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Fuming over plumes

Anjana Ahuja

The Times , 18th September 2003


MOST MODERN scientific disputes are conducted almost clandestinely, their scandal-making potential confined to the pages of academic journals. These are where scholars line up to swear their allegiances, politely sitting on one side of a fence while acknowledging the right of their opponents to sit on the other, and willing to swap sides in the face of fresh evidence.

Not so for the geological community, which has been riven for months by acrimonious arguments over the existence of “mantle plumes”, fountains of rock that supposedly spew periodically from inside the Earth.
The eruptions punch through the land above, creating huge humps in the landscape. Plate tectonics, which describes the features of the Earth’s crust in terms of several interlocking plates that move relative to each other, struggles to explain why volcanic islands such as Hawaii should appear in the middle of plates as well as at the turbulent edges, where plates crash into and slide beneath each other.

Hot, unpredictable plumes from the deep interior provided the perfect solution, and have become a plank of modern geological theory.

Now, bolstered by the fact that experiments have not yet proved their existence, a small but vocal group of geologists is raising the heretical possibility that mantle plumes do not exist at all. Led by Dr Don Anderson, from the California Institute of Technology, and Dr Gillian Foulger, from Durham University, the self-styled aplumatics are pitting themselves as Davids against the “assume-a-plume” Goliaths.

Plate tectonics, they argue, can also explain anomalous, mid-plate island chains such as Hawaii. There is no need to complicate matters, they say, with the addition of hot plumes rising mysteriously from nearly 3,000km (1,864 miles) down in the bowels of the planet. There is, they protest, no evidence of narrow columns piercing the whole mantle and emerging at the Earth’s surface, and, moreover, that unbelievably high pressures in the mantle stop rock from rising, let alone in the suggested plumes. But they cast their net of criticism considerably wider — they accuse the plumatic lobby of discarding or ignoring evidence that does not fit with their cause, and complain that Earth-sciences journals are run by editorial boards with a vested interest in keeping plumes alive.


As unfavourable pieces of evidence trickle in, Foulger fumed recently, “plume enthusiasts have responded to these challenges with creativity. Carefully truncated cross sections, with colour scales cranked up, give noisy images the illusion of strong anomalies traversing the mantle... In 2002 several hundred papers about them are listed by the Science Citation Index, so the subject is paying a lot of mortgages. The assume-a-plume approach has also relieved researchers of the hard work of thinking up new theories — a welcome relief in these days when we are all expected to publish six papers a year or else.”

Anderson even ranks it alongside some of the most notorious frauds and misconceptions in scientific history: “(Plume theory) has so many variants, exceptions, rationalisations, ad hoc adjustments and failed predictions that it is unsatisfactory at the most basic level... It is the same kind of wishful thinking and self-delusion that we associate with Ptolemy, Piltdown, phlogiston, polywater and cold fusion.”

No wonder then that the forum for these outpourings, the newsletter of the Geological Society of London, gives warning that any contributions might be amended “for possibly legal reasons”.

So outraged was Andy Saunders, professor of geochemistry at Leicester University, that he felt compelled to stick up for the plume model, which he says still convinces most of the geoscience community. “A number of us were taken aback at the virulence of these attacks,” Saunders says. “Normally I wouldn’t get involved but someone had to stand up to it.”

And so Saunders entered the fray by penning an article: “Like the electron, which is never seen, but whose effects are predictable and observed, plumes are elusive, hidden, enigmatic, and important... 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.” Saunders then goes on to rubbish the idea that “there is a mafia out to silence the anti-plume lobby”.

Saunders rejects the idea that plumes are not supported by evidence — he says that seismic tomography, which can map the speed of seismic waves through rocks, shows that the waves slow down beneath Hawaii, a so-called hotspot (Iceland and Yellowstone National Park are other examples of hotspots).

This suggests, according to Saunders, the existence of an upwelling of buoyant, probably hot, rock beneath the island, which fits with his working definition of a plume. “If it was not for this plume, we wouldn’t have Hawaii,” he concludes. Foulger has suggested the island may have resulted from a tear in the plate — Saunders says it is theoretically possible, but that no such puncture has been observed.

Why should the debate have become so impassioned? “I don’t know,” Saunders says. “Sometimes it looks good to be an iconoclast. If you are proven right, you are seen as a forerunner in the brave new world. Perhaps people are just frustrated with what is a very old model. A lot of the evidence is indirect.

“These things are happening where you can’t see them. But you cannot see the Earth’s core, and nobody is saying it does not exist.”