Beyond plate tectonics

Posted on 2 August, 2015 by Metageologist

Plate tectonics is the core unifying concept that has underpinned our understanding of the solid earth for over 50 years. To describe your research as moving “beyond plate tectonics” is quite a claim, but Trond Torsvik and the group he leads have some remarkable science to back it up. By tracking the movement of the earth’s plates over half a billion years they trace the effects of hot plumes of rock rising from the edges of structures sitting just above the earth’s core. Their research seeks to explain the origin of diamonds, immense volcanic eruptions linked to mass extinction events, the break-up of continents and how shifts in the earth’s axis caused glaciation in Greenland.

Dance of the plates

Trond Torsvik is a Norwegian scientist with a background in palaeomagnetism – studying fossils of the earth’s past magnetic field frozen in rocks – to trace the past locations of continents. Palaeomagnetism can tell you the latitude at which an ancient rock formed1. Torsvik worked with those in other disciplines – palaeontology and geology – to trace the slow joining and splitting of ancient continents.

This research (which involved many other scientists) has given us a pretty good view of how the earth’s plates moved around over the last 500 million years. But these movements are only the surface expression of the flow of the underlying rocks, the earth’s mantle. Now, as director of the Centre for Earth Evolution and Dynamics at the University of Oslo (CEED) Torsvik seeks to produce an integrated understanding of deep mantle flow – mantle dynamics – and how it drives plate tectonics and other surface processes.

Undoing subduction

The earth’s mantle convects. Although made of solid rock, over geological time-scales it flows like a liquid and we understand the physics of this process well enough to produce computer models of it. One important factor is subduction – as oceanic crust cools it sinks back into the mantle, changing the patterns of flow.

Based on our understanding of how the continents moved in the past, the CEED group (Bernhard Steinberger in particular) have calculated where ancient subduction zones were and therefore where the subducted plates ended up in the deep earth. These models of ancient mantle flow and subduction link our surface observations with deep-earth processes.

The diagrams below show how subduction zones have moved over time. The outline of the continents is fixed, representing a stable reference frame. The coloured lines show how subduction zones at the edges of plates have moved over time2.

The red lines correspond to modern subduction zones, but the colour coding shows how where they used to be in the past. Note how the western edge of the North America plate has moved east over time3. Also note how it shows the subduction zone that used to exist north of the Indian plate and which ceased around 60 million years ago as India and Asia collided (as the oceanic plate in between was completely subducted).

Steinberger, B., & Torsvik, T. (2012) figure 2b

Here we have the same picture, but starting from 140 million years ago and moving back to 300 million years ago, the beginning of the Permian. These are the subduction zones that surrounded the ancient continent of Pangea.

Steinberger, B., & Torsvik, T. (2012) figure 2a

The diagrams aren’t showing it directly, but they remind us that the oceanic crust that passed through these subduction zones is still down there in mantle; imagine the series of coloured lines as sheet descending down into the earth – that is a rough image of what is down there.

Deep structures affect the surface

Mantle plumes have long been suggested as the cause of chains of volcanic islands (like Hawaii). Many believe the concept has been overused and that some proposed plumes don’t exist – this is a controversial area. Torsvik and CEED have taken the debate forward by presenting a testable hypothesis – that big plumes form around the edge of structures at the base of the mantle and that this has been happening for hundreds of millions of years.

Seismic tomography shows mysterious lumps at the very base of the mantle. They are called Large Low Shear Velocity Provinces (LLSVPs) and one sits under Africa and another under the Pacific. They are probably patches of different composition, but no-one knows for sure.

The CEED group believe these LLSVPs haven’t moved for a long time, so they took their models of plate movements to show how surface features have moved over them over time. They also plotted the locations of unusual volcanic features called kimberlites and vast piles of lava called Large Igneous Provinces (LIPs). The diagram below shows an example from 160 million years ago – here they’ve plotted the ancient location of the continents, plus that of the LLSVPs (in red). Note that kimberlites are found where areas of craton (thick old continental plate shown as grey areas) are above the edges of an LLSVP. Kimberlites are the host rocks for diamonds, so this result is not of purely academic interest.

