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Friday, December 2, 2011

Driving Forces of Plate Tectonics

Diverting attention here at the blog again away from mineral resources & environment (I seem to do that more often than not lately!), I'm writing up a post on the driving forces of plate tectonics.  No time for uber-basic stuff here; gotta get deep in a hurry.

Yesterday, a geology student at a university tweeted that his physics professor had told him that plate tectonics was driven by the gravitational force from the moon.  Wowsa, that's awful bad.  Downright ridiculous.  A number of folks in the geotwittersphere have jumped in on the conversation, pointing out how silly this is.

However, within this conversation, a common misconception has arisen that could benefit from clarification, and that is the idea that mantle convection is the main driving force of lithospheric plate motions.  That's not entirely true either.  That model was initially assumed as true when plate tectonics was proposed, but has been discarded.  This model is still sometimes shown in introductory geology textbooks, and hence it still is propagated.  For example, in the text "The Changing Earth", 4th ed., by Monroe & Wicander, published by Thomson Brooks/Cole, 2006, Figure 1.9 explicitly shows mantle convection cells that are intimately related to lithospheric spreading centers and subduction zones.  The caption to the figure states "Earth's plates are thought to move as a result of underlying mantle convection cells in which warm material from deep within Earth rises toward the surface, cools, and then, upon losing heat, descends back into the interior.  The movement of these convection cells is thought to be the mechanism responsible for the movement of Earth's plates, as shown in this diagrammatic cross section."  This image below, from a web search (http://www.yorku.ca/esse/veo/earth/image/1-3-2.JPG) demonstrates this model, and is very similar to MW Fig. 1.9 (probably from an earlier edition of the textbook):


This isn't the only introductory text that makes this error, and it certainly isn't the only error found in many introductory textbooks.  It's still a good textbook - but this figure is wrong.  In fact, if you google "mantle convection", you'll see lots of images & sites that continue this error of directly linking mantle convection with plate motion.  

To correct these issues, however, let's take a look at an extended quote from "Earth Structure", 2nd ed., written by Van der Pluijm & Marshak (W.W. Norton, 2004), from section 14.10, pp. 364-365:
"The question of what drives the plates remains controversial to this day.  In the years immediately following the proposal of plate tectonics, many geoscientists tacitly accepted a convection-cell model, which stated that convection-driven flow in the mantle drives the plates.  In this model, plates were carried along the back of flowing asthenosphere, which was thought to circulate in simple elliptical (in cross section) paths; upwelling (upward flow) of hot asthenosphere presumably occurred at mid-ocean ridges, while down-welling (downward flow) of hot asthenosphere occurred at the margins of oceans or at subduction zones.  In this model, the flowing asthenosphere exerts basal drag, a shear stress, on the base of the plate, which is sufficient to move the plate.  This image of plate motion, however, was eventually discarded for, while it is clear that the mantle does convect, it is impossible to devise a global geometry of convection cells that can explian the observed geometry of plate boundaries that now exist on Earth.  Subsequent calculations showed that two other forces, ridge push and slab pull, play a major role in driving plates."
Van der Pluijm & Marshak then go on to describe ridge-push force & slab-pull force in more detail.  This chapter is a great starting point for anyone who is interested in learning more about plate driving forces, and there are a number of references given at the end of the chapter for further reading.  Notably, Marshak has also written an introductory geology textbook "Earth: Portrait of a Planet" where this is explained correctly, but I think this might be the only introductory textbook out there to get this right!

It is certainly true that without a flowing, convecting asthenosphere, you cannot have a subducting lithosphere.  Hence, mantle convection is intimately tied to plate tectonic motions.  But it is overly simplistic to state that mantle convection drives plate tectonics.  The two systems, mantle convection and lithospheric plate movement, are related and connected to one another, but they are not a simple, single system.  Plate tectonic motion is really a passive system, largely driven by gravitational forces acting on materials that become unstable due to their density.  Once a lithospheric plate begins to subduct, the density of the plate pulls the plate down, exerting a pulling force on the rest of the plate at the surface, much like a table cloth hanging off one one end of a table too far and the whole thing slides off.  The effects of slab-pull & ridge-push forces can be seen in the rates of plate motions.  For example, the Pacific plate is bounded by the East Pacific rise and other ocean ridges to the east, and by a number of subduction zones on the western side.  The Pacific plate is cruising along at a quick pace of ~9 cm/year, largely due to the added force of slab pull.  Conversely, the in the Atlantic where subduction zones are very minor, the spreading rate is less than 1/3 of that, at only ~2.5 cm/year.