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Mantle plumes! 13 May, 2011

Posted by Simon Nickerson in Uncategorized.
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Time to jump away from human evolution and look at a different field.

“Mantle plumes” is not a phrase that you hear very often, but it’s at the heart of an important controversy in geophysics. All over the world there are long chains of volcanic mountains that can’t be explained by classical plate tectonics, and the debate about what’s responsible for them is growing in intensity.

This post will require a little more background than usual, so let’s dive in. We’ll start at one my favourite places in the world – Hawaii. Most people think of Hawaii as a small chain of islands, known for active volcanoes like Kilauea (click here for a live picture of Kilauea’s Puʻu ʻŌʻō crater) . But if you look under the Pacific ocean, you see that the Hawaiian islands are just the end of a very long chain of mountains, most of which are completely submerged. It’s known as the Hawaiian-Emperor seamount chain, and it stretches nearly 6,000km from Hawaii to the Kamchatka Peninsula in Russia.

The Hawaiian - Emperor seamount chain

Hawaii sits in the middle of the Pacific plate, far away from the edge of the plate known as the “Pacific Ring of Fire”. It gets its name from the many volcanoes found there, and they are easily understood by classical plate tectonics – as one plate dives under another, rock is melted and it rises up to form a volcano near the plate boundary. For something like the Hawaiian – Emperor seamount chain, though, a new theory is required.

The standard explanation for how such a long chain of volcanic mountains forms in the middle of a plate is called the hotspot theory. Hotspots are excessively hot places under the earth’s surface that seem to stay fixed in place while continental plates drift over them. Directly above the hotspot, a volcanic mountain is formed. As the plate moves, that mountain moves away from the hotspot and ceases to be volcanic. A new volcano then forms over the hotspot, and the process continues.

A hotspot producing a chain of volcanoes

But how do you explain the hotspots? That’s the controversial part of this story.  The standard theory says that they are caused by mantle plumes – rising columns of hot rock that start from deep within the earth, perhaps as deep as the earth’s core.

Mantle plumes rising from the earth's core. Source: http://faculty.weber.edu/bdattilo/shknbk/notes/htsptplm.htm

Opponents of this theory, who are in the minority, say mantle plumes simply don’t exist, and that the heat responsible for the volcanoes is generated much closer to the earth’s surface, just under the plate. They have organised themselves and maintain the website MantlePlumes.org to present their views.

As the debate intensified, each side organised conferences that tended to exclude their opponents. Eventually, in 2003, the Geological Society of London hosted The Great Plumes Debate in an attempt to bring both sides together. Some of the comments received at their website is eye-opening.

There are those who think the geophysical community has accepted a mantle plume model that is so flexible and poorly defined that it doesn’t actually explain anything:

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 to a point where plumes don’t have to meet any particular test. Hence the hypothesis now always works… but increasingly doesn’t tell us anything or predict anything.

(emphasis mine)

And there are others who think that the anti-plumers aren’t as ostracised as they claim, and the differences between the camps may not be all that fundamental:

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… Hardly a case of being stifled at birth by evil editors keen to hang on to the old ways!

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.

(emphasis mine)

I think what strikes me about this controversy is how it has bubbled away without much public attention. Usually the mass media plays an important role in these debates, either by prolonging them in order to attract readers, or simply by increasing the bitterness as each side sees their opponent’s views get some publicity. The mantle plumes debate has developed differently. It will be interesting to see how it plays out, and I’ll look for any signs that it might be heading for a resolution.

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