The Great Plumes Debate 2003 - Short communications
Plumes and hotspots
From Rex Pilger*
Sir, Since W. Jason Morgan (WJM) introduced the mantle plume hypothesis1,2, subsequent research has tended to focus on the two distinct facets of the proposal: (1) Hotspots form an absolute (I prefer to use "proper") reference frame. (2) Hotspots represent cylindrical plumes flowing from the base of the mantle. Yet, these two facets are coupled intimately if we attempt to rethink the kinematic logic of the hypothesis. Evidence assembled by WJM and subsequently supported by Minster et al.3 suggested that contemporary hotspot traces represent a consistent reference frame. If indeed hotspots represent a global system, then they must originate from the lower mantle, deeper than the deepest subduction zones; subduction zones are in motion relative to one another, requiring relative motion of the shallow mesosphere on either side of the zones. Further a deep source would require very rapid transport to the base of the lithosphere - thus cylindrical plumes of rapidly rising material.
There was a logical flow to WJM's deep mantle plume hypothesis, dependent upon evidence for hotspots forming a globally consistent proper reference frame. However, as we fast-forward some thirty years, we now realize that the hotspots of the Atlantic and Indian Ocean (except for the northernmost Atlantic) form a proper reference frame, as do the hotspots of the Pacific Ocean, but the two hotspots systems are essentially independent. The recent global reference frame is a contemporary accident. For years the inconsistency between the two sets of hotspots was explained in terms of a possible hidden plate boundary in Antarctica - a boundary that can now be shown to have inadequate displacement to explain the discrepancy.4,5 In other words, the Pacific Ocean and Atlantic-Indian Ocean hotspot systems are separate and distinct, for at least the last 80 m.y. Therefore, following the kinematic logic above, the hotspot reference frame(s) no longer need be within deeper mantle.
There is evidence that the hotspot reference frames are in fact shallow: (1) Intracontinental contemporary stress6 and paleostress7 fields are consistent in orientation with motions in the hotspot reference frame. (2) The oldest portions of minor hotspot traces of the eastern Pacific Ocean are genetically related to the age-thickness structure of the overlying Pacific plate.7 (3) Cross-grain gravity lineations of the eastern Pacific, which parallel plate motions in the hotspot frame8 are restricted in their occurrence to thinner, younger plate, similar to the minor hotspot traces.9 (4) Motion of the Pacific (or "Hawaiian") hotspot set relative to the Atlantic-Indian Ocean ("Tristan") set between 80 and 25 Ma is subparallel with motion of the North and South American plates relative to the Tristan set - in other words, the Cordilleran and Andean subduction zones appear to have displaced the hotspot set beneath the Pacific Ocean plates as they advanced to the west.7
In sum, hotspot frames are shallow and minor hotspots originate at shallow depths. The big hotspots may originate from deeper mantle via WJM's plumes, but their shallow-most portions are fixed within the local proper reference frame set - within what are termed "mesoplates", of which there are three: Hawaiian, Tristan, and "Icelandic".7 Mesoplate boundaries are largely determined by deep subduction zones and relative motions of the mesoplates.
I leave seismic, geochemical and petrologic arguments for and against plumes to others. However, I recall a conversation with a prominent basalt petrologist: I expressed the hope that petrologists such as he would provide the constraints we geophysicists need to understand the mantle. With a distressed look, he said that, on the contrary, it was his hope that geophysicists would provide the constraints that petrologists need.
*Landmark Graphics Corporation, 1805 Shea Center Drive, Suite 400, Highlands Ranch, Colorado 80129 United States of America (303) 675-2446
References cited:
W. J. Morgan, Nature 230, 42-43 (1971).
W. J. Morgan, Geol. Soc. Am. Mem. 132, 7-22 (1972).
J. B. Minster, T. H. Jordan, P. Molnar, E. Haines, Geophys. J. Royal Astron. Soc. 36, 541-576 (1974).
S. C. Cande, C. A. Raymond, J. Stock, W. Haxby, Science 270, 947-953 (1995).
C. A. Raymond, J. M. Stock, S. C. Cande, Geophys. Mon. 121, Am. Geophys. Union, 359-375 (2000).
M. L. Zoback, et al., Nature 341, 291-298 (1989).
R. H. Pilger, Geokinematics, Prelude to Geodynamics, Springer-Verlag, Berlin, 338 p. (2003).
W. F. Haxby, and J. F. Weissel, J. Geophys. Res. 91, 3507-3520 (1986).
R. H. Pilger, unpublished ms (2003).
Plumes, cells and the mantle wind of change
From Don Findlay
Sir, Professor Saunders's promising argument (see below) that "the evidence from Hawaii proves the existence of a discrete, focused, persistent and deeply sourced convective mantle upwelling..." for the Pacific founders on the word 'convective', thus hiding the titillating plumage of the model beneath the drab garb of plate tectonics. That is, if he intends to support his argument by the use of his posted images. Diapir rise is not convection, and neither does it imply it, yet his images clearly show that a collapsing diapir at the terminus of its natural reach is the concept under discussion. Rightly, I think.
