Figure 2: A 'snapshot' view of a theoretical situation involving several plumes in varying stages of development distributed around the globe is shown in the figure (right), kindly provided by Paul Tackley of UCLA. This may represent a typical pattern of plume distribution at any period of geological time.
The model proposes that asthenosphere beneath a thick craton is insulated as if by a thermal blanket. It warms as heat convects from depth, and then flows laterally towards cooler regions until it reaches the edge of the craton, where it ascends beneath thinner lithosphere. Here it decompresses and melts. Melting is focussed along the margin, forming a LIP. This model entirely plausible but, unlike the plume model, it has five first order problems in accounting for LIPs.
- It is difficult to see how the edge effect can explain the abrupt flare-up of LIPs; there is no obvious on-off switch.
- Such models cannot work for ocean plateaus such as the Ontong Java.
- Edge-effect models advocate a lithosphere as thin as 50 km in order to operate (any thicker, and melt is not produced in sufficient quantity), which is probably far too thin to be realistic.
- We do not find LIPs along every rifted craton margin.
- The high temperatures required by picrites are unlikely to be achieved by edge-effect models. Interestingly, the ?edge effect? espoused by King and Anderson can be accommodated nicely within the plume model, inasmuch as hot plume mantle rising beneath a craton may flow laterally into regions of thinner lithosphere, where it melts16.
There are many other provinces I could have mentioned in support of the plume model; and I deliberately have not marshalled the geochemical arguments. Contrary to what has been said in recent letters in Geoscientist, I do not believe that there is a mafia out to silence the anti-plume lobby. I, for one, welcome the opportunity for this debate.
Few supporters of mantle plume theory push for every volcanic field to be plume-related, just as few supporters of the ophiolite model advocated that every pillow basalt represents an ancient ocean. Yes, the plume model may allow some to hide behind a convenient paradigm, but what is new there?
I find it curious that a scientific theory or model can cause so much anguish among some of my colleagues in the Earth Science community1. Perhaps similar angst was caused in the early days of plate tectonic theory. Many aspects of both plate tectonic and plume models remain imperfectly understood, yet because something doesn't fit our prejudice, we don't reject the entire theory outright; the model is refined to accommodate new data. The terms 'ugly' and 'inelegant', as used by some critics of plume theory2, are highly subjective epithets, easily returned to source, but serve little purpose.
My challenge to the Beyond-the-Plume lobby is this: present us with a viable, workable, and testable alternative hypothesis (or hypotheses) that accommodates the observations and data, and that sits comfortably with our understanding of the Earth's thermal and chemical structure.
This article was improved by comments from Tiffany Barry, Mike Branney, Laurence Coogan, Richard England, Godfrey Fitton, Ian Hill, Pamela Kempton, Andrew Kerr, Mike Norry, Marc Reichow, Steve Temperley, Pat Thompson, and Rosalind White. Defending such an embracing theory in limited space is fraught with difficulties, not least the sins of omission and error. These are mine alone. Andy Saunders.
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