AW#50 – fieldwork is thirsty work

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.

What came from outer space

I admit it. I’m geocentric. Not in the old-fashioned sense, I’m not that eccentric. I don’t believe the earth is the physical centre of the universe, but it certainly feels that way. The universe, space, the wonders of the solar system are all very well, but emotionally they are all ‘out there’ somewhere. A Twentieth Century geological education contributed to this – everything on the earth was explained by something else on the earth. For example we were taught about the increasingly odd forms of late Cretaceous ammonites. The implication was that their extinction was due to them becoming decadent and depraved, like the late Roman Empire. When the theory that they and the dinosaurs where killed by a meteorite impact arrived, it felt somehow unnecessary (and yet now seems so obvious).

Which is by way of justifying why my response to Dana Hunter’s Accretionary Wedge call for ‘out of this world’ posts on exogeology has its feet planted firmly on terra firma. One of the most interesting strands of studies of this planet in the last 20 years has been discovering the many terrestrial features caused by external influence. Often ‘it came from outer space’ is science fact, not science fiction.

Meteorites are the most dramatic way in which ‘space’ affects the earth. Lumps of rock and/or iron hitting the earth at cosmic velocities (more than 11 kilometres a second) leave some dramatic traces.

Meteor Crater

Meteor Crater, Arizona. Courtesy of Scott Tanis on Flickr. http://www.flickr.com/photos/8376919@N02/3905452932

What is striking about meteor craters on the earth is how few there are. There are so many craters on Mars that studies of its geology use their areal density to estimate the age of the surface. A bit like people’s skin, old craggy areas look different to smooth new ones. By contrast, there are fewer than two hundred impacts recognised on earth. Partly this is due to a thicker atmosphere, mostly due to high rates of erosion and geological activity and partly because there are many more traces of impact left to be found.

Most large round features on earth have been investigated. Some small craters no doubt remain to be found but there are other ways to find traces of ancient impacts. Some believe that we should be looking in the sedimentary record and that many more impact craters remain to be found, hidden away in oil company seismic data.

In the last few decades geologists have recognised a range of distinctive features found in rocks that have suffered/enjoyed an impact. Structural features within the impact include shatter-cones, fractured rock and faulting. On a microscopic scale, features such as shocked quartz are distinctive traces of the sudden massive stresses of impact. For large impacts, the energy turns into heat that melts and vaporises rocks. If a big enough hole is made, there will be extensive metamorphic changes to deep rocks due to the pressure release of removing kilometres of rock.

Another way to identify past impacts is to find material thrown out beyond the crater area. This can include thick layers of fragments and glass, called suevite. Molten glass may be thrown up into space, falling again as spheres of glass over a wide area of the earth. These may be quite thin layers in sedimentary sequences. These surely represent the most extensive record of past impacts but not many have been recognised to date, so keep your eyes peeled. For recent impacts, the glass lumps may still lie on the surface, where they are known as tektites. The largest set of tektites is found in a vast ‘strewn field’ stretching from south-east Asia into Antarctica and yet no-one has yet the crater that was surely formed at the same time.

The hunt for impacts continues. In the last few months researchers have published evidence for two more. A huge old one in Greenland has long been eroded away, but there are lots of distinctive features in the deep rocks that sat below it, which is all that remains. A smaller more recent one in Canada shows contorted strata and lots of pretty shatter cones (fractures formed by the impact).

shatter cones

Shatter cones from Prince Albert crater. Courtesy of University of Saskatchewan. http://www.flickr.com/photos/usask/7644732242/in/set-72157630747745754/

The most inventive method of finding evidence of past impacts is to study ancient myths. Some link myths about ‘fire from the sky’ to impacts that occurred in human history and seek to explain flood myths by a cometary impact.

My awareness of my own geocentric bias makes me sceptical of such things. If  I was an astrophysicist, aware of the sheer volume of space debris, I would be much more inclined to explain features of the earth in terms of impacts. Perhaps too much so. I read someone once trying to explain the South African Cape Fold belt as caused by the Vredefort impact. This was nonsense.

Of course this is how science progresses, by bringing perspectives from different disciplines together as a way of stimulating new research. The link between Chixulub and the K-T extinction is pretty clear (but still debated). What is interesting about many mass extinctions is that there are just so many plausible mechanisms. The Deccan Traps may have killed the dinosaurs just as much as a meteorite. In a similar way the end-Jurassic extinction is close in time both to an impact and extensive vulcanism. The only way to take this argument further is by careful study of the evidence, most of which sits in the rocks.