Convection? Inscripted in the ocean floors from the ridges to the subduction zones of plate tectonics, there is no geological evidence for any more than a single half-cycle of 'convective overturn' (which is, in effect, diapir rise), and the modelled images cited above reinforce the point. Moreover spatially, oceanic ridges once supposedly defining upwelling convection do not sit easily with the tomography-defined 'plumes' of today. So, what about convection? How do the ridges that once evidenced ‘convection’ match with the plumes of diapir rise today? Where, for example, is the 'convective return' for the "convective mantle upwelling" of the Hawaiian plume mentioned by Professor Saunders, presuming it is different from the subduction zone partnered with the East Pacific Rise?
It seems in this discussion we are finally (and not before time) witnessing the troubled transmutation from the CONVECTIVE cell of yesterday to the DIAPIRIC plume of today. This lexical pas-de-deux is the middle step in the conceptual dance that plate tectonics must execute if it is to get out of the blind alley it has landed itself in through overlooking some obvious and fundamental aspects of ocean floor structure.
Once the words/steps are in place, the ground will be set for the meme-driven machinery of consensus to change, from convection-driven plate tectonics to plume/diapir-driven (driven? ...or following?) Earth Expansion. The change in thinking from convection cell to mantle plume is substantial, and caution should be observed in the transfer of plate-baggage. Moreover, the question whether plumes are potentially 'drivers' or 'followers' in the larger global dynamics should be addressed.
An alternative to the alternative - Prof. Andy Saunders (Leicester University, UK) defends the plume hypothesis and challenges its detractors to come up with an alternative
"Ptolemy, Piltdown, phlogiston, polywater and... plumes?" by Don L Anderson (CalTech, Pasadena, Ca) - from the August 2003 issue of Geoscientist
Expansive view
From Don Findlay*
Sir, Have plumes had their day? Certainly. Within a plate tectonic paradigm at any rate, which requires recycling of mantle diapirs as convection cells. The main plume among them however, the Pacific, will almost certainly gain respectable rebirth as the driver of Earth expansion, to which plate tectonics is inevitably and inexorably headed. Not only plumes (in the sense of plate tectonics), but plate tectonics itself, has had its day.
Tough, but it's the way it is.
*http://users.indigo.net.au/don/
Stop that whining
from Dr. Stuart A. Weinstein, Geophysicist.
Sir, The fundamental problems that never seem to be addressed by those who suggest hotspot volcanism sources are shallow are mutli-fold. One of the fundamental problems with placing hotspot sources in the shallow mantle is that it provides no explantion for hotspot tracks which have gaps. When a ridge migrates over a hotspot, spreading produces a break in the track, but hotspot activity as far as we can tell is not interupted by the process. If the source was shallow, one would expect it to be exhumed. If it is a crack, one wonders why the crack doesn't eventually spread into the rifted region. Second, there is the age progression. The age progression switches polarity. That is, suppose plate motion is to the north, the hotspot track gets older northward. After a ridge migrates over it, and relative motion switches direction, the hotspot track gets older southward. An example of a scenario like this is exhibited by the Kerguelen hotspot and resulting tracks.
Next, we have the longevity of hotspots. Hotspots can remain active for well over 100 myr. It remains to be demonstrated whether or not "crack" volcanism can remain active that long. If crack volcanism exists, than why are the basalts produced ocean island basalts and not mid-ocean ridge basalts?
I have by no means exhausted my criticisms. I also appreciate Foulger's wanting to make the task of mantle convectioneers easier. However, simplicity at the cost of violating other observations is never worth it.
I am not claiming that all hotspots that have been so designated over the past decades are indeed formed as a result of plume activity, but to make the claim that plumes don't exist or that the crack hypothesis is better than the plume hypothesis, may get rapid attention from the press, but less rapid consideration from the geodynamical community. In which case I suggest Foulger spend more time crafting her hypothesis, addressing objections, and spend less time whining that reviewers are holding back progress. Don Anderson has been discounting the plume hypothesis for decades, and I haven't heard him whine about reviewers holding back progress.
