It’s a simple and well-known picture. Volcanoes form either at plate boundaries due to subduction or inside plates due to mantle plumes. Invoking plumes – columns of hot rock rising from deep in the mantle – is an awfully useful way of explaining oddly-placed volcanoes, both ancient and modern.
Too useful, many people think. The concept has been abused. See Erik Lundin’s excellent critique in “52 things you should know about Geology“: “A concept that is granted the freedom of perpetual ad hoc amendments has the ability to explain anything … But such a concept can neither be falsified not used predictively. In the long run it may be wiser to ask yourself ‘Is there an alternative explanation?’ rather than simply shrugging, ‘Plumes do that’.
How else to melt the mantle?
The best place to find alternative explanations is mantleplumes.org a site dedicated to “discussing the origin of “hotspot” volcanism”. The site lists many mechanisms, but I’m going to focus on just two: edge-driven convection and shear-driven upwelling.
It’s not that hard to melt the mantle. It’s everywhere fairly close to its melting point and it gets hotter the deeper you go. A key point to understand is that most of it only stays solid because of the intense pressure it is under. As mantle quickly rises up beneath mid-ocean ridges it melts because it stays hot as the pressure reduces. All the atoms that were squeezed tightly together in solid crystal lattices manage to break free into a liquid state, once the earth’s grip lessens a little.
Almost all matter behaves like this, but it doesn’t feel like common sense because we are most familiar with the freezing and melting of water, which is weird and works the opposite way round (which is why ice floats). I labour the point because both of today’s mechanisms are ways of creating upwellings1 – areas where hot mantle material rises up and so is prone to melting.
Edge-driven convection (EDC) is flow caused around the edges of continents. Continents have deep cold roots to them, like icebergs2. A convection cell is set up with a zone of upwelling about 600km from the craton edge. It wouldn’t be surprising if you find some volcanic rocks above here.
The above model assumes nothing is moving, but we know that there will often be flow of the mantle relative to the plates. If there is mantle flow across an edge (for example a craton edge) then material will flow up. This is one way of producing shear-driven upwellings (SDU)3
So far, so theoretical. Let’s go to Australia and look at some rocks.
The Newer Volcanics Province is an active (but dormant) volcanic area in Victoria, Australia. To get a great overview of its many great volcanic features, check out this post (from which the photos here come). The lava is basaltic in composition – just what you’d expect from melting of mantle, but we are a long way from a plate boundary.
A recent paper in Geology studies what’s going on deep beneath the lava. Rhodri Davies and Nicholas Rawlinson of ANU, Canberra and Aberdeen universities start off with a spot of 3-D seismic tomography. Previous workers through they could dimly see a plume beneath, but armed with a new seismic data set from the (wonderfully-named) WOMBAT project they show there is no plume. Instead they show a shallow low-velocity anomaly underneath the NVP, consistent with region of hotter mantle, perhaps containing a small proportion of magma.
Having made the plume vanish, they turn to modelling of the mantle flow, based on their new improved knowledge of what is down there.
This area of Australia sits outside of the deep cratonic root. It’s like a thin ledge sticking out from the side of the iceberg. Therefore the edge of the deep root, that might cause EDC is to the north, allowing the upward return flow to sit directly beneath the NVP. Their models also include relative plate motion (how the plate is moving relative to the mantle beneath). This allows them to model the effects of SDU as well.
The modelling results produce a region of upwelling with velocities between 1 and 2 cm a year – fast moving for mantle – sitting directly underneath the NVP. This neatly explains the NVP, without any need to invoke plumes.
The mechanism is neat, but begs the question as to why there isn’t a line of volcanoes all around cratonic roots. Addressing this question, they point out the interaction of SDU and EDC. Under the NVP the two effects are complimentary – upwelling is increased where the mantle is flowing away from step. Also the edge here isn’t straight – 3D effects are important. Finally, mantle composition varies. So-called ‘fertile’ mantle may melt under conditions where mantle that’s already had melt extracted would not.
Are plumes dead?
There’s a compelling model here for explaining many volcanic hot-spots around the world with no need for plumes. Do we need plumes at all? Gillian Foulger, the force behind mantleplumes.org certainly doesn’t think so. Also Don Anderson of Caltech who recently had the posthumous last word at the AGU annual meeting last year.
Their views may prevail in time, but for the moment most of us still believe in plumes. Explaining how small-scale convection causes a minor volcanic field in one place doesn’t explain continental flood basalts like the Deccan or Siberian Traps. You know, the ones that cause mass extinctions and thickly cover vast areas.
But clearly plumes and not the only game in town. To progress, ideas involving plumes need to be anchored in our understanding of the deep earth, to be falsifiable and have predictive power. Recent research aims to do just that. Watch this space.
Davies D.R. (2014). On the origin of recent intraplate volcanism in Australia, Geology, 42 (12) 1031-1034. DOI: http://dx.doi.org/10.1130/g36093.1