Hot spot volcanoes: no plumes required?

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’.

mt-noora-16

Layers of volcanic debris and ash from the Newer Volcanics Province, Australia. Spot the bombs. With permission from Stephanie Sykora

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.

EDGEFig2_500

Diagram showing edge-driven convection. Reproduced with permission from mantleplumes.org

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

SCUFig1_600

Diagram showing mechanisms of shear-driven upwelling. I discuss the left-hand example. Taken with permission from mantleplumes.org.

So far, so theoretical. Let’s go to Australia and look at some rocks.

Welling-up down-under

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.

"Volcanic bomb with a olivine-rich xenoliths from the mantle at Mt. Noorat – Victoria, Australia" Courtesy of Stephanie Sykora

A piece of mantle that flowed upwards and melted: “Volcanic bomb with a olivine-rich xenoliths from the mantle at Mt. Noorat – Victoria, Australia” Courtesy of Stephanie Sykora

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.

References

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

Categories: Deep earth

Great Geology in Google Maps: dunes

Google Maps is a great resource, particularly in satellite view. My favourite way to enjoy it is via the Chrome extension “Earth View from Google Maps“. This pops up a gorgeous image in every new tab. Many show human landscapes, but every now and then one appears that catches this geologist’s eye. This post is the first in a series exploring and celebrating these images.

This view is of the Rub’ al Khali or ‘Empty Quarter’ of the Arabian Peninsula – the largest sand desert in the world.

In dry environments sand is moved not by water but by wind. The characteristic landform is the sand dune. Common in deserts on earth, they are also found on Mars and even comets.
The shapes of dunes depends on the supply of sand, but above all the wind. Wind strength and direction, averaged over the year, will determine the shape of a dune. Various types of dune are recognised. The ones in this image are complex. Further east of here the dunes are clearly linear features, but here the lines are broken up into loops reminiscent of arabic script.

Look carefully and you’ll see that there are dunes upon dunes. The surface of the large sand bodies are covered in ridges and patterns with a wide variety of shapes. These themselves may have small ripples on, only visible if you visit in person.
 

This image is from the ‘Grand Erg Oriental’ in the Sahara desert in Algeria. It shows star dunes, which form when the wind is variable and simply piles the sand up into mounds 100s metres tall.

Both these areas of sand (by chance) sit above oil-fields, the oil-bearing rocks sitting deep under the ground. Ancient desert sands are of interest to oil geologists as the grains are very round and form sandstones that can contain a lot of liquid.
Ancient ‘aeolian sandstones’ are basically fossilised sand dunes and are often red. The ‘red sandstones’ of the UK, much Triassic sandstone in Europe and the classic Navajo Sandstone in the US are of this type.

Next time you see a red sandstone with big swooping cross bedding, think of these pictures.

Categories: Great Geology in Google earth, sediments

Tasting the earth: mantle geochemistry

If seismologists listen to the earth then geochemists taste it.

Like experts blind-tasting a glass of wine and recognising where it came from, geochemists studying the deep earth aim to find out where a particular liquid came from. Their liquid – basaltic magma formed from melting of the mantle rocks – is now solid, so ‘tasting it’ involves dissolving it in Hydrofluoric acid or vapourising it in the bowels of a machine with an unlovely name.

A wine buff can sniff out where a wine came from because they’ve already sampled lots of known vintages. Geochemists have a much harder job – basalt samples don’t have labels. They are formed from melting of the rocks below, but was the material that melted from the deep earth or shallow? Is it from oceanic crust that’s been subducted and remelted or material that’s sat around since the earth was formed?

Mantle geochemists still have more questions than answers, but that’s because what they do is really hard. They are like first-time wine-tasters who’ve been given anonymous bottles and only a fuzzy satellite image of France to work with.

Image stolen from the Basalt winery. I'm sure they won't mind.

Image stolen from the Basalt winery. I’m sure they won’t mind.

What to sample?

Most basalt is produced at mid-ocean ridges, where oceanic plates move apart and the underlying shallow mantle rises up, decompresses and melts. Known as MORB1 this is plonk. Widely produced, homogenised and of little interest to the true connoisseur.