Where Popper fails us
From John Hernlund*
Sir, I would like to speak up against some of the non-plume ideas expressed in this particular thread of letters. I enjoy Gillian Foulger's work, and wish her great success in improving our understanding of volcanism on the Earth (and perhaps other planets). I hope this continued discussion will lead us all to a better watering hole. Here, I will only address the shortcomings of this particular approach, based on an appeal to Popper's philosophy of science (Ref. #1).
The rhetorical devices used in Dr. Foulger's piece to argue against the plume hypothesis are clever, but flawed at a fundamental level. These devices are also over-used in Professor Foulger's own recent publications, some of which I have had the opportunity to read, and enjoy. The problem here is the following: Dr. Foulger’s piece presents the Modus Tollens argument:
• If deep-seated mantle plumes, then A,B,C,D,...
• Not A,B,C,D,...
• Therefore, not plumes
Recalling our college logic classes, we know that such an argument is valid, so long as the premises are true. Here is where we encounter the essential problem in Professor Foulger's piece: none of the proposed premises bear much resemblance to the truth. Thus she proceeds by handicapping the hypothesis by her own false premises, which she then proceeds to attack. When persons point out that Dr. Foulger's premises are incorrect, she then accuses them of making “ad hoc” adjustments to the hypotheses. Perhaps the essential problem is that nobody is properly taught the plume hypothesis as a student, thus perpetuating these types of misunderstandings.
If we are going to try falsifying a hypothesis, then we ought to be working on the right premises, otherwise we cannot make any firm progress.
But this presents other problems as well. Perhaps a brief (and extreme) example helps. If I want to test the hypothesis that it is not raining outside today, then I must construct a test, preferably using a solid logical format. If I present this argument:
• If it is raining outside today, then George Bush has oranges for breakfast
• George Bush didn't have oranges for breakfast.
• Therefore it is not raining outside.
This is another modus tollens argument, where I have made the absurdity of the premise fairly clear. Obviously, somebody eating oranges 3000 miles away doesn’t have anything to do with whether it is raining outside. This is where Popper failed us. It is the construction of the premises to be tested that confounds scientific progress, and it is where the true business of science is forged (e.g. Ref. #2). However, you can also see why it is absurd to suggest that a better premise is a sign of a "troubled hypothesis" in this case. We cannot, on the basis of the above argument, reject the idea of rain fall. Just make a better test, e.g. going outside and see if you get wet! Is that ad hoc? Or is that just common sense? Cheers!
References:
Popper, Karl, The Logic of Scientific Discovery, Hutchinson, London, 1959.
Laudan, Larry, Progress and its Problems: Toward a Theory of Scientific Growth. Univ. Calif. Press, Berkeley, 1977.
*UCLA
Gillian Foulger replies:
Prof. Hemlund has misunderstood my article.
I do not assert that because the predictions of the plume hypothesis are not borne out, therefore plumes do not exist.
I assert that because the predictions of the plume hypothesis are not borne out, it is unsafe to assume, a priori, that this model is correct.
I point out that the plume hypothesis has become so flexible that it seems nothing could be observed that could rule it out. Thus it appears that the plume hypothesis, as it is applied today, is unfalsifyable. If so, then it is unscientific, and is hindering our progress in understanding the Earth because it removes the perception of need to develop and examine alternative theories.
I hope this clarification is useful.
Plumes in a balloon
From Seth Stein*
Sir, From my perspective as a former editor (JGR), reviewer, and author on some hotspot-related papers, I wouldn't blame anyone for the state of thinking and publications about hotspots and mantle plumes except ourselves, as in Pogo’s dictum “we have met the enemy and they are us.” In the absence of any other clear model, we’ve accepted very vague ideas about plumes and allowed them to be the null hypothesis for excess ridge or intraplate volcanism. The hypothesis evolved from fairly rigorous criteria (deep low velocity anomaly at present site of volcanism with age progression and near-fixity) to a point where plumes don’t have to meet any particular test. Hence the hypothesis now always works with appropriate site-specific modifications, but increasingly doesn't tell us anything or predict anything, especially about structures formed in the past. It does, however, make it harder to offer non-plume explanations.
This is reminiscent of the joke where lost balloonists call to the ground "where are we?" and are told "you're in a balloon". The answer is absolutely true and totally useless.
Fortunately, we're starting to ask the hard questions needed to decide what it means to say that something is a plume, and to decide if, when, and where the concept is useful.