Torsvik, T., et. al (2010), figure 2

This pattern holds when the analysis is done for other periods in the past, also when looking at modern active hotspots. Put all the data together and the pattern is quite impressive. Note that kimberlites and hotspots are not shown in their current position4 but the continents are.

Torsvik, T., et. al (2010) figure 1

This is a startling result. The fit isn’t perfect (the white dots don’t fit the pattern) but nothing on this messy planet of ours ever is.

So why are LIPs and kimberlites associated with the edges of the LLSVPs? The linking factor is deep plumes, which interact with deep continental lithosphere to produce kimberlites (and bring diamonds to the surface). Big plumes cause LIPs and the one shown above around the location of modern-day St Petersburg is the Siberian Traps which caused the largest mass extinction ever know at the Permian-Triassic boundary.

Surface processes affect the deep earth

What links plumes and the edges of the LLSVPs? Think back to those diagrams of ancient subduction zones and those curtains of ancient oceanic crust sinking into the mantle. Modelling of mantle flow through time shows that the ancient subducted crust reaches the base of the mantle where it pushes up against the LLSVPs. The flow of heat from interior of the earth to the surface drives the hot material rising up through the mantle but the interaction between plate and LLSVPs provides plausible mechanisms to get plumes started – the sinking plate pushes on the edge of an LLSVP and creates domes that turn into plumes.

What I like about this work is that by presenting a clear mechanism and predictions of how the deep and surface earth work together it is eminently testable. If mantle plumes form at the edge of LLSVPs, how does this affect the chemistry of the molten rocks that reach the surface? Perhaps one side contains the LLSVP material and another not. Any new seismic tomography data can be compared with the computer models that underlie this research. Does this research give us a new way to find diamond deposits? Finding answers to any of these questions will either help confirm the hypothesis or take research in new and interesting directions.

Our wobbly world

So much science, so little time! But allow me to test your attention span a little more and talk about my favourite example of how research from CEED links the surface and the depths of this planet.

The presence or absence of ice on this planet is one of the longer-term climatic cycles observable in the fossil record. For all of the last half-billion years, glaciation has been restricted to the southern hemisphere – until the last few millions years. Climate is the major control over glaciation, but a paper this year points to three ways in which deep earth processes caused glaciation in Greenland to start.

Steinberger, B., et. al, figure 5

Firstly, Greenland is unusually high (and so cold) – this is because the deep plume now centred on Iceland thinned the Greenland lithosphere and, from five million years ago, fresh ‘plume pulses’ pushed it up. Secondly, standard plate-tectonics has caused it to drift north (blue points and lines in diagram) by 6 degrees. Thirdly and most mind-bogglingly, changes in the distribution of density of the earth’s interior have caused the earth’s pole of rotation to move closer to Greenland by 12 degrees (green points are observation, pink are theoretical calculations).

If you’ve ever pushed a barrel or ball part-full of water, you’ve some sense of what lies behind the third cause, known as “true-polar wander”. Classroom globes have have a solid rod down the earth’s axis, but the real earth does not – it rotates around an axis called the ‘maximum moment of inertia’ that is determined by the distribution of mass within the planet. If this distribution of mass changes over time, then the axis changes and the poles shift to compensate. Modelling suggests that the shift of the north pole towards Greenland was caused by increased subduction under East Asia and South America.

Plate tectonics explains subduction. But models that show subduction tweaking the earth’s axis to bring glaciers or tickling the deep earth to create mantle plumes that can kill off nearly all life, break up super-continents, and send diamonds tinkling up to the surface. That really is going beyond plate tectonics.

References & image credits

This post is necessarily a skim over large amounts of complicated research. If you don’t believe it’s true, at least read the papers yourself. All are available online.

Source of images are in the image text. All either from open-source papers or produced under fair-use.