Basalt from oceanic islands (OIB) is mantle geochemists’ favourite tipple. Found only in select areas far from plate boundaries it has many flavours but can be distinguished from MORB by a trained nose. Thought to be formed by material rising up in hot plumes from the deep mantle it carries whiffs of what is lurking down there.

Of particular interest at the moment are dark intense picritic lavas. Formed under higher temperatures in smaller batches they tell us more about what happens when a mantle plume first nears the surface.

sniffing wine

Tasting

The process of producing basalt from the mantle is complex, depending on the composition and mineralogy of the melting material plus the depth and pressure. Also a lot may happen to the magma before it cools as the surface as lava. Iceland has rhyolite lava flows – very different in composition to basalt, but ultimately formed from mantle melt2.

So to study the mantle that was melted tasting the basic chemistry of the lava is not enough as changes due to later processing can obscure the smell of the source material. More sensitive mechanical noses are required, that can sniff trace elements or isotopes that may be unchanged by later processing and hold the tang of the source mantle.

Terroir

Mantle composition, as inferred from basaltic melt, is very variable, leading to the identification of a ‘zoo’ of acronyms, from DMM and HIMU (sources for MORB) to EM and FOZO (for OIB).

A key concept is ‘enrichment’. Particular elements are ‘incompatible’ which means that if they are in a rock that melts, they are strongly partitioned into the melt. As the ‘enriched’ melt moves away you are left with a ‘depleted’ residue. Continental crust is extremely enriched, oceanic crust less so.

For this reason the churned up mantle contains portions which are depleted by having had oceanic crust melted from it (DMM) and other enriched portions which contain recycled oceanic crust (HIMU). Small amounts of continental crust may enter the mantle – perhaps the mantle frozen to the base of continents may fall off. Also continental material (sediment, stones frozen into icebergs, the Titanic) may end up on ocean floor destined to be subducted. EM and FOZO are sources that may have been enriched in this way.

Primitivo

Geochemists don’t just worry about the mantle, but the whole earth. Chondritic meteorites have long been thought to be a model of the bulk chemistry of the earth. Strip out iron and other elements into the core, account for the enriched crust and you can calculate the bulk composition of the mantle.3.

Compare known mantle compositions with the theoretical bulk composition and you get a gap, leading to the idea of a hidden reservoir of ‘primitive’ composition (e.g. closer to chondritic). Conceptually this is similar to the idea of ‘dark matter’ in physics – a thing invented to explain inconsistent pieces of evidence, but for which there is no direct evidence. Only time will tell if hidden reservoirs in the mantle will be found or go the way of the luminiferous aether.

Basalt in a vineyard

Basalt in a vineyard

Paradoxes and problems

The idea of hidden reservoirs was extremely popular over 20 years ago, when it seemed that subducting plates stopped at 660km depth, where a ‘phase change’ in minerals alters the stiffness of the flowing mantle. This suggested that the lower mantle could be of very different composition. But modern seismic imaging suggests whole-mantle convection is possible, suggesting that over billions of years the mantle will have been thoroughly stirred – with the exception of a mysterious layer at the base of the mantle.

Mantle geochemists often talk of ‘paradoxes’ – patterns of ratios between elements and isotopes that aren’t consistent. There is a lead paradox, and an Argon one, plus a ‘heat-Helium imbalance’. Explaining these in terms of a primitive reservoir is one way, but others are possible. Let’s look at Helium.

Helium comes in two flavours. The first 3He is just two protons and a neutron and from the earth’s point of view it’s primoridal, it’s always been there and never changes. In contrast, when the great hulking nuclei of Thorium and Uranium fall apart they leave small fragments – making 4He in the alchemical process of radioactive decay.

The ratio of the two Helium isotopes is fairly consistent for MORB sources, but wildly variable for OIB. Material with a high ratio has been interpreted in terms of a primitive reservoir, rich in primoridal 3He. An alternative explanation is that the source is extremely low in 4He due to it being depleted in Uranium/Thorium. Or maybe the 3He bubbled up from the core.

