No, I’m not talking about THAT controversy.  What I am talking about is a current controversy – or rather, scientific debate – that is currently happening with regard to plate tectonics. Yes, good ol’ plate tectonics is having a bit of a moment at the moment, with the publication of a paper by Anderson and Natland (2014, PNAS) on the evidence for – or rather, lack of evidence for – mantle plumes and other features of deep-convective plate tectonics.

Anderson and Natland argue that while the theory of hot-spot volcanism may explain features like the Emperor Seamount chain, there is not evidence that hot-spot volcanism actually exists. The existence of  hot, thin, mantle-deep plumes is not supported by the latest geophysical surveys, and indeed if anything like this exists it is shallower, wider, and more extensive.  This is part of a larger campaign by Anderson (and Natland, and some others) to dismantle (sic) an important tenet of plate tectonics – that is, that plate tectonics is largely driven by mantle convection (or, that plate tectonics is the surface manifestation of convection in the mantle). Instead, they have posited a slab-pull heavy version of plate tectonics, which is driven mainly by external cooling of the crust, causing the cold, dense crust to sink into the hotter mantle, pulling the connected plate into the gap via hinge-creep or hinge roll-back, and … well, here’s a summary, via Hamilton:

Plate motions are driven by subduction, the passive sinking of oceanic lithosphere that is gravitationally  unstable because of cooling from the top, and are self-organizing. Hinges rollback: slabs sink subvertically, and inclinations are transient positions,not trajectories. Broadside-sinking slabs push upper mantle back into oceans from whence they came, forcing spreading therein, pull overriding plates, and are plated down on the discontinuity near 660 km and overpassed by continental plates. Material from, and displaced rearward by, sunken slabs is cycled into enlarging oceans behind continents. (Hamilton

This isn’t a complete revolution in thinking about plate tectonics – the basic facts of moving plates on the surface doesn’t change. However, this does change the mechanism by which plates are moving. And before we think that’s not important, remember that Wegner’s original idea of Continental Drift was dismissed because it lacked a mechanism. So, this is certainly worth exploring.

The question now becomes – what do I teach my students? Do we teach an orthodoxy that may  be overturned in the coming years? Do we teach a new version that has yet to gain full acceptance in the scientific community (and I’m in no position to say whether it will or won’t supplant the current orthodoxy)?

Actually, though, this fits in well with how I have been teaching plate tectonics.  I use plate tectonics as an example of the scientific method – of how science works and how scientists think. It is a very good example of the difference between observation, hypothesis, and theory – we have certain observations, like magnetic anomalies, ages, and heat flux that tell us sea-floor spreading is real; we see where volcanoes are, and where earthquakes occur; heck, now, with satellites and frickin’ laser beams we can measure the movement of the continents! These are all observations that support the hypothesis that the continents move, the oceans spread outward from the mid-ocean ridges, and that sea floor is destroyed at convergent subduction margins. The theory part is what drives this – mantle convection. I already talk about the difference between shallow and deep convection, if only in passing to say “hey, we’re still working on this”.  But now, this can take on a new life. Is the Emperor Seamount chain really the remnants of a hot-spot, or is it a crack propogating along lines of stress in the Pacific plate?  If we no longer have evidence of hot-spots, then hot-spot volcanism (and that theory of the ESC) is unsupported. I feel like I’m living a real-life version of one of Kuhn’s Scientific Revolutions.

May we live in interesting times, indeed.

References

Anderson and Natland (2014) Mantle updrafts and mechanisms of oceanic volcanism, PNAS 111:41

Stuart et al. (2007) Propogation of the Hawaiian-Emperor volcano chain by Pacific plate cooling stress, GSA Special Publications 2007 430.

Hamilton (2002) The Closed Upper-Mantle Circulation of Plate Tectonics, Plate Boundary Zones, Geodynamics Series 30, AGU

http://www.agu.org/books/gd/v030/gd030p0359/gd030p0359.pdf