This Nature paper links LLSVPs, diamonds, plumes and LIPs.
Torsvik, T., Burke, K., Steinberger, B., Webb, S., & Ashwal, L. (2010). Diamonds sampled by plumes from the core–mantle boundary Nature, 466 (7304), 352-355 DOI: 10.1038/nature09216

This details the mathematical models linking subduction, LLSVPs and the initiation of plumes.
Steinberger, B., & Torsvik, T. (2012). A geodynamic model of plumes from the margins of Large Low Shear Velocity Provinces Geochemistry, Geophysics, Geosystems, 13 (1) DOI: 10.1029/2011GC003808

This links deep-earth processes to the onset of glaciation in Greeland.
Steinberger, B., Spakman, W., Japsen, P., & Torsvik, T. (2015). The key role of global solid-Earth processes in preconditioning Greenland’s glaciation since the Pliocene Terra Nova, 27 (1), 1-8 DOI: 10.1111/ter.12133

This contains the detail about true polar wander.
Steinberger, B., & Torsvik, T. (2010). Toward an explanation for the present and past locations of the poles Geochemistry, Geophysics, Geosystems, 11 (6) DOI: 10.1029/2009GC002889

Looking from the sky at diamonds

Posted on 22 February, 2015 by Metageologist

When a geologist looks at Google Maps images, we usually filter out any human activity. But in the case of mines, that would be a mistake – the holes humans dig can tell us about the geology.

What’s this? A big circular hole in the ground and large piles of bluish mine waste. The shadows are on the northern side, showing we are in the Southern hemisphere. This is the Letlhakane mine in Botswana.

And here’s another one, in the northern hemisphere. The hole here is half a kilometre deep (it’s an older mine, the Mirny mine in Siberia. They used jet engines to melt the soil so they could dig).

And again, the Diavik mine in Canada. Note that the hole is considerably deeper than the lake it sits in. They really wanted to dig that hole and yet most of the its contents are sitting in those big blue piles.

Finally the oldest example in the world, the Big Hole in Kimberley. The piles of dirt were put back in the hole, or covered in houses. Mining underneath here reached a kilometre depth.

We’ve been looking at some famous diamond mines. Diamonds form deep in the earth and those worth digging holes for only reach the surface via weird fizzy molten rock called kimberlite. This magma zips up from 100km depth to the surface in only hours. Travelling at depth along a narrow crack (or dyke in geo-speak1) when the magma reaches the surface it forms a carrot-shaped pipe. The magma solidifies, peppered with diamonds that formed at depth and were pulled up inside it.

The pipe is circular in cross-section, so as the miners dig out the kimberlite they leave a circular hole. The vast majority of what is dug out is waste – only the precious diamonds are extracted. Kimberlite is bluish in colour, as you can see from the piles of it above.

The Big Hole in Kimberley was the first kimberlite pipe ever identified2, in the 19th Century. Diamonds found before then were from placer deposits, river gravels that contain diamonds eroded out of kimberlite pipes. Diamond placer deposits were first discovered in India and then Brazil. But the biggest area for modern mining is on Namibia’s Skeleton Coast.

Southern Africa’s Orange river rises on the Kapvaal Craton, an area rich in kimberlite pipes. For 100 million years it has flowed across the continent into the Atlantic ocean, leaving thick placer deposits. These have since been pushed around since by wind and ocean waves to cover a wide area.

Starting at the beginning of the Twentieth Century, German settlers found diamonds and the government designated a huge area of land as Sperrgebiet – forbidden territory. This whole area remains closed, but active mining is concentrated the southern end, on the coast, where the diamonds are concentrated in ancient beach sands.

The mine here looks very different from the others, no round hole or blue kimberlite (but look for the regular patterns on the spoil heaps). Like the other mines, this one was caused by the desire for the beauty and strength of diamonds.

Six amazing facts about what’s under your feet

Posted on 11 June, 2014 by Metageologist

Just 100s km away from you are right now there is a place where rocks can flow, where once-living matter is turned in jewels, from where deadly plumes once killed nearly all life on earth. We can never visit this place – even though it’s right under you.

1. Human beings will never reach the deep earth

We’ve been to the moon and Mars is almost in our reach but we are never going to the deep earth. The deepest hole ever drilled got just over 12km below the surface – that’s a paltry 0.2% of the 6380km to the earth’s centre. It gets harder to drill the further you go. The rock was already 180°C when they stopped drilling and it gets hotter as you go down, getting as high as maybe 6000°C (as hot as the surface of the sun).

