until the major breakthroughs of the early sixties.

Two- and three-dimensional imaging of Earth's interior ([[seismic tomography]]) shows a varying lateral density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this varying lateral density is [[mantle convection]] from buoyancy forces.{{sfn|Tanimoto|Lay|2000}}

How mantle convection directly and indirectly relates to plate motion is a matter of ongoing study and discussion in [[geodynamics]]. Somehow, this [[energy]] must be transferred to the lithosphere for tectonic plates to move. There are essentially two main types of mechanisms that are thought to exist related to the dynamics of the mantle that influence plate motion which are primary (through the large scale convection cells) or secondary. The secondary mechanisms view plate motion driven by friction between the convection currents in the asthenosphere and the more rigid overlying lithosphere. This is due to the inflow of mantle material related to the downward pull on plates in subduction zones at ocean trenches. Slab pull may occur in a geodynamic setting where basal tractions continue to act on the plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab). Furthermore, slabs that are broken off and sink into the mantle can cause viscous mantle forces driving plates through slab suction.

==== Plume tectonics ====
In the theory of [[plume tectonics]] followed by numerous researchers during the 1990s, a modified concept of mantle convection currents is used. It asserts that super plumes rise from the deeper mantle and are the drivers or substitutes of the major convection cells. These ideas find their roots in the early 1930s in the works of [[Vladimir Belousov|Beloussov]] and [[Reinout Willem van Bemmelen|van Bemmelen]], which were initially opposed to plate tectonics and placed the mechanism in a fixed frame of vertical movements. Van Bemmelen later modified the concept in his "Undation Models" and used "Mantle Blisters" as the driving force for horizontal movements, invoking gravitational forces away from the regional crustal doming.{{sfn|Van Bemmelen|1976}}{{sfn|Van Bemmelen|1972}}

The theories find resonance in the modern theories which envisage [[hotspot (geology)|hot spots]] or [[mantle plumes]] which remain fixed and are overridden by oceanic and continental lithosphere plates over time and leave their traces in the geological record (though these phenomena are not invoked as real driving mechanisms, but rather as modulators).

The mechanism is still advocated to explain the break-up of supercontinents during specific geological epochs.{{sfn|Segev|2002}} It has followers amongst the scientists involved in the [[Expanding Earth|theory of Earth expansion]].{{sfn|Maruyama|1994}}{{sfn|Yuen|Maruyama|Karato|Windley|2007}}{{sfn|Wezel|1988}}

==== Surge tectonics ====
Another theory is that the mantle flows neither in cells nor large plumes but rather as a series of channels just below Earth's crust, which then provide basal friction to the lithosphere. This theory, called "surge tectonics", was popularized during the 1980s and 1990s.{{sfn|Meyerhoff|Taner|Morris|Agocs|1996}} Recent research, based on three-dimensional computer modelling, suggests that plate geometry is governed by a feedback between mantle convection patterns and the strength of the lithosphere.{{sfn|Mallard|Coltice|Seton|Müller|2016}}

=== Driving forces related to gravity ===
Forces related to gravity are invoked as secondary phenomena within the framework of a more general driving mechanism such as the various forms of mantle dynamics described above. In modern views, gravity is invoked as the major driving force, through slab pull along subduction zones.

Gravitational sliding away from a spreading ridge is one of the proposed driving forces: plate motion is driven by the higher elevation of plates at ocean ridges.{{sfn|Spence|1987}}{{sfn|White|McKenzie|1989}} As oceanic lithosphere is formed at spreading ridges from hot mantle material, it gradually cools and thickens with age (and thus adds distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with increased distance from the ridge axis.

This force is regarded as a secondary force and is often referred to as "[[ridge push force|ridge push]]". This is a misnomer as there is no force "pushing" horizontally, indeed tensional features are dominant along ridges. It is more accurate to refer to this mechanism as "gravitational sliding", since the topography across the whole plate can vary considerably and spreading ridges are only the most prominent feature. Other mechanisms generating this gravitational secondary force include [[Lithospheric flexure|flexural bulging]] of the lithosphere before it dives underneath an adjacent plate, producing a clear topographical feature that can offset, or at least affect, the influence of topographical ocean ridges. [[Mantle plume]]s and hot spots are also postulated to impinge on the underside of tectonic plates.

[[Slab pull]]: Scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly cause motion by friction along the base of the lithosphere. Slab pull is therefore most widely thought to be the greatest force acting on the plates. In this understanding, plate motion is mostly driven by the weight of cold, dense plates sinking into the mantle at trenches.{{sfn|Conrad|Lithgow-Bertelloni|2002}} Recent models indicate that [[back-arc basin|trench suction]] plays an important role as well. However, the fact that the [[North American plate]] is nowhere being subducted, although it is in motion, presents a problem. The same holds for the African, [[Eurasian plate|Eurasian]], and [[Antarctic plate|Antarctic]] plates.

