SP E C I A L F E A T U R E : E AR TH

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Why does plate tectonics occuronly on Earth?Paula Martin, Jeroen van Hunen, Stephen Parman andJon Davidson

Department of Earth Sciences, Durham University, Science Laboratories, South Road,Durham DH1 3LE, UK

E-mail: paula.martin@durham.ac.uk, jeroen.van-hunen@durham.ac.uk,stephen.parman@durham.ac.uk and j.p.davidson@durham.ac.uk

AbstractPlate tectonics governs the topography and motions of the surface of Earth,and the loss of heat from Earth’s interior, but appears to be found uniquely onEarth in the Solar System. Why does plate tectonics occur only on Earth?This is one of the major questions in earth and planetary sciences research,and raises a wide range of related questions: has plate tectonics ever occurredon other planets in the past? How did plate tectonics start on Earth? Will itever end? In the absence of plate tectonics, how do planets lose their heat?This article provides a brief introduction to the ways in which planets losetheir heat and discusses our current understanding of plate tectonics and thechallenges that lie ahead.

IntroductionPlate tectonics governs the nature and shape of thesurface of Earth, from ocean basins to mountainranges. It also governs the motions of the surfaceof Earth, providing a range of natural hazards suchas earthquakes and volcanic eruptions. It is afamiliar component of the National Curriculum,and a major field of on-going scientific research.

This article focuses on the five largest silicatebodies in the Solar System, namely Mercury,Venus, Earth, the Moon and Mars, collectivelyreferred to as ‘terrestrial bodies’ (figure 1).Terrestrial bodies can be thought of as a seriesof approximately spherical layers, defined eitherchemically or mechanically. For example, startingat the centre and working outwards, Earth ischemically composed of an inner core, outer core,mantle, and crust; it is mechanically composedof an inner core, outer core, lower mantle, uppermantle, asthenosphere and lithosphere (figure 2).

The lithosphere is composed of the crust and therigid uppermost part of the mantle, and is the‘plate’ of plate tectonics. Although it is alsosolid, in contrast to the rigid lithosphere, theunderlying asthenosphere is plastic (i.e. it can flowon geological timescales).

Plate tectonics only occurs on Earth. Wedo not know exactly why. We have looked forplate tectonics on all of the other terrestrial bodiesin the Solar System (i.e. terrestrial planets andsatellites), and found that it is unique to Earth. Thisis puzzling. Why should this process be uniqueto Earth? How did it get started? Will it everstop? Why does it not happen anywhere else?This article begins with a consideration of howplanets lose their heat, putting plate tectonics intothe larger context, followed by a brief summary ofwhat we do and do not know about plate tectonics,and ends with a look at how we hope to find outmore about plate tectonics in the future.

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Why does plate tectonics occur only on Earth?

Figure 1. The silicate bodies of the Solar System (Mercury, Venus, Earth, the Moon and Mars). Image courtesyNASA/JPL-Caltech.

How do planets lose their heat?Plate tectonics is the primary mechanism throughwhich Earth loses its heat. This raises the question:in the absence of plate tectonics, how do theother terrestrial bodies lose their heat? Terrestrialbodies are generally thought to have been initiallyhot, and gradually cooling, with many planetaryprocesses (e.g. volcanism and tectonism) beingdriven by this cooling. The sources of heat withinplanetary bodies can be categorized as eitherprimordial (i.e. inherited from processes occurringduring formation) or the result of radioactivedecay. Heat is transferred within planetary bodiesand eventually lost to space through a combinationof convection, conduction and radiation. Differentmethods of heat loss dominate in the differentlayers of planetary bodies, and at the boundariesbetween these layers. For example, it is estimatedthat every year Earth loses 4.2 × 1013 W, or42 TW, of heat: 32 TW conducted through thelithosphere, and up to 10 TW lost by, for example,hydrothermal activity at mid-ocean ridges [1].

