Folded sediments from the Welsh coast

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My life is currently in a phase that isn’t compatible with many trips to the field. No complaints, but this does mean a lack of opportunities to take geological photos. So when my mum told returned from a geological field trip to Pembrokeshire in Wales, I was soon pestering for a copy of her pictures. I was not disappointed, as you shall soon see.

Sticking out of the southwestern corner of Wales, Pembrokeshire contains a range of rocks, from Precambrian to Carboniferous. The above photo is of Devonian rocks of the ‘Old Red Sandstone’. These were deposited on the bones of the Caledonian orogeny in a continental setting (hence the red colour).

These sandstones started off as flat layers sitting on top of older folded sediments. Geology being complex, they didn’t stay flat for too long. The Variscan orogeny, which along with the American Alleghenian orogeny helped form the supercontinent of Pangea, caused folding of sediments across South Wales.

Here’s another example of the folding, from St Ann’s Head (click on it for a big view).

Let’s bring out some of the structure.

Note how the lines are of different shapes. Think about the folding process. These rocks are barely metamorphosed, they are in essences still layers of sandstone. As they are folded they’d much rather not change shape or thickness. Rocks like this often deform by a mechanism called flexural slip, where the layers slide past each other.

From a geometrical point of view, how do these different shaped folds sit alongside each other. Well, they  don’t – at some point the layers need to fracture.

Here’s a more detailed view of the left-hand fold.

In the core of the fold, the layers just don’t match up. I’ve put a red line at the most obvious mismatch – this is likely the location of a minor fault.

In yellow, I’ve highlighted some of the axial-planar cleavage visible in the rocks. Cleavage is where rocks have planes of weakness in them, as a result of deformation. They are a result of alignment or dissolution of minerals as a result of the deformation of the rock. Note how the angle of the cleavage relates to the fold, not the layers. In the core of the old (the axis) they are perpendicular to the bedding and on the limbs of the fold they are at an angle. Some of the lines are curvy, which is cleavage refraction, where the angle varies depending on the physical properties of the rock.

The cleavage, the folding and the minor faulting all formed at the same time – different ways the rock tried to deal with being squashed.

Here’s another Pembrokeshire fold, here in Carboniferous sediments. The style of the fold is completely different – much more angular, with deformation concentrated in the hinge of the fold. The limbs have been rotated, but may otherwise be little deformed.

AW#50 – fieldwork is thirsty work

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Evelyn over at Georneys is hosting this months Accretionary Wedge. Where she asks us to:

Share a fun moment from geology field camp or a geology field trip. You can share a story, a picture, a song, a slogan, a page from your field notebook– anything you like! 

When thinking through possible stories, a clear theme appeared. Drink – grog, booze, mother’s ruin, the sauce, a wee tincture, a cleansing pint, the devil’s own buttermilk. Call it what you will, it seems clear that a day in the field makes many geologists very thirsty indeed. It’s a way to relax after a day of physical and mental effort. A way to create social bonds. A pub provides a forum to talk, speculate, educate as a group.

Enough excuses, to the stories.

Guinness is not always good for you

The west of Ireland is one of the world’s best places to drink and has fantastic Geology. A traditional Irish pub like Hilary’s bar in Leenane, Connemara is a great place. A paradise where the Guinness is lined up on the bar for the full 5 minute settle. Where a smile and slight hand gesture are all that’s required for the barman to set up the next round. If you’re particularly lucky your pint will be brought to your table by a smiling 10 year old and there’ll be no strain on your tired legs. A word of advice – don’t buy a whiskey chaser with your Guinness. The ‘old fellas’ may do it, washing down the dregs of the porter with some cheap whiskey like Paddy, but they don’t have to walk up hills the next day or explain transpression to perplexed students.

It is possible to have too much of a good thing, even Guinness. I learnt this on a weekend trip to Donegal in NW Ireland in the company of some Ulster geologists. The rocks were fantastic- syntectonic granites, sub-solidus fabrics, glacially scoured outcrops as far as the eye could see. During the day I built up some field-work-credibility by not wearing my waterproof on the principle that the frequent showers were only short and the wind would dry me off in between. Being near the Atlantic coast, you could see the day’s showers laid out to the west and plan accordingly.

