Where on Google Earth #322

I got WoGE #321 rather quickly, so I feel a little guilty. Perhaps because I was actually trying to work at the time. But I saw @FelixBossert tweet about it, took a peek and instantly recognised it as a drumlin field from the British Isles. Spotting the bit of sea on the right hand side helped me narrow it down quickly to Donegal in the north of Ireland. As it turned out I’ve drunk Guinness, if not in the actual image, at least nearby.
To assuage my irrational guilt, I’ve tried to put this post up nice and quick. Also I’ve chosen an area that, as far as I’m aware, none of you players of the game have drunk Guinness in. I won’t guess how hard it is, but I’ll not invoke the Schott Rule, so no restrictions.

If you have no idea what I am talking about, you are in for a treat! This is a game that is an excellent excuse to trawl through Google Earth. Rules are best expressed here, but in summary, look at the entire Earth’s surface until you find the area shown in this picture. Then find out about its Geology. Then add the location and Geological description in the comments. If you are first, you get the honour of hosting the next one. If you are a veteran WoGEr, this is the first one I’ve hosted on my new blog, so please look around while you’re firing up Google Earth.
I’m looking for a mention of a fairly specific piece of Geology, but this may be a little ambitious of me. We’ll see, but enjoy the lovely picture any way (click on it for a bigger one). The image is rotated so that the North arrow is pointing to somewhere between 9 and 10 o’clock. Or a vertical arrow on the image points east, if that makes more sense.

PS this post vanished and was hosted by Felix for a short while due to technical difficulties, thanks again Felix.

 

Metamorphism: open or closed?

Thermobarometry is the art of inferring Pressure and Temperature conditions by relating the composition of a metamorphic rock to a thermodynamic model, assuming chemical equilibrium was achieved. That’s the single sentence summary, but as usual, things are more complicated if you look in more detail. I’ve discussed elsewhere reasons why chemical equilibrium may not be achieved – in summary, where the rocks are not hot for long enough, or where reaction kinetics (the details of how minerals grow)  control metamorphic mineral growth. Can we at least be certain of the composition of the rock? That must stay the same, surely?

Nope.

Granulite facies sillimanite-garnet-spinel metasediment. Where did all of this stuff come from?

A major assumption when making thermobarometric calculations is that you are dealing with a closed system, that you have the same bunch of atoms at the end as you do in the beginning. Let’s think about this assumption of a ‘closed system’ in more detail. A first complication to mention is that metamorphic petrologists tend to make an exception for fluids. We don’t expect a rock that’s been through chlorite breakdown to leave a puddle on the desk – we know the water produced by the reaction is long gone (but we do account for it in our thermodynamic models).

Consider a closed system – how big is it? What scale are we talking about? Metamorphic mineral growth usually requires the movement of atoms, from a phase that is breaking down into one that is growing. So on the scale of a mineral grain we are not dealing with a closed system at all. To do metamorphic petrology though, we assume that the system is closed on the scale of a rock sample, or maybe thin-section. This seems reasonable if you think about metamorphosed sediments. Some sediments can’t make their minds up, are they are sandstone, or a mudstone? They end up being a bit of both, with alternating layers of dramatically different composition. When such rocks get metamorphosed, they still have layers, the compositional differences haven’t been completely homogenised. In a gneiss, centimetre scale layers in the sediment protolith aren’t preserved but metre scale layers often are. This is despite the intense ‘mixing’ that comes from the multiple phases of deformation these rocks have often enjoyed.

I’ve not presented any real proof here of course, just some hand-waving. Evidence that metamorphic rocks are not completely homogenised on a metre scale doesn’t mean that they are completely closed systems. Maybe only quartz doesn’t move, but other atoms zip around like billy-o over great distances?

Let’s think about things from the other side. What mechanisms could drive open system behaviour over >1m scale? How do you shift atoms around within solid rock?