There are three primary modes of planetarycooling: magma ocean, stagnant lid, and platetectonics. Regardless of the mode of planetarycooling, all bodies lose heat from their surfaceto some degree via radiation. All terrestrialbodies are thought to undergo a short-livedmagma ocean stage early in their evolution.The name ‘magma ocean’ refers to the stagewhen a body is so hot that the surface is

partially or largely molten, and heat loss fromthe surface is primarily through small-scaleconvection (figure 3). When a body has cooledsufficiently, the surface solidifies and the commonmode of heat loss is stagnant lid behaviour, whereheat loss from the surface is primarily throughconduction (although there could probably alsobe significant heat loss through widespread large-scale extrusive volcanism during this stage). It ispossible that other intermediate stages might haveexisted between the magma ocean and stagnantlid stages. These intermediate stages wouldprobably have been on a significantly smaller scalethan the current plate tectonics regime, and mayhave involved, for example, a relatively mushylithosphere that could deform and subsequentlyform small-scale downwellings or drips in contrastto the large-scale downwellings (subduction)associated with plate tectonics. Alternativelyto the stagnant lid regime, if the conditionsare appropriate, a body may begin to loseheat via plate tectonics. It is theoreticallypossible that a body may alternate between astagnant lid regime and a plate tectonics regime;this has never been observed, but the lack ofobservation may simply be a reflection of thelong timescales involved. Ultimately, whenthey have become sufficiently cool, the fate ofall terrestrial bodies is to continue to cool byconduction alone; they may then be considered tobe inactive or dead (i.e. lacking in any force to

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drive planetary processes such as volcanism andtectonism).

What are the conditions necessary for platetectonics? This question may be thought of asa Goldilocks problem: everything needs to bejust right. First, the planetary body in questionmust have cooled sufficiently so that it is toocold to sustain a magma ocean. Second, thereneeds to be sufficient heat within the interior ofthe body to prevent the existence of a stagnantlid, i.e. sufficient heat to maintain convectionwithin the upper layers of the body. Third,the lithosphere needs to be cool enough, denseenough, strong enough and thin enough to subduct.Finally, probably the most important ingredientfor successful plate tectonics is liquid water,which is readily available only on Earth, noton the other terrestrial bodies. This too is aGoldilocks problem: the Earth may be at justthe right distance from the Sun to have a surfacetemperature between 0 and 100 ◦C, and thereforebe a stable environment for liquid water. So far,all of the necessary conditions for plate tectonicshave been found together only on Earth. In thenext section we discuss our current understanding

Figure 3. Artist’s conception of a planetary magmaocean. Image courtesy NASA/JPL-Caltech.

of plate tectonics, based on our only observedexample: Earth.

What do we know about plate tectonics onEarth?Plate tectonics is a theory that has been developedto explain the observed evidence for large-scalemotions of Earth’s lithosphere. The developmentof the theory of plate tectonics, including the

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combination of concepts such as continental driftand seafloor spreading, is a very interestingillustration of how science works. A simpleintroduction to the development of the theory ofplate tectonics can be found in a variety of books;see, for example, [3]. A comprehensive review,including a wide selection of papers describingthe development of the theory and the personalstories of the scientists involved, written by thescientists themselves, is given in Plate Tectonics:An Insider’s History of the Modern Theory of theEarth, edited by Oreskes [4].

On Earth, the lithosphere is divided intorigid plates, separated by linear features thatare identified by their appearance on mapsshowing the locations of major tectonic events(e.g. earthquakes) and topographic features such asmountain chains, volcanoes and oceanic trenches,as illustrated in figure 4 [5]. There are atotal of seven major tectonic plates, and severalminor tectonic plates on Earth, which all movein relation to one another at typical rates of afew centimetres per year. The plate boundariesmay be categorized into one of three types:convergent or destructive boundaries, divergentor constructive boundaries, and transform orconservative boundaries.

Tectonic plates are created at mid-oceanridges (where a gap is continuously renewedwhen two plates move away from each other)and destroyed at subduction zones where theplates sink into the mantle. At the constructiveboundaries (mid-ocean ridges), melting resultsin new buoyant, basaltic crust, which today istypically about 7 km thick. This crust andunderlying mantle material quickly lose their heatto the surface, and become the lithosphere. Thislithosphere continues to cool, and becomes thickerand denser. After about 20 million years ofcooling, the lithosphere is already denser than theunderlying mantle, and ‘ready’ to sink down. Thissinking (called subduction), however, has to bepostponed until the plate meets another one at asubduction zone. The lithosphere does not simplysink under gravity when it is sufficiently densebecause of a variety of other factors, includingthe energy required to bend the plate, and thefact that it is often attached in some way tosomething else (for example, the cold, dense edgesof the oceanic plates in the North Atlantic areattached to the buoyant continental plates that form