Tasting the earth does not give you all the answers, but it is vital part of the picture. As I continue my tour of the deep earth, geochemistry will often have an important role to play. The difference between OIB and MORB is a powerful argument in the armoury of those who favour mantle plumes and as seismologists start to see odd things at the base of the mantle, getting a whiff of the chemistry here becomes very important.

Tasting ‘black cherries’, ‘tar’ or ‘cat-pee’ in wine is a clever trick. Tasting blobs of 4.5 billion year-old rock or recycled oceanic crust in basalt is even cleverer. Cheers!

Further reading

This is a good overview, if a little old.

 

Categories: Deep earth, geochemistry

Metamorphic petrology under stress: round 2

Back in August I wrote about an extremely important paper by John Wheeler of Liverpool University called “Dramatic effects of stress on metamorphic reactions”. This uses a theoretical approach to show that differential stress (squashing rocks) is a very important control on metamorphic reactions. If true, this would imply that many estimates of depth of metamorphic conditions (that ignore the squashing) are wrong. Maybe eclogites don’t form at great depth after all.

Counterblast

Conceived as a provocative paper, it’s no surprise to find a “Comment” on it in the latest edition of Geology. Written by Raymond Fletcher of Penn State, it aims to “show
that Wheeler’s claims do not have a sound basis” by constructing a more complete mathematical model “for metamorphic reaction and pressure solution” (the two processes that Wheeler’s original paper wound together).

For both our sakes, I’m not going to get into the detail of the mathematics (all papers are open-source, so you can read it yourselves). It isn’t massively complicated maths – single lines of algebra only –  but what matters here is the assumptions and simplifications made and whether they are valid.

Fletcher’s comment picks on one aspect of the original paper – that in the section quantified the effects of differential stress, it focussed on a single way in which atoms can rearrange themselves called incongruent pressure solution. Fletcher’s set of equations are a more complete model that shows that Wheeler’s results are merely a ‘special case’ leading to ‘contrived outcomes’.

 A spirited defence

‘Comments’ on papers are often followed by a ‘reply’ from the original author. As here where the ‘comment’ is negative, they are effectively a form of public combat.

Wheeler is uniformly polite and positive. He starts by thanking Fletcher for his stimulating Comment and listing the ways in which they agree. Then this:

“But it is inappropriate to say that I am wrong, first because his model is not of the incongruent pressure solution (IPS) pathway, second because it actually contains
confirmation of some of my claims, and thirdly because it is extremely
restricted in scope.”

He then proceeds to show that Fletcher’s model doesn’t just model a single pathway and the he lists the assumptions made by Fletcher and demolishes each one. For this audience member, Wheeler starts to win the battle by the depth of context he brings to the discussion.

For each assumption he refers to existing research into real world complications. These include: defining 3-D stress as a single term in an equation is complicated – taking the simple average of the 3 dimensions is not correct; fluid pressure may control reactions, not stress; the topology of grains is important and the one chosen by Fletcher extremely unrealistic; diffusion of atoms is often a limiting factor in metamorphism; porphyroblasts often grown in specific shapes – ‘interfacial’ kinetics may also be important.

For this (slightly biased) reader the knock-out blow was the fact that 3 times the research into these complications is his own allowing Wheeler to write that Fletcher “may well have rediscovered the sorts of problems described above (Ford and Wheeler, 2004) but by ignoring these he reduces the value of his assertion that…” .

All good replies to comments look to the future:

In summary, Fletcher’s model is too restricted in scope to undermine my conclusions: we agree that a more general model is required. I challenge him and other interested readers (including myself) to construct such a model, which would be of great benefit to understanding how metamorphism and deformation interact.”

Science in action and in the open

I remind you again  of the important implications of Wheeler’s paper – existing estimates of metamorphic conditions – used to build tectonic models – are suspect. To quote another article in Geology discussing it “the potential inaccuracy of depth estimates based on minerals would question current paradigms in geology“.

Wheeler’s original paper was a huge challenge to metamorphic petrology. It has withstood the first attempt to refute it. This is science in action, in open source papers for all to view. I hope you’ll read the papers yourself and we can follow the unfolding story together.

Categories: metamorphism