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Water plays a crucial role in plate tectonics by easing the brittle and ductile deformation of the Earth's lithosphere. Water lowers the Mohr-Coulomb for brittle fractures. During ductile deformation, it lubricates different deformational regimes. This is also important for the deformation in the asthenosphere.

In addition, water plays an important role in the formation of water-bearing minerals and the fluxing of melts, thereby substantially changing the rheology of the lithosphere and asthenosphere.

Water-bearing minerals of the mantle.

So, what would plate tectonics look like on a waterless Earth? Would it even work? Would there be different rates, or totally different processes taking over, similar to what is seen on adjacent solar system bodies?

Sources: V.S. Solomatov. The role of liquid water in maintaining plate tectonics and the regulation of surface temperature
See also: Plate tectonics and water

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    $\begingroup$ Also see Fei et al., "Small effect of water on upper-mantle rheology based on silicon self-diffusion coefficients," Nature 498, 213–215 (2013) which concludes "that the effect of water on upper-mantle rheology is very small". The scientific jury may still be out on this one. $\endgroup$ – David Hammen May 4 '14 at 19:28
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    $\begingroup$ Per the 2nd answer on physics.se, "Venus" is the traditional response to "what would an Earth-like planet with less water look like?". (I think an answer here with an ES slant would be good! Just - that's the extent of my ability to chip in on this this evening, heh.) $\endgroup$ – kaberett May 4 '14 at 21:13
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    $\begingroup$ Seems like if water plays no role, you'd have to be able to explain why oceanic plates subduct under continental plates without water playing a role, right? $\endgroup$ – blunders May 5 '14 at 2:05
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    $\begingroup$ @blunders Oceanic crust is mafic and denser than continental crust. $\endgroup$ – stali Dec 17 '14 at 5:52
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    $\begingroup$ This document people.earth.yale.edu/sites/default/files/korenaga13a.pdf may provide some insights $\endgroup$ – user889 Dec 20 '14 at 4:43
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One way of looking at this question is to determine if Earth would possess an active-lid (multiple plate), stagnant-lid (single plate) or an intermediary state (e.g. episodic or sluggish) tectonic regime. This is one of the reasons comparisons are often made with Venus, especially in regards to the main difference been that Venus has a stagnant-lid with possible episodic active tectonic regime (see question and answer to Can Venus be considered to be tectonically active?, compared to Earth's active-lid tectonic regime.

However, there are other differences between Venus and Earth that also may account for the differences in tectonic regimes, asides from the relative absence of water on Venus. Some modelling by Bouffard, (2013) indicated that different plastic yield strengths of lithospheric rheology could account for the different tectonic regimes.

How water interacts oceanic lithosphere is a vital consideration for this kind of question, considering that most spreading ridges and all subduction zones are based on the oceanic lithosphere.

This answer is based on how the absence of water may have prevented a key theorised tectonic mechanism from even starting.

A crucial role for water is modelled by Regenauer et al. (2001) for the initiation of subduction and as they assert the beginning of plate tectonics, based on the theory that long term build up of sediment at a passive continental-oceanic boundary (e.g North American east coast) may have caused the strain to build up to a point where the oceanic crust 'breaks' and starts subducting, as shown in the diagram below:

enter image description here

Image source

In the absence of water, this process of subduction initiation (should it be true) could not have started; as the authors believe could be the case for Venus where volcanism is evident, but the mechanism appears to be hotspot activity through the current stagnant-lid tectonic regime observed there - if this theory were true, then the absence of water would mean that Earth would potentially have had a stagnant-lid, punctured by hotspot activity.

References

Bouffard, 2013, Role of water in the tectonics of Earth and Venus, University of Lyon

Regenaeur et al. 2001, The Initiation of Subduction: Criticality by Addition of Water? Science

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    $\begingroup$ take the bounty... this answer earned it. :) $\endgroup$ – Azzie Rogers Dec 23 '14 at 20:57
  • $\begingroup$ @AzzieRogers thank you for that :) - it was fun to research this, learnt something new! $\endgroup$ – user889 Dec 23 '14 at 21:05
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In my opinion, the role of water in plate tectonics is to serve the plate driving mechanism. A spreading ridge is heated from bottom and effectively cooled on top by water.

Moon/Sun induced deformations of the ridge have to cause hot material to be transported upwards according to the second low of thermodynamics. Basically, the transported material cools and solidifies thus pushing the sides of the ridge apart on the ridge deformations induced by Moon/Sun.

I called the theory "Active Fracture Tectonics". You may wish to look through my blogs sukhotinsky.blogspot.com or divergent-boundaries.blogspot.com to get the idea in details.

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The short answer is that we don't really know for sure.

In the larger scheme of things, a plate is simply a thin (and cool) boundary layer at the top of a convecting cell. Plates are a result of mantle convection and their composition will have little effect on convection itself.

At the same time water tends to lower melting point of mantle rocks which can have many important consequences, e.g., back arc volcanism (as water gets transported to great depths during subduction), onset of instabilities etc.

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  • $\begingroup$ Why the downvotes? A comment may have been more helpful. $\endgroup$ – Isopycnal Oscillation Dec 23 '14 at 7:50
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    $\begingroup$ @IsopycnalOscillation only the first line is somewhat relevant to the question. The rest is unrelated. $\endgroup$ – Gimelist Dec 24 '14 at 7:20
  • $\begingroup$ @Michael thanks, it is not my area so to me it was not clear. $\endgroup$ – Isopycnal Oscillation Dec 24 '14 at 7:29
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    $\begingroup$ @IsopycnalOscillation It is relevant though the reasons are subtle. See plate tectonics depends on mantle convection which depends on the viscosity of mantle rocks and onset of Rayleigh-Taylor instabilities. Water can alter both viscosity of olivine as well lower its melting point which in turn will have an effect on the onset of convection as well as its evolution. Now how drastic this effect is unknown. The only insight we have is from numerical simulations. It is my area :). $\endgroup$ – stali Dec 24 '14 at 15:22
  • $\begingroup$ @stali i was interested because you mentioned instabilities. Which you expanded on in the comment - thanks. I have studied and simulated Rayleigh-Taylor instabilities in Boussinesq fluids, and it is interesting to see the same phenomena at those scales and with markedly different materials. $\endgroup$ – Isopycnal Oscillation Jan 1 '15 at 9:16

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