The critical importance of water to plate tectonics

The late Professor J. Tuzo Wilson, formerly of the University of Toronto and the Ontario Science Centre, was one of the fathers of plate tectonics, and was the author of numerous papers which were critical in the formation of the theory in the early and mid 1960s.  He is credited with developing the idea that at various times in the past the continents have combined and separated and then combined again (see eg. Wilson, 1966).  The mechanism for this process is known as the Wilson Cycle . 

A critical part of the Wilson Cycle, and of Plate Tectonics as a whole, is the initiation of subduction.  This process is illustrated to the right in very simple terms.  At a passive margin (where oceanic and continental plates meet but there is no subduction) such as the east coast of North America, erosion from the continent is deposited into the ocean as a very thick (10 to 20 km) sequence of continental shelf sediments (A).  Over a period of tens to hundreds of millions of years this sediment pushes down on the oceanic crust (B and C), and eventually the strain becomes so great that the oceanic crust breaks and subduction begins to take place (D).

In a recent paper in Science geologists from the Institute of Geophysics in Zurich and from two U.S. universities describe the results of some digital modelling of a situation similar to that shown in the diagram above.  Based on our understanding of the physical properties of the lithospheric mantle, oceanic crust and the overlying sediments, they have been able to show that if the lithosphere is essentially dry it is unlikely fail in such as way as to allow subduction to be initiated.  Wet lithosphere, on the other hand, will fail through its entire thickness under similar conditions, leading to the development of a fault-like zone several hundred metres in width - and then to subduction. 

 The authors make the following statement: 

Venus appears to have a hot interior like the earth’s and there is lots of evidence of volcanism, but there is currently no plate tectonic activity on Venus (although there might have been in the past).  It is possible that the lack of water on Venus is one of the reasons for this fundamental difference between these two sisters of the solar system.

References 

Wilson, J.T., Did the Atlantic close and then re-open? Nature, V. 211, p. 676-681, (1966)

Ragenauer-Lieb, K., Yuen, D. and Branlund, J., The initiation of subduction: criticalilty by addition of water? Science, B. 294, p. 578-580, (October 2001)

Steven Earle, 2001. Return to Earth Science News