*Department of Geological Sciences Northwestern University, Evanston, IL 60208
Hotspots fight back!
From Dr Ray Kent (University of Leicester)
Sir, I was intrigued to see alternatives to the mantle hotspot hypothesis being presented as 'Radically different, alternative models [that] ... if substantiated ... may lead to a first-order paradigm shift.' This suggests that such alternative hypotheses offer fresh, exciting insights; they are poised to flood the literature with new data and interpretations in order to brush away the established order of things. There is even some suggestion of long-lived conspiracies between referees and journal editors to stifle emerging non-plume hypotheses in their infancy, and thereby to suppress 'the truth'.
The reality is somewhat different. Some folk have never accepted the plume hypothesis, and their views, far from being suppressed, are widely published in the geological literature. For example, the 'big daddy' of non-plume hypotheses, Don Anderson, is credited with having published more than 30 'non-plume' papers in the past 8 years, most of which appear in respected journals. Similarly, Alan Smith, Gill Foulger and others allied to the 'Andersonian' cause have produced a rash of papers in the past 15 years, all championing non-plume models. Hardly a case of being stifled at birth by evil editors keen to hang on to the old ways!
One might also object to the suggestion that non-plume models are somehow 'new', and hence it is only a matter of time before such hypotheses are proven. There were non-plume models long before the first plume model, and the two have existed (more or less peaceably) side-by-side for forty years. Thus far, neither side has been able to prove their case conclusively. This suggests to me that 'the truth' is probably somewhere between the two opposite ends of the spectrum, i.e. some but not all of the largest volcanic outpourings were fed by hotspots in the mantle, whereas other, smaller, outpourings have their origin in plate tectonics.
To conclude, the best way forward is to continue talking to one another rather than throwing stones at one another's glass houses. This means genuine debate must occur, preferably in a nice (plume-top) setting like Hawaii ... !
Deccan claptrap
From Dr Hetu Sheth (Department of Earth Sciences, Indian Institute of Technology (IIT) Bombay,)
Sir: Proposing mantle plumes as explanations for anything and everything has been fashionable in recent years for Earth scientists. The original plume hypothesis posulated plumes that were deep-sourced, narrow, fixed, "enriched", abnormally hot, etc. Competing non-plume models were rejected without sufficient consideration. Today's plumes can be hot or cool. They can be hydrous while at the same time hot. They can be fixed or swaying in the mantle, narrow or very broad ("superplumes"), with cylindrical, tabular or any other imaginable shape (as required), and deriving from any depth in the mantle. They can be "enriched" or "depleted", with just the chemical flavour required to explain the data. Large volumes of homogeneous magma in a volcanic province become "compelling evidence for a plume origin", but if significant chemical heterogeneity is found, it becomes "evidence that the plume is heterogeneous". Ad hoc escape routes abound. Entire plume heads can be missing (e.g., Hawaii), or plume tails (e.g., Siberia, Ontong Java).
I started my student career as a plume enthusiast but soon felt the idea was nonsense. Four years ago I published in mainstream journals against the plume head-tail model for the Deccan Traps of India (despite hostile reviewers, some of them understandably anonymous). I hoped for debate and reconsideration. What I have seen since is total silence and indifference, however. Papers citing the Deccan as an excellent example of a plume-generated province are pouring in every month even now. In fact, I do not know of a single flood basalt-hotspot track pair where all the predictions of the plume head-tail model are fulfilled without requiring special pleading.
Gill Foulger's article in your magazine is excellent, as are the letters by readers. Thank you for your coverage of our views.
Plumes - the truth will out
Reply by Gillian Foulger to John Wright (see below)
Dear John, Thank you for your supportive words in response to my recent article Plumes, Plates and Popper in Geoscientist. You are expressing sentiments shared by many geologists who think the Earth is made of rocks rather than numbers on a spreadsheet or resolution kernels in a computer's memory.
The plume hypothesis has disenfranchised geologists in the same way that Kelvin disenfranchised them in the early 20th Century by his authoritarian insistence that, despite their observations, the Earth had to be very young because it was still warm. Geologists have been persuaded that their observations are inconsequential to identifying the source of volcanic regions by the insistence that it is so deep in the Earth than only geophysics can reach it. Then, when geophysics cannot find it, we are told that it must be because it is too narrow to be detected. However, if regions of anomalous volcanism are indeed sourced at shallow depth, it may be that geology and geochemistry are capable of probing essentially the entire depth interval of any consequence to what is at the surface. I encourage geologists to step outside of the plume framework and interpret their observations in the way the data suggest, rather than forcing them into an ill-fitting, a priori model.