Study of earth’s early history removes any lingering doubts that earth can be studied in isolation from its surroundings. The earth formed within a dusty disc around the new sun 4.56  billion years ago. During the earth’s first few 100 million years it was constantly being struck by other pieces of debris. The best current theory for the formation of the moon is the ‘giant impact hypothesis’. This suggests that the proto-earth was struck by another proto-planet the size of Mars. The impact resulted in two separate blobs which formed the earth and the moon. The energy of such an impact left both bodies completely covered in a magma ocean. Any water in the earth would be boiled off, meaning that our atmosphere and oceans are all derived from water from comets that have hit the earth since. We are all made of star dust, but let’s not forget the comet juice.

My favourite link between the earth and beyond is only an idea so far, but a beautiful one. On earth we find rare meteorites that came from Mars and the moon. When one day we study the moon in more detail, perhaps we’ll find pieces of earth on there. The period when the most impacts hit earth (sending bits flying off) is also the time when we have the fewest rocks preserved on earth. What if the oldest earth rock still in existence is actually to be found on the moon?

To end, a look at earth from the outside to remind me that our planet, endlessly fascinating as it is, is only a tiny dot in space.  ‎

Picture of the earth and moon taken from Juno probe. Courtesy of Nasa http://www.nasa.gov/mission_pages/juno/news/juno20110830.html

 

Geological pilgrimage – Assynt, Scotland

In Accretionary Wege #45 Denise Tang asked for “Geological Pilgrimage – the sacred geological place that you must visit at least once in your lifetime “.

For me, and dare I say it for any British educated hard-rock geologist the answer has to the Assynt district, in Sutherland, Scotland.

Denise asks for somewhere relatively remote and hard to get to. In British terms, Assynt is  as far away from ‘civilisation’ as it gets. It is almost at the very northerly point of Britain. From the bits of Scotland where most people live (the Midland Valley) it takes most of a day to drive there. The final part of the journey is on single-track roads that weave their way through an amazing undulating landscape. It is in the bit of Scotland that is more Scandinavian than British – Sutherland means ‘south-land’ in the language of the Vikings, but it is far enough North that I’ve seen the aurora borealis there.

To the Geology. This photo encapsulates the main geological features of the western half of Assynt.

This is the a view of the amazing mountain of Suilven. You’ll have spotted that it is made up of sedimentary rocks – Proterozoic rocks called the Torridonian. These sit on Lewisian Gneiss, Archaean and Proterozoic basement gneisses, visible in the foreground hillock. If you home sits on the the Canadian or Scandinavian shields then these are familiar rocks, but for the British Isles these are exotically old and exotically high-grade.

Side view of a sheath fold, Lewisian Gneiss

Intense glacial scouring makes this an amazing landscape. The gneisses form a rugged craggy ‘cnoc and lochan’ landform with lots of little lakes and rock hillocks. The Torridonian forms spectacular mountains (well, hills really) that rise above. In Assynt the Torridonian also includes a layer of suevite, recording a major meteorite impact somewhere nearby.

Why should you care about this area? Well, it is a UNESCO Geopark, for one and perhaps the main reason for this is that it contains the Moine Thrust Zone. This is a major thrust structure, marking the edge of the Ordovician/Silurian Caledonian orogeny. It puts the Moine Schists (lateral equivalents of the Torridonian, now strongly deformed and metamorphosed) over undeformed ‘foreland’ rocks. The foreland contains Cambro-Ordovician sediments which sit unconformably over both the Lewisian and the Torridonian. Assynt is a particularly interesting part of the Moine thrust zone as it is a culmination – there is a big package of thrust slices between the undisturbed foreland and the Moine schists. The Cambro-Ordovician sediments are an important part of the picture as they have a regular stratigraphy made up of varied distinctive rock types. This makes the wild structure of this ‘zone of complexity’ much easier to map.

In the early 20th Century, Peach and Horne of the (state-funded) British Geological Survey produced a classic report on the area. This proved without any doubt the reality of thrust faulting and is a significant event in the history of geology as well as a classic of fieldwork. The rock type mylonite (characteristically formed at thrust contacts) was first identified and named (by others) in the Moine thrust zone just north of Assynt.

As well as showing the complex geometry of thrusting in three dimensions via detailed mapping and copious cross-sections, Peach and Horne also had influential thoughts on the broader tectonic implications of what they found.

In Assynt the ‘zone of complication’ is awesomely complicated. The structure is truly three-dimensional. Well four-dimensional really, as structures cross-cut and debate rages over the sequence of thrusting over time.

Here’s an picture of a relatively simple area (the Glencoul thrust), to give you a taste.

Starting from the base moving up, first you see a hummocky area of Lewisian Gneiss. Then there is a crag of slightly-pinkish layered Cambrian quartzites. There is an unconformity at the base of the crag – it was once a rocky seafloor that became covered in sand. There is a step in the slope and the final rock unit appears – hummocky Lewisian Gneiss again!