Firstly, let’s get granite out of the way. If you heat rocks enough they melt and you get migmatites. Sometimes the pockets of melt manage to meet up and the melt is able to move up and away. Granite plutons can be huge, so this is an important mechanism of mass transfer on a crustal scale. Like with fluids earlier, this is something metamorphic petrologists have a handle on. The whole concept of restite in migmatites is an acknowledgement that the composition of the rock has changed. The reactions that create the melt can themselves be modelled, and so on.

Metasomatism is the chemical alteration of a rock. It is applied to cases where the composition has clearly been altered, which is what distinguishes it from metamorphism. Classic examples are skarn, or greisen, where rocks in contact metamorphic aureoles have their composition dramatically altered by hot fluids carrying other elements. Cases where elements such as gold are dissolved in fluids and then transported and concentrated are of obvious interest to economic geologists.

Around granite plutons the effects can be dramatic, but fluids are known to be present in many types of metamorphism. Indeed the presence of a fluid phase is an important factor in reaction kinetics, as a means of moving atoms around to help drive metamorphic reactions. This being the case, why should this only occur on scales of <1m? What if metasomatism was the norm in metamorphism, only with effects too subtle to spot?

Its an interesting possibility and some recent research is finding evidence that some textures previously interpreted as metamorphic are actually metasomatic. This is worrying for thermobarometry – if mineral growth is driven by fluid-induced compositional change, rather than by changing P-T conditions, then standard estimates of P-T may well be wrong.

Is this is a big problem? Well, thermobarometry is proven to be a useful tool for understanding mountain belts, but it’s a reminder that our understanding is incomplete. I think of my own PhD field area. The syntectonic gabbros had large areas of ‘Acid Gneiss’. This rock-type was rich on quartz, but also contained amphibole and calcic plagioclase. It is most likely a metasomatic rock, with silica being added to a gabbro host. An area of ‘andesine porphyroblastic gneiss’ in the aureole was suspiciously homogeneous and hard to link to a sedimentary protolith. Hmmmm. There may yet be a fantastic case-study of metasomatism to be made in these rocks, yet in my thesis I merely describe and move on. I didn’t have any conceptual or analytical tools available to me to make anything more of them.  My feeling is that one day someone will come up with a technique (stable isotopes? fluid inclusions?) that will revolutionise our understanding of hot rocks and show quite how far from being closed systems they really are.

One last quirk. Even if a rock sample is a closed system, can we simply take its bulk composition and plug it into THERMOCALC? Think of the classic Mn profile in Garnet, with lots in the core. Once it is in there, it is ‘locked away’ and doesn’t take part in metamorphic reactions, (until the garnet is broken down or higher temperatures allow the Manganese to diffuse away). Until this happens, metamorphic reactions are taking place in a system with low-Mn, compared to the bulk composition. This is a general problem. We know that systematic compositional variations in metamorphic mineral grains are common. This causes an effect analogous to fractional crystallisation in cooling magma – the composition active in metamorphic reactions (‘effective bulk composition’) changes over time. What’s a metamorphic petrologist to do? Dave Waters has a fine summary of these issues of effective bulk composition. In summary, study your rocks carefully and think carefully.

Labradorite: butterflies and poetry

Here’s my contribution to Ron Schott’s labradorite geomeme (inspired by Sandatlas’s Lunar Anorthosite).

One place that labradorite is popular is in posh office-buildings. I work in such a place and I snuck down to the canteen restaurant and took a picture. No shoes for scale this time, as they serve food just out of shot. The Schiller effect / labradorescence is there in the outcrop, honest.

Labradorite, ready for business

The rock type is most likely larvikite, a popular ornamental stone in the UK, quarried in (you guessed it) Larvik in Norway.

My favourite things about labradorite involve butterflies and poetry.

The optical effect that makes it so lovely, is due to exsolution in feldspar, a phenomena that results in lots of parallel layers of slightly different composition. The thickness of the layers depends on the composition of the feldspar, with labradorite, this thickness is very small, sufficiently close to the wavelength of light to cause beauty.