Europe and North America). When two platesdo meet at a subduction zone, one plate bendsdown below the other into the mantle, and itshigh density (from cooling, and further increasedby the transformation of basaltic crust to muchdenser eclogite below 40 km depth) will providethe gravitational force to sink further down. Thissinking plate (called the ‘slab’) pulls the attachedplate at the surface towards the subduction zone,and this process is called ‘slab pull’. Slab pullis the dominant driving force of plate tectonics,providing 90% of the force required to drive theplate tectonic process (figure 5) (Stern, 2007).

Water plays a dominant role in the totalprocess of plate tectonics: for example, bylubricating the sliding of tectonic plates past eachother at subduction zones; by rapidly coolingtectonic plates near the ridges by hydrothermalcirculation; by speeding up the transformationof basalt to eclogite; and by facilitating bendingof plates into the subduction zone by hydrousweakening and chemical alteration in bendingcracks.

What do we not know about platetectonics?There is still a lot that we do not know about platetectonics. For example:

• How and when did plate tectonics start on theEarth? Did it simply ‘turn on’, or was there a‘spluttering’ period when it started andstopped before finally getting going?

• Is plate tectonics a continuous process thatwill continue for the foreseeable future, or adiscontinuous process that stops and starts?If it is a discontinuous process, how manytimes in Earth’s history has it actually startedand stopped?

• How does subduction begin (when platetectonics first began, and even today)?

• Why do subduction zones have arc shapes?They are called arcs because of their shape;several theories have been suggested toexplain the arc shape, but none of thesesuggestions can explain all of theobservations.

• Why do subduction zones move around?They move both towards and away from thesubducting plate, and very little correlationexists with, for example, plate age or platemotion.

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• Why do only some tectonic plates have asubducting slab?

• Why do plates without a subducting slab(e.g. the North and South American plates)move with significant speed (up to5 cm yr−1)? This is particularly confusing aswe do know that, in general, plate motion isprimarily driven by slab pull. Does theunderlying mantle play a role (i.e. is it doingsomething more than just passively sittingthere)?

• Did plate tectonics look different in the past?For example, was there always the samerange in sizes of tectonic plates?

• How did plate tectonics influence thegeneration of continental crust? This isparticularly interesting, as it is the buoyantcontinental crust that rises above sea leveland forms the ‘life rafts’ on which we live.

We are pursuing a number of lines of evidencein an attempt to answer the above questions. Forexample, the critical examination of ophiolites(which are pieces of oceanic crust that have been

thrust up onto continental crust, for example, asseen on Cyprus) and transform faults (for example,the San Andreas fault, USA) will allow us to

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develop a better understanding of processes atplate boundaries.

Is there any evidence for plate tectonics onother planets?There is no conclusive evidence for plate tectonicson any other planets [6]. Both the Moon andMercury are significantly smaller than Earth, andtherefore it is likely that they lost all of theirinternal heat at a much faster rate, largely becauseof their greater surface area to volume ratio.Now, they both have a single lithospheric plate,continuing to cool through conduction alone, andare considered to be geologically inactive. Thereis no evidence to suggest that plate tectonics everoperated on either the Moon or Mercury.

Venus shows no evidence of active platetectonics, although the surface does appear to berelatively young based on the lack of a significantnumber of impact craters (we do not yet have anysamples that may be used to date the surface ofVenus by any other methods). The evolution of thesurface of Venus remains a hotly debated issue andthe subject of substantial on-going research. Venusis similar in size to Earth, and so the questionof why Venus shows no evidence for active platetectonics is intriguing. It has been suggested thatthe key difference between Venus and Earth maybe the lack of water on Venus, as on Earth waterplays an important role in the evolution of thesurface, particularly in plate tectonics.

In contrast to Venus, Mars is considerablysmaller than Earth, but does have water (mostlyin the form of ice). Some surface features havebeen interpreted as indicating the possibility ofplate tectonics operating on Mars in the past.For example, it has been suggested that magneticpatterns observed by the Mars Global Surveyorspacecraft may indicate that a process similar toplate tectonics may have operated on Mars in thepast. However, other surface features have beeninterpreted as indicating that plate tectonics hasnot operated on Mars. For example, it has beensuggested that the enormous size of volcanoessuch as Olympus Mons may indicate that theMartian crust has remained stationary over themagma source for a protracted period of time,whereas on Earth the movement of tectonic platesover magma sources results in linear tracks ofrelatively small volcanoes on the surface (forexample, the chain of Hawaiian islands). There

remains no evidence for coherent planet-wideplate tectonics at any time in the history of Mars.