Gillian R. Foulger 4 June, 2003 http://www.mantleplumes.org
The plume's a balloon
Open Letter to Dr Gillian Foulger, from Prof. John Wright (Open University)
Dear Gillian, Oh, I did so greatly enjoy your blast (Geoscientist May 2003, p. 16-17) at the plume hypothesis - which even now is difficult to dignify with the status of theory, given the inherent inconsistencies you describe. Some thirty years ago (golly, judging from the photograph, that's before you were born!), I railed against some of the worst excesses promulgated by advocates of the hypothesis (refs. 1-3), who seemed to see plumes where none existed. My particular beef was the proposition that a plume gave rise to the (granitic) White Mountain magmas in New Hampshire, and that the same plume is now under the (basaltic) Azores volcanoes. Equally hare-brained (to me) was the suggestion that another plume was responsible for the Nigerian Younger Granites, and that this one ended up under Ascension Island. Mind you, I cannot deny that the hypothesis has a superficial appeal, even for me, especially as regards the Hawaiian island chain.
However, I readily concede that it's the only example that seems to work at all, and I understand there are several other island chains in the Pacific that are without regular age progression and may lack convincing evidence of underlying plumes.
But where do you go from here? You have the enormous advantage of youth on your side, for I have to confess that, with advancing years I have become less and less familiar with the relevant literature; in addition to which, you and your colleagues must know and understand a hell of a lot more than we did back in the 1970s. All the same, you have a hard row to hoe, a long steep hill to climb, if you hope to convince the Earth science fraternity that plumes are a figment of their collective imagination. Can you really bring them round to your view that the mode of convection that drives plate tectonics is the "only game in town"? Plans to send a probe to the centre of the Earth may appear to have overtones of lunacy (ref. 4), but such a project might perhaps contribute to resolving such questions. Anyway, all power to your elbow, and best wishes for success.
References
Nature, vol. 244, p. 565-7 (1973)
Ibid. vol. 247, p. 25-6 (1974),
Ibid. vol. 248, p. 365-6 (1974).
Ibid. vol. 423, p. 239 (2003).
Non-plumian professing
From Tony Doré (Statoil)
Sir, I thoroughly enjoyed Gill Foulger's recent article on mantle plumes ("Plumes, plates and Popper", Geoscientist v. 13 no. 5).
I have been puzzled about Iceland for a long time. Specifically, what bothers me is the assumption of an Iceland hotspot track based on working out where the plume centre "must have been" assuming a fixed hotspot reference frame. So by that reckoning, the plume started in the Labrador Sea at about 60 Ma, and moved southeastward across Greenland until by some amazing coincidence it resides under the spreading ridge/plate boundary at present day. And yet, if you look at the hotspot "track" in the present ocean, it is distributed symmetrically either side of Iceland (the Greenland Faroes Ridge), not just on one side as it would be if it had migrated in from the northwest. Furthermore, if you look at the Thulean volcanism, representing earliest known "plume" activity in the Paleocene, it extends approximately the same distance into the continents either side of the N Atlantic (NW to the Baffin Island coast, SE to Lundy Island) - i.e. it is symmetrically distributed about Iceland again. All of this empirical evidence cries out that the hotspot must have always resided under the plate boundary. Conversely, there is hardly a scrap of corroborative evidence for a hotspot trail moving across Greenland. But, if you accept these observations, a deep-seated plume fixed relative to the moving plates cannot be the explanation for Iceland. I have put this evidence to experts on the Iceland hotspot (I am not one) and, although they don't deny it, they seem unwilling to take this issue on.
I still think the Hawaiian seamount chain is best explained by the plate moving over a fixed heat source. However, other hotspots such as Iceland don't fit the plume dogma - and especially the fixed hotspot reference frame - at all well. It's great to see somebody tackling this issue head on.
Alan Smith, CIE-UNAM, Temixco, Morelos, Mexico writes:
Sir, I have long considered the plume model to be a bandwagon characterised by a monkeys-'n'-typewriters-style approach of looking for every possible variation without ever pausing to consider the possibility of fundamental flaws in the whole concept. The monopoly it has enjoyed is a result of the peer review system whereby any criticism or presentation of an alternative to a ruling theory needs a majority vote from proponents of the ruling theory, and editors allow reviewers to reject manuscripts for the most trivial of reasons. To say the plume concept has served the geological community well is overgenerous, more like it has set back a true understanding of the geodynamics of the Earth's interior by three decades.
