<p>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.  </p>

<p>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. </p>

<p>I called the theory "Active Fracture Tectonics". You may wish to look through my blogs <a href="http://sukhotinsky.blogspot.com/" rel="nofollow">sukhotinsky.blogspot.com</a> or <a href="http://divergent-boundaries.blogspot.com/" rel="nofollow">divergent-boundaries.blogspot.com</a> to get the idea in details.</p>

CC BY-SA 3.0
2014-12-21T11:55:44.740Z
2014-12-21T11:55:44.740Z
Sergey S.
<p>The short answer is that we don't really know for sure.</p>

<p>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. </p>

<p>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.</p>

CC BY-SA 3.0
2014-12-23T07:50:02.693Z
Why the downvotes? A comment may have been more helpful.
Isopycnal Oscillation
CC BY-SA 3.0
2014-12-24T07:20:56.697Z
@IsopycnalOscillation only the first line is somewhat relevant to the question. The rest is unrelated.
Gimelist
CC BY-SA 3.0
2014-12-24T07:29:26.720Z
@Michael thanks, it is not my area so to me it was not clear.
Isopycnal Oscillation
CC BY-SA 3.0
2014-12-24T15:22:39.457Z
@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 :).
stali
CC BY-SA 3.0
2015-01-01T09:16:55.833Z
@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.
Isopycnal Oscillation
CC BY-SA 3.0
2014-12-21T16:38:38.440Z
2014-12-21T16:38:38.440Z
stali
<p>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.</p>

<p>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.</p>

<p><img src="https://i.sstatic.net/f7YJ7.gif" alt="Water-bearing minerals of the mantle."></p>

<p>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?</p>

<p><sub>
Sources:
<a href="http://adsabs.harvard.edu/abs/2001AGUFM.U21A..09S" rel="nofollow noreferrer">V.S. Solomatov. <em>The role of liquid water in maintaining plate tectonics and the regulation of surface temperature</em></a>
<br>
See also:
<a href="https://physics.stackexchange.com/questions/48318/plate-tectonics-and-water">Plate tectonics and water</a>
</sub></p>

CC BY-SA 3.0
2014-05-04T19:28:11.990Z
Also see [Fei et al., "Small effect of water on upper-mantle rheology based on silicon self-diffusion coefficients," Nature 498, 213–215 (2013)](http://www.nature.com/nature/journal/v498/n7453/abs/nature12193.html) which concludes "that the effect of water on upper-mantle rheology is very small". The scientific jury may still be out on this one.
David Hammen
CC BY-SA 3.0
2014-05-04T21:13:40.300Z
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.)
kaberett
CC BY-SA 3.0
2014-05-05T02:05:37.253Z
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?
blunders
CC BY-SA 3.0
2014-12-17T05:52:48.053Z
@blunders Oceanic crust is mafic and denser than continental crust.
stali
CC BY-SA 3.0
2014-12-19T13:14:59.867Z
very similar question here: http://physics.stackexchange.com/questions/48318/plate-tectonics-and-water
DavePhD
<plate-tectonics><hypothetical><water>
How would plate tectonics differ if Earth had no water?
tobias47n9e
# How would plate tectonics differ if Earth had no water?

(Asked by: username_0 on 2014-05-04)

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.](https://i.sstatic.net/f7YJ7.gif)

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*](http://adsabs.harvard.edu/abs/2001AGUFM.U21A..09S)
  
See also:
[Plate tectonics and water](https://physics.stackexchange.com/questions/48318/plate-tectonics-and-water)

**Tags:** plate-tectonics, hypothetical, water

> **Comments:**
> **username_1**: Also see [Fei et al., "Small effect of water on upper-mantle rheology based on silicon self-diffusion coefficients," Nature 498, 213–215 (2013)](http://www.nature.com/nature/journal/v498/n7453/abs/nature12193.html) which concludes "that the effect of water on upper-mantle rheology is very small". The scientific jury may still be out on this one.  
> **username_2**: 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.)  
> **username_3**: 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?  
> **username_4**: @username_3 Oceanic crust is mafic and denser than continental crust.  
> **username_5**: very similar question here: http://physics.stackexchange.com/questions/48318/plate-tectonics-and-water  

---

## 2 answers

### Answer by username_6 on 2014-12-21. Score: 0

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](http://sukhotinsky.blogspot.com/) or [divergent-boundaries.blogspot.com](http://divergent-boundaries.blogspot.com/) to get the idea in details.


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### Answer by username_4 on 2014-12-21. Score: -1

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.

> **Comments:**
> **username_7**: Why the downvotes? A comment may have been more helpful.  
> **username_8**: @username_7 only the first line is somewhat relevant to the question. The rest is unrelated.  
> **username_7**: @Michael thanks, it is not my area so to me it was not clear.  
> **username_4**: @username_7 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 :).  
> **username_7**: @username_4 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.  

---