We could *maybe* send a probe down there. In a paper in Nature, David Stevenson came up with the most plausible plan to date. It involves making a crack in the earth (with a nuclear explosion) and filling it with 100,000 tonnes of liquid iron and a small probe. The probe (might) then sink down to the edge of the earth’s core. No-one is planning to try this out at the moment, but it would make for an interesting Kickstarter project.

There is a lot of heat in the earth. Consider that scene in Star Wars when the Millennium Falcon comes out of hyperspace and finds just spinning rocks where Alderaan used to be. If a Death Star destroyed earth you wouldn’t get an asteroid belt immediately afterwards, instead you’d get a extremely hot cloud of liquid, or even vaporised rock. Even Han Solo would have trouble dodging that.

2. A spinning ball of molten metal

The earth’s structure is a bit like that of a peach. There’s a thin skin, a thick juicy layer and a core that’s surprisingly large and totally different from the stuff above it.

We are the thin layer of mould on the surface of our giant peach. The skin is the earth’s crust (that we can’t even drill through). The bulk of the earth is called the mantle and its made up of a dark heavy rock called peridotite. We can never get to the earth’s core, but we know (from indirect observations and by analogy with meteorites) that it’s made of iron and nickel – totally different from the silica-rich rock above. The outer layer of the core is molten, but at the very centre of the earth its solid, made out of giant crystals of iron and nickel.

A giant sphere full of 15 tons of liquid sodium, used to simulate the earth’s core. Source.

The spinning liquid outer core produces the magnetic field that twists your compass and tells dogs which direction to pee in. One way to understand the core better is to simulate it in the lab, by creating a giant sphere containing spinning liquid sodium. It’s also a good way for scientists to look a bit more like Bond villains standing next to their super-weapon.

3. You can’t handle the pressure!

It seems counter-intuitive. The earth gets hotter the deeper you go, yet the outer core is molten and the inner solid. Normally things melt when they get hotter, so why is that?

Pressure.

We’re familiar with the fact that pressure increases in the deep sea, as there is a lot of water above, pushing down. It’s the same in the earth, only rock is heavier and there are thousands of kilometres above, pushing down.

The pressures get so intense that the only way to reproduce them in the lab is to take an tiny unfortunate sample of rock, put it between two diamonds and squeeeeze. To reach the required temperatures, you also fire lasers at it, through the diamonds. Occasionally the diamonds can’t handle the pressure and they explode, sometimes popping loudly or emitting a flash of light.

Diamond damaged by laser fire. Image from Wendy Panero.

Diamond damaged by laser fire in a diamond anvil. Image from Wendy Panero.

Pile of diamond dust after ‘blowout’ of the anvil. Source.

Under high pressure, it’s easier for materials to be solid rather than liquid, as the atoms prefer to be tightly packed together. The metal in the earth’s core is solid at the centre because the pressure is higher there.

4. Solid rock that flows

We know from listening to earthquake waves that pass through it that the earth’s mantle is solid. It’s incredibly hot, but the pressure means that – give or take a few percent in places – there is no liquid rock down there.

Knowing this held back acceptance of the idea that continents move. The geological evidence was known, but it was thought that continents couldn’t move because they were attached to the mantle and ‘solid rock can’t flow’. Only it can.

Hit a piece of the mantle with a hammer and it will break (or break your hammer – it’s tough stuff). But heat it up and push it the same way for millions of years and it will flow. It’s made of crystals, endless ranks of atoms lined up in rigid patterns. Tiny gaps in these patterns allow atoms to slip past each other and slightly change the shape of the crystal. Countless of these tiny steps, in myriad of crystals over millions of years allows the earth’s mantle to flow, convecting in majestic patterns driven by heat leaving the earth.

Patterns of plate tectonics on the surface are driven by this flow. The most dramatic example being subduction, where crust sinks down into the mantle, sometimes sinking deep down to the edge of the core.