Gravitational sliding away from mantle doming: According to older theories, one of the driving mechanisms of the plates is the existence of large scale asthenosphere/mantle domes which cause the gravitational sliding of lithosphere plates away from them (see the paragraph on Mantle Mechanisms). This gravitational sliding represents a secondary phenomenon of this basically vertically oriented mechanism. It finds its roots in the Undation Model of [[Reinout Willem van Bemmelen|van Bemmelen]]. This can act on various scales, from the small scale of one island arc up to the larger scale of an entire ocean basin.{{sfn|Spence|1987}}{{sfn|White|McKenzie|1989}}{{sfn|Segev|2002}}

=== Driving forces related to Earth rotation ===
[[Alfred Wegener]], being a [[meteorologist]], had proposed [[tidal force]]s and [[centrifugal force]]s as the main driving mechanisms behind [[continental drift]]; however, these forces were considered far too small to cause continental motion as the concept was of continents plowing through oceanic crust.<ref>{{Cite web |title=Alfred Wegener (1880–1930) |url=http://www.ucmp.berkeley.edu/history/wegener.html |url-status=dead |archive-url=https://web.archive.org/web/20171208011353/http://www.ucmp.berkeley.edu/history/wegener.html |archive-date=2017-12-08 |access-date=2010-06-18 |publisher=[[University of California Museum of Paleontology]]}}</ref> Therefore, Wegener later changed his position and asserted that convection currents are the main driving force of plate tectonics in the last edition of his book in 1929.

However, in the plate tectonics context (accepted since the [[seafloor spreading]] proposals of Heezen, Hess, Dietz, Morley, Vine, and Matthews (see below) during the early 1960s), the oceanic crust is suggested to be in motion ''with'' the continents, which caused the proposals related to Earth rotation to be reconsidered. In more recent literature, these driving forces are:
# Tidal drag due to the gravitational force the [[Moon]] (and the [[Sun]]) exerts on the crust of Earth.<ref>{{Cite web |last=Neith, Katie |date=April 15, 2011 |title=Caltech Researchers Use GPS Data to Model Effects of Tidal Loads on Earth's Surface |url=http://media.caltech.edu/press_releases/13411 |url-status=dead |archive-url=https://web.archive.org/web/20111019023322/http://media.caltech.edu/press_releases/13411 |archive-date=October 19, 2011 |access-date=August 15, 2012 |publisher=[[California Institute of Technology|Caltech]]}}</ref>
# Global deformation of the [[geoid]] due to small displacements of the rotational pole with respect to Earth's crust.
# Other smaller deformation effects of the crust due to wobbles and spin movements of Earth's rotation on a smaller timescale.

Forces that are small and generally negligible are:

# The [[Coriolis force]].<ref name="Ricard">{{Cite encyclopedia |year=2009 |title=Treatise on Geophysics: Mantle Dynamics |publisher=Elsevier Science |last=Ricard |first=Y. |editor-last=Bercovici |editor-first=David |volume=7 |page=36 |isbn=978-0-444-53580-1 |chapter=2. Physics of Mantle Convection |editor2-last=Schubert |editor2-first=Gerald |chapter-url=https://books.google.com/books?id=bIHNCgAAQBAJ}}</ref><ref name="Glatzmaier2013">{{Cite book |last=Glatzmaier |first=Gary A. |url=https://books.google.com/books?id=RY-GAAAAQBAJ&pg=PR4 |title=Introduction to Modeling Convection in Planets and Stars: Magnetic Field, Density Stratification, Rotation |publisher=[[Princeton University Press]] |year=2013 |isbn=978-1-4008-4890-4 |page=149}}</ref>
# The [[centrifugal force]], which is treated as a slight modification of gravity.<ref name=Ricard/><ref name=Glatzmaier2013/>{{rp|249}}

For these mechanisms to be overall valid, systematic relationships should exist all over the globe between the orientation and kinematics of deformation and the geographical [[latitude|latitudinal]] and [[longitude|longitudinal]] grid of Earth itself. Systematic relations studies in the second half of the nineteenth century and the first half of the twentieth century underlined exactly the opposite: that the plates had not moved in time, that the deformation grid was fixed with respect to Earth's [[equator]] and axis, and that gravitational driving forces were generally acting vertically and caused only local horizontal movements (the so-called pre-plate tectonic, "fixist theories"). Later studies (discussed below on this page), therefore, invoked many of the relationships recognized during this pre-plate tectonics period to support their theories (see reviews of these various mechanisms related to Earth rotation in the work of van Dijk and collaborators).<ref>{{harvnb|van Dijk|1992}}, {{harvnb|van Dijk|Okkes|1990}}.</ref>

==== Possible tidal effect on plate tectonics{{anchor|Tidal effect}} ====
{{see also|Tidal triggering of earthquakes}}