Time to test plume alternatives
From Rex Pilger (received 19.9.03)
Sir, The arguments against deep mantle plumes continue to accumulate. Natland's1 summary of the arguments: geothermal, geochemical, petrologic, and tomographic must be challenging to plume proponents. But, does the alternative hypothesis, intraplate stress fields, buy us anything more?
In papers2-5 published some twenty years ago, I argued for stress fields along the same lines, following the suggestions of Solomon and Sleep.6 What was most bothersome about the stress argument at that time still applies today: We have examples of hotspot traces that clearly formed in semi-symmetrical fashion on opposite sides of a spreading centre: I showed via plate reconstructions that the Tuamotu and Nazca ridges are "mirror-images", just as Morgan7 had originally proposed (however, Morgan's other suggestion, that the Line Islands represent continuation of the Tuamotu trace, appears to be incorrect).
If we have mirror image traces that formed from a common source centered on a spreading centre (we might add Iceland, Walvis-RioGrande, and Kerguelen-Ninetyeast to the list), how can a shear stress field propagate across a spreading centre, producing anomalous volcanism on either flank? I suggested that once melting results due to plate fracturing and thinning, the melting anomaly might become self-sustaining, fed from below; thus when it encounters the spreading centre, it produces enough additional volcanism to generate the twin traces. However, I was never completely persuaded of the viability of this proposal. After all, the Tuamotu trace initiates abruptly across the Marquesas fracture zone, and the nature of Andean subduction beneath Peru indicates that the mirror image of the Tuamotu trace exists on the subducting Nazca plate. I don't think plate fracturing is enough.
In my previous communication,8 I reviewed some newly recognised evidence9 that demonstrates persuasively (to me at least), that virtually all of the Cenozoic hotspots/traces of the Pacific plate owe their origin and persistence to lithospheric structure. That is, variations in plate thickness (controlled by age) provide the context for the hotspots to emerge, perhaps with the additional control of variable fertility of the underlying asthenosphere and mesosphere. In other words, all of these hotspots (except Hawaii and Louisville) are demonstrably shallow in origin. And, the only reason we can't include the two bigger, long-lived hotspots is because their oldest extents are hidden in subduction zones, along with the plate structure.
I want to encourage the geochemical and petrological modellers to get to work testing what I have empirically shown. Allow the effect of sudden depressurisation of asthenosphere/mesosphere due to replacement of thicker, overlying lithosphere by thinner lithosphere (as a result of movement over the mesosphere of a fracture zone separating older from younger plate). Use the thermal models of plate thickening versus age, and try different possible mesosphere petrology recipes. See if you can produce the volumes of magma that we observe. And, see if you can produce the cross-grain gravity lineations10 while you're at it, from the same plate-thickness control.11 Develop the plate thickness-partial melting idea and, as an alternative or as an addition, the intraplate stress models advocated by others.
It's time to move on: let's begin testing alternatives to plumes. The energy devoted to plume plundering is better devoted to developing and testing viable alternatives, at least until we have indisputable tomographic evidence of the reality of deep mantle plumes. Whether we accept Kuhn's scientific revolution paradigm, our human experience tells us an existing theory isn't buried until a better model is established.
Rex H. Pilger, Ph.D., Senior R&D Program Manager , Landmark Graphics Corporation, 1805 Shea Center Drive, Suite 400, Highlands Ranch, Colorado 80129, (303) 675-2446, wirefree: (303) 589-3802; Email rpilger@lgc.com
References:
1. Natland, J., 2003, If not plumes... what else? , Letters, GSL, September 19, 2003, http://www.geolsoc.org.uk/template.cfm?name=Plumes39847538497.
2. Pilger, R. H., and Handschumacher, D. W., GSA Bull., 92, 437-446, 1981.
3. Pilger, R. H., GSA Bull., 92, 448-456, 1981.
4. Pilger, R. H., JGR, 87, 1825-1834, 1982.
5. Pilger, R. H., 1984, JGSL, 141, 793-802, 1984.
6. Solomon, S. C., and Sleep, N. H, JGR, 79, 2557-2567., 1975.
7. W. J. Morgan, GSA Mem. 132, 7-22,1972.
8. Pilger, R. H., Plumes and Hotspots, Letters, Discussion, GSL, August 11, 2003, http://www.geolsoc.org.uk/template.cfm?name=Letters.