Cut-away diagram showing modern convection from computer modelling by Fabio Crameri. Red is rising plumes, blue sinking plates.

Cut-away diagram showing convection in the earth. Red is rising plumes, blue and yellow sinking plates. Image generated from computer modelling by Mario Crameri.

5. Death from below

The link between meteorite impacts and mass extinctions is well known – the image of a dinosaur looking up at an incoming fireball is almost a cliche. However some scientists think they should be looking down, and that the deep earth has caused more extinctions than impacts have.

The event that did for the dinosaurs, at the end of the Cretaceous (66 million years ago) is just the most glamorous of many mass extinction events. The biggest was at the end of the Permian (252 million years) – up to 96% of marine species became extinct. There is no good evidence for a meteorite impact at the end of the Permian, but there is a huge pile of lava, covering much of Siberia. This was caused by a massive mantle plume, a column of hot rock that started at the base of mantle.

To kill nearly anything you need to foul the sky and poison the seas. Volcanoes give off noxious gases which in normal amounts don’t cause problems. But covering 2 million square kilometres of land in lava is not normal. However it was done – by pumping out massive quantities of ash or by producing carbon dioxide by burning nearby coal deposits (or some other way) – the link between mantle plumes, lava and death seems pretty certain.

The end-Cretaceous extinction saw a meteorite impact, but it also saw a massive outpouring of lava from the mantle, this time in India, at exactly the same time. There are those who argue that the death of the dinosaurs is as much due to the deep earth as it is a rock from space.

A huge outpouring of lava. From Wikipedia

A huge outpouring of lava. Image from Wikipedia

6. Visitors from the deep

A lot of what we know about the deep earth comes from indirect measurements, like when a doctor uses a stethoscope or MRI scanner to ‘look’ inside your body. But sometimes to know what’s really going on you need a direct sample from deep inside – a biopsy. To do this for the heavenly body we sit on – in other words to perform a geopsy1 – you need a special kind of volcanic rock. When some molten rock rises from the mantle, it contains crystals that were formed at depth, but carried up in it. The most interesting, most glamorous, most beautiful, and most valuable of these exotic visitors are diamonds.

I could go on about diamonds for a long time (in fact, I already have) but forget that they can be made billions of years old, and may contain traces of an ancient oxygen-free atmosphere, instead let’s focus on the fact that some diamonds contain carbon that was once part of a living thing.

When carbon has passed through a process of photosynthesis, its isotopes have a distinctive pattern . Finding this ‘light carbon’ in diamonds allows us to tell an amazing story. Something was once alive2, died and formed black mud on the ocean floor. This was then forced into the mantle where it sank deeper and deeper. At some point the carbon started flowing up (as part of some sort of fluid) and got pulled into a growing diamond which then got caught up in some fizzy magma that, within hours or days, pushed up to near the earth’s surface, where humans could find it and marvel.

Human beings can never reach the deep earth. Alive. The carbon in our bodies might though. Just arrange to die and get buried in the right bit of the sea bed and part of you might one day end up in the deep earth.

References

What to know more? Don’t believe these amazing facts are true? Either way, read these links for more details.

The idea of how to get a probe into the deep Earth come from a pukka scientific paper.

The Death Star isn’t really real (honest), but scientists model the liquid/gas rock that would happen if the earth was destroyed when they model a collision with another planet. Something that may have happened long ago to create the moon.

If you want to see that huge sphere containing liquid sodium in action, see it here.

My information about exploding diamonds came from Mary Panero, who is on Twitter @mineraltoPlanet

All you ever wanted to know about the Siberian Traps and the end-Permian event can be found on a great Leicester University site.

Scientists who believe mantle plumes rather than the meteorite killed the dinosaurs point to evidence from India. The lava (known as the Deccan Traps) is seen to happen at the same time as the extinctions, but the layer of Iridium enrichment comes after the extinction. More details here.

More more information about diamonds, there’s a useful recent review of the science.

Story of an atom: diamond

Posted on 15 December, 2013 by Metageologist

This is the third part of a story told to me by a Carbon atom in my brain. It started with her tale of how she ended up on earth, followed by an inside view of the Carbon cycle.