Of an evening the rounds of Guinness kept arriving. Even if I was nearly a pint behind another one would appear. Not to keep up would be a clear dereliction of duty, a breach of a pub’s unwritten rules. My usual strategy of switching to gin and tonic, while acceptable in Oxford, would here only invite scorn and derison. So I kept up, but it was a heavy weekend. By the end my soft Sassenach nature came through. My liver cleared the alcohol but the whatever-the-hell-makes-Guinness-black stuff must have remained lodged in my body. Higher level brain functions took several days to return, even after the drinking stopped. Until then I wandered my field area, looking blankly at outcrops and wondering what had happening to me.

Whisky on the rocks

Scotland is one of the best places to drink and has fantastic Geology. A note of advice to university administrators. If you put students in self-catering accommodation in Mull and then pay them an excessive daily subsistence rate, they will spend it on Tennants 80-shilling and bottles of whisky. A least we did. Today’s students may have to put it aside to offset their huge debts, I fear.

One memorable field trip was to Assynt, land of wonders. We were booked to stay in Achmelvich youth hostel, where the washing facilities consisted of one cold tap. In a field. When we arrived it turned out to be double-booked, so instead we had to stay in the Inchnadamph hotel, famous haunt of Peach and Horne. It was the start of the season (Easter) and the beds were still damp, but we didn’t care because there was a bar. Forget any memories of the film Trainspotting, Scotland has many good bars and they all have a shelf entirely full of whisky bottles, each one a different single malt. We hopped our way along it of an evening, wandering into the night, sniffing whisky and enjoying a rather good display of the Northern Lights that the hotel had kindly laid on for us.

Image from http://www.theputechan.co.uk/ so it’s probably Dalradian schist.

Oceanic crust – down to the core

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Almost all of what I write about in this blog concerns only 1% of the earth’s volume. All crust, all sedimentary rocks, the glories of mountain building, all occupy an insignificant portion of the earth. It’s the only bit we can get to – in geology, we are the 1%. This post is all about the 99%, the earth’s interior, the mantle and core, which is remarkably different to here. The inner core reaches 5430°C (as hot as the surface of the sun) and pressure is multiple millions times atmospheric pressure. Forget your Jules Verne, we are never going to reach the centre of the earth*.

We aren’t isolated from the interior though, it affects us in many ways – from the earth’s magnetic field to plate tectonics and volcanic islands. The influence goes the other way too. Oceanic crust is constantly being pushed down into the mantle (maybe 20% by volume over time) and it goes a surprisingly long way. Rocks that form part of the familiar 1% start on the surface but travel down into the extreme conditions of the deep interior.

Cross section of earth’s interior. Image from Wikipedia

Deeper and deeper

In a previous post, we traced the journey oceanic crust takes, from its birth in a mid-ocean ridge down into the mantle in a subduction zone. We left the story at 250km depth.

Here our crust has been transformed. Its minerals have changed into new ones, stable at these depths. Metamorphism and partial melting have taken away most of its water and some other elements that have large ions. It consists largely of garnet and pyroxene. The material surrounding it, called peridotite may in addition contain olivine. As it descends further, at around 2cm a year, our plate continues to change.

A huge amount of what we know about the deep earth comes from listening to earthquakes. Seismologists can identify the location of earthquakes within the sinking slab. This pattern of earthquakes (called the Benioff zone) shows the shape of the subduction zone. Seismic waves can be used in multiple ways to identify the properties of the materials they pass through. The liquid outer and solid inner core were identified this way in the 1930s.

We can also now spot sudden changes in the speed at which the waves pass (seismic velocity). This implies a sudden change in the properties of the rock. In the mantle, there are multiple places where we see such a change. They are known as discontinuities and they occur at specific depths.