Discussion and conclusionsWe have made substantial progress in understand-ing plate tectonics since the early development andacceptance of the theory in the 1960s. We havesolved many puzzles in this field. For example, wenow know that the lithosphere and asthenospherebehave relatively independently, in contrast to theoriginal idea that the motion of the tectonic plateswas controlled by motion in the asthenosphere. Wealso know that the motion of the tectonic plates hassignificant control over motion in the mantle, notother way around (i.e. the location of downwellingslabs at subduction zones form and control the lo-cations of the downwelling zones within the man-tle). We also know that mid-oceanic ridges spreadpassively, and do not provide a significant contri-bution to driving plate tectonics.

We have also identified new puzzles that weare only just beginning to address. For example,Earth is unique in that it has plate tectonics, butalso in that it has continents, and in that it haslife. Are these issues related? There is no clearconsensus on these issues, as we do not yet fullyunderstand how continental crust is formed. Wedo not know whether it would be possible to havea world with plate tectonics, but no continents, orconversely a world with continents but no platetectonics. The relation between plate tectonicsand life is even more speculative, and this iscurrently discussed as a chicken and egg problem:do we need plate tectonics in order for there tobe life on Earth, or do we need life in order forthere to be plate tectonics on Earth? Of course,although our attempts to address this puzzle aremore speculative, this puzzle is also very exciting!

There is still a lot that we do not know aboutplate tectonics, and there are many tasks that lieahead for geophysicists. For example, why dosubduction zones have arc shapes, and why dothey move around? What are the differencesbetween the various tectonic plates, what causesthose differences, and do those differences controlthe process of plate tectonics in any way? Howand when did plate tectonics start on Earth? Thislast question is the one that we are most likely tobe able to answer in the near future. If we canunderstand how plate tectonics started on Earth itwill help us to figure out why it does not occur

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on any of the other terrestrial bodies in the SolarSystem. Will we ever find another planet that doeshave plate tectonics, or is Earth not just uniquewithin the Solar System, but also within the wideruniverse? If you want to find out the answers tothese kinds of questions, become a geophysicist!

Acknowledgments

The images in figures 1–3 in this article arecourtesy of NASA/JPL-Caltech. The NASAPlanetary Photojournal is an excellent resourcebank containing thousands of images, completewith their original release captions [7].

Received 8 January 2008doi:10.1088/0031-9120/43/2/002

References

[1] Anderson D L 2007 New Theory of the Earth(Cambridge: Cambridge University Press)

[2] Stern R J 2007 When and how did plate tectonicsbegin? Theoretical and empirical considerationsChin. Sci. Bull. 52 578–91

[3] van Andel T H 1994 New Views on an Old Planet:A History of Global Change (Cambridge:Cambridge University Press)

[4] Oreskes N 2001 Plate Tectonics: An Insider’sHistory of the Modern Theory of the Earth(Oxford: Westview Press)

[5] Davidson J P, Reed W E and Davis P M 2001Exploring Earth 2nd edn (Englewood Cliffs, NJ:Prentice-Hall) (ISBN 0-13-018372-5)

[6] Beatty J K, Petersen C and Chaikin A 1999 TheNew Solar System 4th edn (Cambridge:Cambridge University Press)

[7] http://photojournal.jpl.nasa.gov/index.html

Paula Martin is the Science OutreachCo-ordinator for Durham University. Hercurrent research is focused on thegeology and geophysics of Venus andMars.

Jeroen van Hunen is a lecturer in EarthSciences at Durham University. Hiscurrent research is focused on dynamicgeophysical models of subduction and theearly Earth.

Steve Parman is a lecturer in EarthSciences at Durham University. Hiscurrent research is focused on thechemical evolution of the Earth’sinterior.

Jon Davidson is a professor of EarthSciences at Durham University. Hiscurrent research is focused on thegeneration and evolution of volcanoes atsubduction zones.

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