So like I said, I’ve fallen into this pattern of cycling around different places, occasionally going underground for a bit. One time I got buried was really special. Just… just a really *deep* experience, you know?

It started off as normal, I was in a bunch of organic matter that’d settled onto the bottom of the sea. Things got slowly hotter and more squashed, but this time it just didn’t stop. Things started falling apart – everyone got antsy and wanted to move around, some atoms badly wanted to escape. The organic gunk I was in broke down. Lots of water started coming out and disappearing upwards, eventually there was just us carbon atoms left. Lots of other atoms around us too, of course, lots of silicate minerals – big structures dominated by Silicon and Oxygen, all in rigid ranks. It got so hot and pressured that even they couldn’t cope and had to start rearranging themselves to get more comfortable. Some minerals just disintegrated and the atoms had to find other minerals to join1. I’d never experienced this before – I’d never been buried so deep.

You know when you’ve drunk too much coffee and you’re stressed? You feel like you’re vibrating really fast but you’re quite uncomfortable and you feel stuck. Same with us atoms in the deep earth. One way to cope is to try and help each other. Take us Carbon atoms in that organic lump. At first we were just jumbled up in a heap, but eventually we sorted ourselves out a more comfortable arrangement – we got together in lots of groups of six, all joined up but flat. If we stacked up like this then things weren’t too bad 2. But it just kept getting more extreme. Eventually every single mineral changed, even quartz – I’ve never seen that before or since 3.

Things kept getting more extreme. All around us atoms kept shuffling around into new configurations that were more comfortable. Hydrogen was very uncomfortable, it became harder and harder for it to find a place in these new arrangements of Silicon and Oxygen and more and more it joined with Oxygen and disappeared upwards. Eventually, some of us Carbon atoms broke up and starting moving upwards in a liquid4.

We travelled a long way like this (but were still very deep) when suddenly: it happened. There was this crystal of carbon – the most amazing thing. You call it a diamond. I’d always been a bit snooty about atoms all locked together in a mineral – fancied myself as a free spirit. But I so wanted to join in this crystal of just carbon. The other Carbons beckoned me in and at first I couldn’t see how it would work. When I’ve been with carbon before, I was joining with 3 other carbons in a flat plane. In this crystal all 4 of my bonds were joined, each with another Carbon. Not just flat too. I only managed to squeeze myself in because we were so tightly squashed together and buzzing around so much 5.

Once I was properly in there, I didn’t mind the conditions. We felt so strong, all together, bound so tightly. After a while I started to lose myself – no more ‘me’, only ‘us’. I was merging myself into a greater thing. One great collective of Carbon, perfectly happy in one eternal unchanging moment….

Sorry, drifting off there. Amazing times, so special. It couldn’t last though: diamonds aren’t forever, not really. The first sign of trouble was when the rock around us started to melt. We ignored it, but suddenly the whole area around us started shooting up through a big crack! The pressure dropped incredibly quickly – we were in a panic because it felt like we might start breaking up – could we stick together in these new conditions? Luckily we quickly cooled as well, making it easier for us to stay together6.

We soon got used to the new conditions – we were still underground after all – and we remained strong, ready for anything. It was a rude surprise when the rock around us got crushed up and we saw daylight for the first time. We were having a lovely time bouncing those photons about through us when this human hand grabbed us and put us in the dark again.

We were still together through all of this, which made what happened such a sudden shock. We were whipped out of the bag and put in a funny metal box. All the air around us disappeared and then ZAP! A huge beam of light hit us. Hit me! There was so much energy that a bunch of us got blown apart, all our bonds broken. There I am, alone and floating in space, just I was in my first memory. I soon hit some weird thing and ended up back here on the surface, going through the same old cycles7. For a while it all seemed so shallow, so temporary, so lonely. I’ve talked to other Carbon atoms about it, but none of them know what I’m talking about.

Still it’s been nice talking to you about it. It looks like you’re about to break up this molecule I’m in, so I’ll be off soon, back out into the atmosphere. Who knows where I’ll end up next!

No atoms were harmed in the making of this story.