The step in the slope marks the position of the Glencoul Thrust, one of the thrust planes that make up the Moine thrust zone. Think about this – all of the upper part of the hillside has been pushed on top of the lower part. If you trace the fault you realise that this would mean 10s of kilometres of horizontal movement. This is an amazing thing. Peach and Horne’s work is important as it settled the matter, proving the reality of thrust faulting, for the first time.

When geologists first starting debating the Moine Thrust, the way Gneiss formed wasn’t understood. Geologists argued that the picture above showed a conformable set of sedimentary rocks. Imagine, if you didn’t know that gneisses aren’t formed on the sea-floor then this makes sense, more sense than the idea of hillsides climbing on top of each other, anyway. Charles Lapworth, who first named mylonite and identified thrusting in Assynt, at one point had nightmares of being bodily caught up in the Moine Thrust, being crushed under what he called the great Earth engine.

You should visit Assynt, if you get the chance, it will give you good dreams, not nightmares.  The site of the North West Highlands Geopark will help you plan your visit. While you are waiting, here are some ways to visit virtually. You can get to geological maps of the area via the BGS geology of Britain viewer.  There’s an introduction to the Geology on Leeds University’s Assynt Geology website.

If you are as obsessed with Assynt as me, you should buy the centenary Special Publication from the London Geol Soc. It’s not cheap, but it is very large and very good. Portions of it are available on the Internet via the BGS open access repository. Other books include Exploring the landscape of Assynt from the BGS which is an informal guide for walkers and “A geological Excursion guide to the North-West Highlands of Scotland” for a more geology focused guide.

To get a sense of the structural complexity, the paper on the Traligill Transverse Zone by Maarten Krabbendam and Graham Leslie is hard to beat. Here’s a taster of some modern BGS cross-sections to close out with.

Image sources

Photo of Suilven from Neil Roger (neil1877) on Flickr. http://www.flickr.com/photos/neil_roger/4129901847/sizes/l/in/photostream/
Sheath fold picture by the author.
Image of Peach and Horne from Wikipedia
Glencoul thrust picture from Shandchem on http://www.flickr.com/photos/14508691@N08/4818337130/sizes/o/in/photostream/
Diagram from Krabbendam and Leslie with kind permission of primary author. Also thanks to Maarten for information of guide books.

Accretionary Wedge #44 – “most important teacher”

Teaching is not just about imparting knowledge. To quote Elli Goeke, an important teacher is a “person has influenced not just my knowledge of a particular subject, but has also changed how I address research or teaching or just life in general (or all three).” These posts will teach you that great teachers can also inspire, instil discipline and generate self-belief. Denise Tang knows a Chinese expression that sums it up: teachers “pass the knowledge and good traditions to the next generation”.

We’ll start with Southern Geologist’s first contribution to an accretionary wedge, on a new blog. The post describes a inspirational introductory course in Geology, which for variety and interest put other sciences in the shade.

Short Geologist picks an anonymous college chemistry teacher whose good teaching cured a phobia of ‘hard’ equation-rich subjects and laid the foundations for successful geological studies.

Denise Tang over at Life as a Geologist owes a debt to Prof. LS Chan, who introduced her to Geology and so “passed the flame”.

My post on John Dewey describes how I learnt the importance of breadth from a remarkable man.

John Adams (the Geologist) was taught geology by not one, not two, but three Ulstermen called Reid and is interested in knowing if anyone else remembers them.

Over at Life in Plane Light, Elli Goeke tells us about three ‘runner-up’ teachers  but settles for Kim Hannula as her most important, someone who is a mentor as well as a teacher.

Hollis over at Plants and Rocks gives us a tribute describing the life and work of Dr Brainerd “Nip” Mears Jr, a man who contributed to our understanding of the Geomorphology of the American west, but who put his students first.

Following a common theme, Casey at Gioscience lists the teachers (at University of North Carolina at Wilmington) who led him to a love of Geology: Dr. W. Burleigh Harris, Dr. David Blake and Dr. Michael Smith.

Moving away from formal education, Dana Hunter offers a characteristically engaging and generously-illustrated story  (also here) about how Lockwood DeWitt fed her geology addiction with a first opportunity to “see some stuff with an actual geologist”. There are some great descriptions of what being taught by a great teacher is like and how they build confidence as well as impart knowledge.

The man himself, Lockwood DeWitt submits a touching eulogy to Harold “Sharkey” Enlows the College teacher “who made me work the hardest, and from whom I learned the most”.

Ann over solved the difficult problem of picking between her university geology teachers by talking instead about an important school teacher, Miss Relic who through belief and encouragement changed for the better the way Ann thought about learning and her own abilities.

Ryan Jackson over at Educated Erosion had no problem choosing Coach Ford, an inspiring High School teacher who set him off on a rocky road.

The next wedge is hosted by Denise Tang and is “Geological Pilgrimage – the sacred geological place that you must visit at least once in your lifetime “. Get thinking…