The shallowest mantle discontinuity is at 300km depth. It’s only seen intermittently and is most common beneath continents and island arcs. It’s been interpreted as a phase change in minerals made of SiO2 (silica). At the surface, the stable form of silica is quartz, but at depth it is transformed, first into coesite (discovered 1953) and then, at 300km depth into a mineral called stishovite (1961). Stishovite is dramatically denser than coesite so the transformation from one into the other changes the rock properties, making it visible to seismologists. Nearly all rocks contain SiO2, but in the mantle it is all locked up into other minerals. Only subducted oceanic crust has enough SiO2 to contain ‘free silica’. For this reason, the existence of the 300 km discontinuity in a particular place is evidence for the presence of subducted oceanic crust [1].

There is not much free silica even in our sinking slab of crust, so as it passes through the 300km discontinuity the transformation is relatively minor. Bigger changes await.

Into the lower mantle

At 410km we are leaving the upper mantle and entering the transition zone, marked by several seismic discontinuities. Here were are reaching pressures of 24GPa (250,000 atmospheres) and temperatures of 1600°C. Now olivine is no longer stable. It transforms itself into other minerals with the same composition (polymorphs), first wadsleyite (1966) and then ringwoodite (1969). 

These minerals with the funny names aren’t found in crustal rocks, aren’t stable under conditions humans can reach. The first line of evidence came from meteorites. Most meteorites are pieces of a rocky planet that formed a core and mantle but was then smashed into pieces by the hurly-burly of the early solar system. Both wadsleyite and ringwoodite were both first recognised in meteorites and are named after Twentieth Century scientists. Samples from meteorites give us an insight into high pressure conditions and a rare opportunity to find actual samples of them. The only other place is tiny pieces inside diamonds.

Blue Ringwoodite. Why are all high-pressure minerals beautiful colours? Image from Wikipedia

At a depth of 650km there is a significant discontinuity that marks the beginning of the lower mantle. This is the point where most of the minerals mentioned above are no longer stable. Under the intense pressure minerals with extremely dense, tightly-packed mineral structure are formed. Here Ferropericlase (aka Magnesiowüstite) and Silicate perovskites become the dominant minerals. Note the silicate perovskites have the same mineral structure as perovskite (a calcium titanium oxide) but a different composition.

It used to be thought that sinking crust couldn’t get through the 650km discontinuity. Maybe some slabs don’t, but ours does, continuing its journey into the lower mantle.

Much of what we know about the lower mantle comes from experimental work. In order to simulate the crushing pressures and high temperatures within the deep earth, experimenters have to take tiny samples and squeeze them between two diamonds and blast them with frickin’ lasers.  These ‘diamond anvils’ allow scientists to peer through to materials under deep-earth conditions, studying the minerals that form there. Only recently have scientists managed to create diamond anvils able to achieve conditions within the deepest parts of the earth. This brings home quite how extreme and alien the deep earth is.

Diagram showing conditions in deep earth, plus conditions reproducible in a Diamond Anvil Cell. Taken from http://www.earth.ox.ac.uk

Research on the deepest earth is cutting edge and not yet settled so the story of our plate’s progress becomes a little vague. A recent addition to the menagerie of extreme minerals is post-perovskite (2004) a mineral stable in the lowest 200 km of the mantle, a material so exotic it doesn’t yet have a proper name (being just ‘stuff perovskite turns into’). It’s been linked to the deepest seismic discontinuity, the D” (D double prime) which is a layer just above the core-mantle boundary. This mysterious boundary zone may be linked to the fate of our oceanic slab. As our crust ends its long journey it encounters a boundary it cannot cross. The core is made of metal, which doesn’t mix with our silicate crust, so the slab ends up lying flat on the base of the mantle, forming the D” layer.

Everything is connected

In one of earth science’s many dizzyingly lovely links between different domains, the very deepest layers of the mantle have dramatic influences on the earth’s surface. Firstly, large igneous provinces – big areas of vulcanism not obviously linked to plate tectonics, seem to originate in mantle plumes that start at the core-mantle boundary (these in turn may cause mass extinctions). One theory is that the presence of old oceanic crust leads to the instabilities that drive plume formation [2]. Here geochemistry, specifically isotope geochemistry, is our tool for understanding places we can’t reach.