9. R. H. Pilger, Geokinematics, Prelude to Geodynamics, Springer-Verlag, Berlin, 338 p. (2003).
10. W. F. Haxby, and J. F. Weissel, J. Geophys. Res. 91, 3507-3520 (1986).
11. R. H. Pilger, unpublished ms (2003).
How can a crack propagate across a ridge axis?
From Norm Sleep* (received 19.9.03)
I spent a lot of time trying to find alternatives to plumes from the 1970s until about 1986. For example, it is easy to show that slab pull should create intraplate tension more or less perpendicular to the Hawaiian chain. Thus there may be a propagating crack in the plate.
This leads to testable predictions. The crack should tap ordinary asthenosphere not much hotter material. The swell will form by delamination of the base of the lithosphere.
I was going to show that the plume was not kinematically compatible with the nose of the swell being upstream from the volcanoes and end the issue once and for all. I used a simple kinematic theory for the interaction of the plume material with plate drag. When I plotted my first guess of the shape of the nose of the swell it was right on. My disproof had instead given an explanation for the (map) shape of the swell. There are of course better dynamic models now, but it does show that antiplume people can change their minds if they form testable hypothesis.
In retrospect, the oblique lineaments of the volcanoes probably indicate that the stress is not exactly perpendicular to the chain. This is bad news for the chain being a propagating crack. A propagating crack should not cross ridge axes like hotspots seem to do.
Approximations to reality
From Norm Sleep* (received Sunday 21 September 03)
Sir, Plumes, like plate tectonics, are an approximation that helps one visualize the complicated flow pattern in the Earth. In analogy, one can say that a tornado is in the atmosphere. The tornado exists and is a useful concept (especially if one lives in a caravan park) but one cannot define exactly when it commences and when it ceases or where its edges are. The flow pattern in and around the tornado is much complicated than a simple vertical updraft.
Rigid plates are a useful approximation, but one that is clearly wrong everywhere if one looks in enough detail. One does not abandon plate tectonics because there are intraplate earthquakes, like in Yorkshire, or because the active zone of the ridge axis is wider than a hammer. Rather, one examines the geology and physics of plate interiors and ridge axes.
Plumes are part of the flow pattern in the mantle that impinges on near surface regions where we see hotspots. The stationary cylindrical vertical plume that is independent of the rest of the mantle is sometimes a useful approximate starting concept, but sometimes a concept that clouds thinking. One can understand a lot about the Earth from kinematic plate tectonics. One can understand other things from plume theory where the top of plumes are sources of hot buoyant material. One must examine the whole Earth to understand other issues like the relative motion of hotspots and the interaction of plates (dead slabs) and plumes at the base of the mantle.
Which hotspots are associated with plumes and where the plumes are at depth are valid unsolved questions. So is the fate of plume material once it reaches sublithospheric depths.
It is also productive to formulate testable quantitative alternatives to plumes that explain hotspots. It is not productive to press a cartoon concept of plumes to the point it cannot work and then say that plumes do not exist.
Reply to Prof. Sleep from Gill Foulger
Sir, Prof. Sleep's latest contribution illustrates well the fundamental problem that restricts the diversity of thought regarding the origins of melt anomalies. It is written as though we may assume that plumes exist without the slightest shadow of doubt. As long as this mind-think is pervasive, and transferred to students, progress will be limited to second-order advances of little consequence.
It does not help to compare plumes to things for which there is abundant evidence, e.g. tornadoes, implying that there is similarly compelling evidence for plumes. It does not help either to plead that it is OK if plumes are only vaguely defined. If the existence of a plume somewhere is to be accepted, convincing evidence for a) high temperatures and b) deep upwelling must be found. Until this is achieved, all the possibilities that are consistent with the observations should be entertained, including the possibility that the melt anomaly may arise from locally fusible mantle at normal mantle temperatures.
Popper, Plumes and Palaeomagnetism
From Ted Irving HonFGS (received 23.9.03)
Sir, Regarding Gillian Foulger's introduction of Popper into the plume debate, and Andy Saunder's question about whether there was similar 'angst in the 'early days of plate tectonics, the answer is Yes. I* introduced Popper into the mid-century mobilism debate, as a consequence of attending, in 1963, a seminar series given by him on the logic of scientific discovery. I found, much to my astonishment, that he was (with his gift for aphorisms) describing rather accurately and eloquently what we was doing. We were then engaged at the Australian end of the palaeomagnetic pole test for continental drift. That the Earth's magnetic field was on average a geocentric axial dipole (GAD) was our basic assumption.