Image source Wiki Commons

The same effect is also found in butterfly wings, which I find rather pleasing. I knew a physicist who was being paid to study this effect in butterflies with a view to commercialising it. I don’t know how he go on, but maybe somebody is about to bring out a labradorescent something-or-other for us to buy.

I’ve always found exsolution rather poetic. At high temperatures, when they form, plagioclase feldspars are as one. Sodium and Calcium atoms mingle freely in the lattice, sharing the same spaces, side by side. They put aside petty differences of mass and rejoice in their similar size and valence, finding strength and common purpose, sharing electrons with their Aluminium, Silicon and Oxygen comrades. As temperatures fall, things change. In a colder world, they slowly, silently start to drift apart, huddling into ghettoes, seeking out only their own kind. The one, single shared lattice is balkanised, split into strips where only Sodium or only Calcium is welcome.  Every plagioclase crystal we see is flawed in this way. Only labradorite, with its beauty, holds an echo of the wonder of the original crystal, where all ions were equal and lived together in peace.

Back down to earth, I have a geeky question to finish. What does labradorite look like in thin-section? Other, lesser plagioclase compositions have rather handsome black and white stripes in crossed-polars. Are these visible in labradorite or are they too small?

Tim Robinson: Geology and place

Aran north

Tim Robinson is a celebrated author and visual artist whose intense engagement with the land beneath his feet is an inspiration to anyone who spends time in wild places. His work is inspired by the idea of ‘the good step’, of reintegrating body and world and achieving a state of consciousness ‘worthy of the ground we walk on’. The maps and books inspired by these abstract ideas contain enormous amounts of very specific information on the geology, archaeology, history, botany, language and folk lore of the West of Ireland.

I first encountered his work in my PhD field area in Connemara. One of the gabbro intrusions I was studying formed a hill called Doughraugh and I wasn’t sure how to pronounce it – the English language being absurdly inconsistent about the pronunciation of ough, as proved by the English towns of Slough (rhymes with cow) and Loughborough (‘luffburra’). I was pointed to Tim Robinson’s beautiful map and gazetteer of Connemara. Along with the location of fairy wells, infant burial grounds and much else, the map shows the placenames in their original Gaelic. Doughraugh, I learnt is a mangling of the Gaelic Duchruach or ‘black stack’ (the ‘ch’ sound is soft as in Scottish ‘loch’ and German ‘ich’).

A black stack (and a castle)

The map improves on the bad job done by an English-speaking cartographer working for the British Ordnance Survey in the Nineteenth century. It uncovers the pleasing fact that the name of the hill reflects the colour of the gabbro that forms it. This is what his work does, again and again and again, his fierce dedication to find out everything about a place creates dizzying connections between forms of knowledge often treated separately. Here the simple fact of a name of a hill is used to spin links between folk memory, past cultural imperialism and the unreflective nature of pyroxene.

When I worked in Connemara I didn’t understand this. It was a time when there were regular terrorist attacks in my country, and the people doing it used the Gaelic language as a badge of difference. Stupidly and wrongly I assumed Tim Robinson was some form of Irish nationalist and looked no further. Consequently I didn’t discover his books until later, when nostalgia for fieldwork and Ireland drew me to them. My love for his work is linked with my personal connection with the places he talks about, no doubt, but you should read them too, even if the closest to Ireland you’ve been is the bottom of a pint of Guinness.

Only a small proportion of his work deals with Geology directly, but he does so accurately and poetically, acknowledging how he draws on his interactions with local experts. Most interestingly, he comes at Geology from the perspective of an artist. For example he writes beautifully of Deep Time, of the awe that contemplating the age of rocks should generate in any thinking person. There is a beautiful section in Stones of Aran: Pilgrimage where he talks about a glacial erratic, covering myth (saints sailing on the stone across the sea) and Geology together. He’s read the scientific papers about the the rock and talks of one exhibiting ‘one of the most lapidary prose styles I’ve ever come across’. He then quotes a chunk of the technical language verbatim and talks about how he, a non-specialist, relates to it – a lovely new perspective on scientific language. Note also the ‘pun’ of using both meanings of the phrase ‘lapidary prose’. Also on Aran, he coins the compelling phrase ‘Aran North’ to talk of the way the joints in the limestone pavement (oriented near to ‘True North’) dominate the geography and define a human frame of reference.