Another link between the surface and the deep relates to the magnetic field that helps homing pigeons and old-fashioned geologists find their way. Some researchers speak of hot dense “thermochemical piles” created by deep subduction which influence heat loss from the outer liquid core. This core creates earth’s magnetic field and a recent paper links patterns of magnetic polarity reversals with these piles and so with patterns of deep subduction and plume formation.

The reality of deep subduction seems clear, even if the details are still coming into focus. A recent comparison of numerical modelling of subduction with cutting edge seismic imaging (seismic tomography) shows a good match [3] – oceanic plates really are going all the way down.

Although the deep earth seems an alien place, turning familiar minerals into substances we are only starting to understand, still it is intimately related to our surface world. A lump of basalt that forms at a mid-ocean ridge, that gets subducted and transformed into new minerals again and again, may ultimately rise up as part of a mantle plume and get melted, ending up as a new form of basalt. Crust and mantle are intimately linked – the 1% and the 99% together make up a single planet, after all.

* I say never, but it may be possible, provided you got hold of a nuclear weapon and a hundred thousand of tonnes of molten iron.

References

Web-links

[1] Evidence for 300km discontinuity (abstract only)

[2] Link between oceanic-island basalts and deep mantle (pdf of paper)

[3] Deep subduction – comparison of geodynamic and tomographic models (open source paper)

Old-skool links

Q. Williams, & J. Revenaugh (2004). Ancient subduction, mantle eclogite, and the 300 km seismic discontinuity Geology DOI: 10.1130/G20968.1

W. M. White (2010). Oceanic Island Basalts and Mantle Plumes: The Geochemical Perspective Annual Review of Earth and Planetary Sciences DOI: 10.1146/annurev-earth-040809-152450

B. Steinberger, T. H. Torsvik, & T. W. Becker (2012). Subduction to the lower mantle – a
comparison between geodynamic and
tomographic models Solid Earth Discuss DOI: 10.5194/sed-4-851-2012

The End of the World is Nigh

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Today’s XKCD cartoon was interesting (as well as the usual funny and clever).

The humour is contrasting the ephemeral world of Hollywood with the eventual fate of the world. My initial reaction, beyond enjoyment of the cartoon, was “what, only 800 million years?”.

Two things about this. First, only a geological training could make me say such a thing. Secondly, that’s really soon! In my head, I had the end of the earth as being 5 billion years away. Given that’s longer than the earth already existed, it was over my geological event horizon. The 800 million year figure is, this being XKCD, entirely accurate, and I have rocks sitting on my desk at work that are older than 800 million years.

The difference between the two numbers is because they are talking about different things. Long before it becomes a red giant (5Ga) the sun will hugely increase its output and burn off the thin blue/green layer on the earth’s surface (800Ma). This I did not know. What surprises me is how much this changes the way I think about things.

It’s not about people. The concept of the Anthropocene is interesting and useful in lots of ways and one of them is the way it jams together the geological and human timescales. We know now we have messed up the earth enough to leave a trace for a very long time – humans are the most remarkable thing to ever happen to this planet – but we know that whatever is around in 10s of millions years time, there won’t be any humans, or if ‘we’ avoid extinction, we won’t be human anymore.

So people and 800Ma are things that don’t match. But, somewhere in my brain was a view of the earth as a system, with a long past and a long future. What I know now is that the future will ‘soon’ be very different. You don’t need to like the Gaia hypothesis to be impressed with the way life and earth have cooperated to moderate temperatures and keep liquid water on the surface of the earth for 4 billion years, during which time the solar luminosity has varied a lot. It seems that the sun will, within less than a billion years, break this nice balance and sterilise the planet. Life, having spent ?3 billion years failing to become leave much of a trace has done some remarkable things in the last 540Ma. Does it really only have 800Ma more years to go? What about plate tectonics? Having chugged away for ?3Ga, will it survive a lack of lubricating water, will subduction zones gum up leading to plume tectonics and massive volcanoes?

We’ll never know of course as we’ll be dead. Ah well.

Anyway, thank you Mr Munroe for expanding my mind. My brain does hurt though. Some white wine I think.