Of the many forces discussed above, tidal force is still highly debated and defended as a possible principal driving force of plate tectonics. The other forces are only used in global geodynamic models not using plate tectonics concepts (therefore beyond the discussions treated in this section) or proposed as minor modulations within the overall plate tectonics model.
In 1973, George W. Moore{{sfn|Moore|1973}} of the [[United States Geological Survey|USGS]] and R. C. Bostrom{{sfn|Bostrom|1971}} presented evidence for a general westward drift of Earth's lithosphere with respect to the mantle, based on the steepness of the subduction zones (shallow dipping towards the east, steeply dipping towards the west). They concluded that tidal forces (the tidal lag or "friction") caused by Earth's rotation and the forces acting upon it by the Moon are a driving force for plate tectonics. As Earth spins eastward beneath the Moon, the Moon's gravity ever so slightly pulls Earth's surface layer back westward, just as proposed by Alfred Wegener (see above). Since 1990 this theory has been mainly advocated by Doglioni and co-workers {{Harv|Doglioni|1990}}, such as in a more recent 2006 study,{{sfn|Scoppola|Boccaletti|Bevis|Carminati|2006}} where scientists reviewed and advocated these ideas. It has been suggested in {{Harvtxt|Lovett|2006}} that this observation may also explain why [[Venus]] and [[Mars]] have no plate tectonics, as Venus has no moon and Mars' moons are too small to have significant tidal effects on the planet. In a paper by Torsvik et al.,{{sfn|Torsvik|Steinberger|Gurnis|Gaina|2010}} it was suggested that, on the other hand, it can easily be observed that many plates are moving north and eastward, and that the dominantly westward motion of the Pacific Ocean basins derives simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces). In the same paper the authors admit, however, that relative to the lower mantle, there is a slight westward component in the motions of all the plates. They demonstrated though that the westward drift, seen only for the past 30&nbsp;Ma, is attributed to the increased dominance of the steadily growing and accelerating Pacific plate. The debate is still open, and a 2022 paper by Hofmeister et al.{{Sfn|Hofmeister|Criss|Criss|2022}} revived the idea of the interaction between the Earth's rotation and the Moon as the main driving force for plate movement.

===Role of water===
The role of water has been proposed to be crucial in plate tectonics on Earth.<ref>{{cite journal | url=https://ui.adsabs.harvard.edu/abs/2001AGUFM.U21A..09S/abstract | bibcode=2001AGUFM.U21A..09S | title=The role of liquid water in maintaining plate tectonics and the regulation of surface temperature | last1=Solomatov | first1=V. S. | journal=AGU Fall Meeting Abstracts | date=2001 | volume=2001 }}</ref><ref>{{cite journal | pmc=5394257 | date=2017 | last1=Tikoo | first1=S. M. | last2=Elkins-Tanton | first2=L. T. | title=The fate of water within Earth and super-Earths and implications for plate tectonics | journal=Philosophical Transactions. Series A, Mathematical, Physical, and Engineering Sciences | volume=375 | issue=2094 | doi=10.1098/rsta.2015.0394 | pmid=28416729 | bibcode=2017RSPTA.37550394T }}</ref><ref>{{cite web |url=https://phys.org/news/2013-06-lubricant-reassessment-role-plate-tectonics.html |title=Water is no lubricant: Reassessment of the role of water in plate tectonics |author=Helmholtz Association of German Research Centres |website=Phys.org |date=12 June 2013 |access-date=13 January 2025}}</ref>

=== Relative significance of each driving force mechanism ===
The [[Euclidean vector|vector]] of a plate's motion is a function of all the forces acting on the plate; however, therein lies the problem regarding the degree to which each process contributes to the overall motion of each tectonic plate.

The diversity of geodynamic settings and the properties of each plate result from the impact of the various processes actively driving each individual plate. One method of dealing with this problem is to consider the relative rate at which each plate is moving as well as the evidence related to the significance of each process to the overall driving force on the plate.

One of the most significant correlations discovered to date is that lithospheric plates attached to downgoing (subducting) plates move much faster than other types of plates. The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than the plates of the Atlantic basin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates. It is thus thought  that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces which determine the motion of plates, except for those plates which are not being subducted.{{sfn|Conrad|Lithgow-Bertelloni|2002}} This view however has been contradicted by a recent study which found that the actual motions of the Pacific plate and other plates associated with the East Pacific Rise do not correlate mainly with either slab pull or slab push, but rather with a mantle convection upwelling whose horizontal spreading along the bases of the various plates drives them along via viscosity-related traction forces.<ref name="Rowley-etal_2016">{{Cite journal |last1=Rowley |first1=David B. |last2=Forte |first2=Alessandro M. |last3=Rowan |first3=Christopher J. |last4=Glišović |first4=Petar |last5=Moucha |first5=Robert |last6=Grand |first6=Stephen P. |last7=Simmons |first7=Nathan A. |year=2016 |title=Kinematics and dynamics of the East Pacific Rise linked to a stable, deep-mantle upwelling |journal=[[Science Advances]] |volume=2 |issue=12 |page=e1601107 |bibcode=2016SciA....2E1107R |doi=10.1126/sciadv.1601107 |pmc=5182052 |pmid=28028535}}</ref> The driving forces of plate motion continue to be active subjects of on-going research within [[geophysics]] and [[tectonophysics]].