His works are subtly biographical. They are not about him, but in talking about human interactions with the natural world he doesn’t leave himself out. They paint a picture (borne out by some email interaction) of a remarkable man dedicated to his art. There is an asceticism and disregard for material things which resonates with these austere post-boom times. Mostly there is a dedication to truth and the importance of finding out everything there is to know. This is a man who spent decades tramping over small areas of Ireland documenting everything that there is to know about them. To do this, he has learnt from the local people, linguists, historians, botanists, archaeologists, geologists and others. There is a passion in his search for knowledge, as a way of bearing witness to, say, the unknown victims of the Irish Famine as much as to the wonders of the natural world.

Tim Robinson’s thirst for knowledge is an inspiration, a reminder that any research into the natural world is important and interesting because it aims to find out what is true. For me it is also a reproach. Returning to Connemara recently as an ex-geologist and the owner of a garden I was astonished to see fuchsia hedges and huge gunnera plants in ditches (essentially wild garden plants). I had no memory of these. As a young geologist I simply filtered them out of my experience: they weren’t rocks and they didn’t matter. Reading Tim Robinson reminds us to turn off the filters, be like young children and realise that everything matters, everything is interesting.

Frightened terrain

A final reason to read Tim Robinson is his use of language. I have no technical skill to judge, but approving quotes from those who do adorn his book covers. His writing is rich and dense with meaning. One anecdote he tells is of climbing a hill just to think of the right adjective to describe the landscape below. He settled on ‘frightened’. Perhaps not every word has received an afternoon’s consideration, but such attention to the quality of writing yields wonders. Here’s a snippet from his latest book, from the chapter called ‘Subduction’.

” … and steps into a zone in which the rocks under-foot seem to be resting for a moment between bouts of hand-to-hand fighting. The outcrops are glacially polished as elsewhere, but in their smooth surfaces one can trace a static turmoil of forms – gnarled lumps, twisted and clenched veins, heaped gobbets – that are evidences of some profound convulsion long anterior to the Ice Ages”.

This is an artist’s perspective on an outcrop but he follows it with an accurate description of the science behind these rocks. He creates art about rocks, but the science is in no way in opposition to this. There is no artificial distinction between Art and Science here, or in any of his work. The two are melded together and reinforce each other in a way I’ve rarely seen elsewhere. This is why Tim Robinson deserves to be widely known – his focus is the West of Ireland but his scope is universal. Read him.

Major works

All of Tim Robinson’s work is widely available, but some are best sourced from his own publishing house.

Connemara: Part 1, Introduction and Gazetteer; Part 2, a 1-inch map

If you ever visit Connemara, pick up one of these. A beautiful map and a gazetteer full of masses of information.

Stones of Aran: Part 1: Pilgrimage; Part 2: Labyrinth

Recently republished in the New York Review of Books – Classic series, these are rich and satisfying books. They are long and not for the faint-hearted but once you are drawn into their world you won’t want to leave. Every hill-side, every field, every step even of his journey across a single remarkable island is drawn into a web of meaning, connecting the natural world, Geological time, human history and memoir.

Connemara Trilogy: Listening to the Wind, The Last Pool of Darkness, A Little Gaelic Kingdom

The best place to start is with his most recent works, the Connemara trilogy. Each deals with a small area of land, but the focus is less intense, broadening out to talk much the area’s history, taking in the Irish Famine, British mistreatment and the struggle to preserve the Irish language. It also includes much about those who have visited Connemara over the years, from feuding botanists to Ludwig Wittgenstein.