By the late 50s we knew that if the GAD was correct then continents had drifted in much and the same fashion as Wegener suggested. We knew from the work of Hospers and others that the GAD was, within our errors, correct for the past 10Ma, but prior to that was less certain, although we had written many papers arguing that it was applicable for the later half of the Phanerozoic. Fixism was then the ruling theory, and our critics, perhaps sensing defeat, fastened on to what they believed was our Achilles' Heel, the magic word 'non-dipole, and used it at every opportunity. However, according to Popper, their non-dipole hypothesis was unfalsifiable and therefore unscientific, because any possible observation could be accomodated by introducing ad hoc further harmonics in the time vaying geomagnetic field.
Happily the 'angst soon evaporated, when that other aspect of paleomagnetism, reversals recorded in the sea floor, confirmed mobilism, and hence the legitimacy of my Popperian argument.
Finally I should note that the 'angst to which Andy Saunders referred is very evident in the current debate about the Late Palaeozoic GAD, Permian palaeogeography, and the relative merits of Pangaea A and B.
*Irving,E., 1964, Paleomagnetism and its Application to Geological and Geophysical Problems, Wiley, p.133.
Equal time plea
From Alexei Ivanov*
Sir, The classical lower mantle plumes of Jason Morgan [1] were used to explain the mechanism of uprising convection in the Earth's mantle and for explaining the unidirectional propagation of volcanism within lithospheric plates. The suggested features of such plumes (e.g. head-and-tail structure, stability in the mantle, excess of temperature etc.), however, have not been confirmed so far. This lack of corroboration has been taken by Gillian Foulger to show that existence of mantle plumes is just a hypothesis - and an unsuccessful one at that. One of the major counterarguments of the plume supporters has been that there is no alternative explanation that is any better. Let me claim the opposite.
Geologists have great expectations of deep seismic tomography of the mantle. Large progress in that field has been achieved in recent years, but plume head and tail structures have not been revealed [2]. Plume supporters generally attribute this to the low resolution of deep seismic tomography. But the question is, why should we waste our time talking about what we cannot see (plumes) and overlook things we can see in these seismic tomography images?
In 2001 Sergei Balyshev and I noted morphological similarity between the low velocity anomaly beneath Africa and high velocity anomalies elsewhere (undoubtedly connected to subducting slabs [3] ). We used a simple model to calculate that subducted slabs can be radioactively heated on a time-scale of 1-2 billions years and hence due to heating can be converted from high-velocity to low-velocity structures. In 2003 Jeroen Ritsema and Richard Allen have written in their outstanding paper [2]: ' [low-velocity anomalies] appear to connect to broad lower-mantle low-velocity anomalies beneath the Pacific and Africa via convoluted paths, reminiscent of high-velocity anomalies in the lower mantle that have been interpreted as remnants of the Farallon and Tethys slabs. They did not go one step further, however, and make the genetic link between those anomalies that are so similar in shape and so different in velocity. Is it right time to make this step?
The idea that low-velocity anomalies are re-heated slabs is not so strange as it sounds. Yet in 1988, Paul Silver and his colleagues [4] suggested a model of penetrative convection in which modern subducting slabs are downwelling convective flow and ancient warmed up slabs are upwelling convective flow. This model with support of radioactive heating of slabs can account for almost all observations thought to be attribution of the plume model [5].
I would like to finish this letter by saying that the plume model is certainly not the only one. The propagating crack model for temporal migration of intraplate volcanism and penetrative convection model for mode of whole-mantle convection (and maybe some other models) should be paid as much attention as plumes have had in the last thirty years. Only comparing of observations with various models can give an answer on how the Earth actually works.
References:
1. Morgan W.J. Convection plumes in the lower mantle. Nature, 1971, v.230, pp. 42-43.
2. Ritsema J., Allen R.M. The elusive mantle plume. Earth Planet. Sci.Let., 2003, v. 207, pp. 1-12.
3. Balyshev S.O., Ivanov A.V. Low-density anomalies in the mantle: ascending plumes and/or heated fossil lithospheric plates? Doklady Earth Sciences, 2001, v. 380, pp. 858-862.
4. Silver P.G., Carlson R.W., Olson P. Deep slabs, geochemical heterogeneity, and the large-scale structure of mantle convection:Investigation of an enduring paradox. Ann. Rev. Earth. Sci., 1988, v.16, pp. 477-541.
5. Ivanov A.V. Plumes or reheated slabs? www.mantleplumes.org October 3, 2003
*Alexei V. Ivanov, Senior Researcher at the Institute of the Earth's Crust, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia