Plume debate continues - The recent defence of plumes by Saunders et al. explains more about why plume advocates believe in plumes than about the real Earth, says Hetu Sheth - 16 September 2003
The latest, lengthy contribution to the plume debate, by Drs. Saunders, Fitton, and Coogan, and several earlier ones on this page by various plumatics, clearly explain why they consider the mantle plume model the best explanation for intraplate volcanism, mantle evolution, etc. Debate is happening and that is good, and I offer the present contribution to keep it going.
Saunders et al. 1 express surprise at the "angst" the plume model has generated within the community of aplumatics. We aplumatics are equally surprised at the deep, religious faith bright, intelligent scientists can have in artificial laboratory experiments that are far from simulating the real Earth and that are consistent with precious few facts. There also seems to be another fundamental difference in the ways aplumatics and plumatics function.
Aplumatics must and do read the plumatics?s publications; plumatics clearly do not need to and do not read the aplumatics. I can justify what I say; only one example will suffice. Saunders et al. cite several flood basalt-hotspot track pairs to argue that the plume head-tail model is valid and the best. This is early nineties? stuff. Saunders et al., like most plumatics I have read, do not deal with more recent literature that completely contradicts this speculation. Mahoney et al. 2 argued long ago, well before plumes took over, that the Rajmahal basalts are very different geochemically from the Ninety East Ridge basalts. Kent et al. 3, which include Saunders, made the same observation in 1997, when the plume model was as strong as ever, but nevertheless required a plume to function as just a heat source. Peate 4 and others have long argued that the Parana basalts are quite different geochemically from the Tristan hotspot.
The Deccan-Réunion case is perhaps the most often cited classical plume head-tail example. Saunders et al. cite my very short contribution to the debate 5 (hardly an appropriate reference) and say that despite what I claim there is a hotspot track leading from the Deccan to Réunion Island. How about discussing my scientific arguments about the Deccan-Réunion connection (or lack thereof)? I suggest they discuss my more detailed web page 6, or my papers 7,8,9 where my detailed arguments have appeared. Saunders et al. also seem to be unaware of the major paper by Burke 10 ? a plumatic himself ? which convincingly argues that the Réunion hotspot cannot be related to the Deccan. Saunders et al. are also perhaps unaware of the absolutely stunning paper by Baksi 11 where he concludes that out of thirty-five Ar-Ar ages for "hotspot tracks" in the Indian and Atlantic Oceans that exist in the literature (and are routinely used by plate motion modellers) only four could be called reliable crystallization ages. The Deccan-Réunion "hotspot track" is one of these dated tracks.
In fact, Sr-Nd isotopic ratios analysed in the ODP Leg 115 basalts by White et al. 12 are strikingly similar to those in post-shield lavas on Mauritius Island 13 that formed just before Réunion and is part of the "track". This means it is not even certain that the ages (of whatever quality) that exist are for shield-stage volcanism. If the Leg 115 basalts are post-shield, as Sheth et al. 13 consider likely, we can?t use their ages to model anything. It would be good if plumatics stopped for a minute and pondered over how much we really know.
The plume model has thrived on ad hoc modification and immunization. Consider Siberia (no hotspot track) or Hawaii (no LIP). Saunders et al. do not consider this and many other problems serious ? the post-Siberian hotspot track must have got destroyed due to subsequent plate/plume movements, the proto-Hawaiian LIP must have got subducted. No pre-volcanic lithospheric uplift occurred in Siberia 14 or in the world?s largest oceanic LIP, the Ontong Java 15. The lithosphere must simply have been very thick and strong then, because a plume is unquestionable to them. Just any observation is consistent with the plume model, and it is impossible to disprove the model. It is pertinent that the most classic alleged products of plumes are all highly anomalous with respect to the model (Siberia, Ontong Java, Hawaii). Of course a model has to change to accommodate newly discovered facts. I am in agreement with Saunders et al. here. The plume model postulates a spherical plume head and I don?t care if were not exactly spherical. But an entire plume head missing does cause concern. I would say that special pleading and immunisation of the plume model have extended far beyond what can be called reasonable scientific modification of a working hypothesis.
Saunders et al. say that the plume model has the edge over other models for explaining lots of things. But this derives naturally from the fact that with the plume model one simply assumes what one needs. Just assume the required size and temperature and petrological-geochemical composition and you have "explained" it. I do not think the plume model "explains" anything. It is the most convenient and handy explanation. What the plume model has thrived on is speculations and assumptions, continuous ad hoc immunisation, and even misinformation. The geology of India and the Deccan, about which I claim to know something, has been terribly ignored in speculative plume papers such as by Cox 16 and Campbell and Griffiths 17. In fact, I am not aware of even one flood basalt ? hotspot pair worldwide that satisfies all the predictions of the plume model without requiring special pleading. If there is one, I am keen to know about it.
To conclude, until Saunders et al., and other plumatics, address the hard questions posed by the aplumatics to them, no progress is going to be achieved by continuous ad hoc modifications of the plume model to fit every anomalous case. The global community has the right to clear answers, not vague speculations, from those who invented plumes and those who have kept invoking them. The plumatics? basic approach can be summarised as "we don?t fully understand plumes but there is nothing better and so plumes are OK". The aplumatics? approach, in contrast, can be described as "we don?t fully understand intraplate volcanism and geodynamics but that doesn?t mean plumes are OK." The two are irreconcilable.
Back to the plume debate home page
1. Saunders, A. D., J. G. Fitton, and L. Coogan,
http://www.geolsoc.org.uk/plumesdebate 2. Mahoney, J. J., J. D. Macdougall, G. W. Lugmair, and K. Gopalan, Nature, 303, 385-389, 1983.
3. Kent, R. W., A. D. Saunders, P. D. Kempton, and N. C. Ghose, in AGU
Geophys. Monogr., 100, 145-182, 1997.
4. Peate, D., in AGU Geophys. Monogr., 100, 217-246, 1997.
5. Sheth,H.C.,
http://www.geolsoc.org.uk/plumesdebate, 2003.
6.
http://www.mantleplumes.org/Deccan.html, 2003.
7. Sheth, H. C., Earth Planet. Sci. Lett., 168, 19-26, 1999.
8. Sheth, H. C., Tectonophysics, 311, 1-29, 1999.
9. Sheth, H. C., Int. Geol. Rev., 42, 1007-1016, 2000.
10. Burke, K. C., The African plate, S. Afr. J. Geol., 99, 41-409, 1996.
11. Baksi, A. K., J. Geol., 107, 13-26, 1999.
12. White, W. M., M. M. Cheatham, and R. A. Duncan, Proc. ODP, Sci. Res., 115, 53-62, 1990.
13. Sheth, H. C., J. J. Mahoney, and A. N. Baxter, Int. Geol. Rev., in press, 2003. (Preprint posted on www.mantleplumes.org).
14.
http://www.mantleplumes.org/Siberia.html, 2003.
15. Tejada, M. L. G., J. J. Mahoney, C. R. Neal, R. A. Duncan, and M. G.
Petterson, J. Petrol., 43, 449-484, 2002.
16. Cox, K. G., Nature, 342, 873-877, 1989.
17. Campbell, I. H., and R. W. Griffiths, Earth Planet. Sci. Lett., 99, 79-93, 1990.
Dr. H. C. Sheth, Assistant Professor, Department of Earth Sciences, Indian Institute of Technology (IIT) Bombay, Powai, Bombay 400 076 India Ph.(91)-22-25767264; Fax: (91)-22-25767253/25723480 Email: hcsheth@iitb.ac.in; Web:
www.geos.iitb.ac.in/hcs.html