This is my contribution to the ‘most important teacher’ Accretionary Wedge.

I’ve had the privilege of learning from many excellent teachers. Choosing one single person to talk about is somewhat arbitrary. I’ve chosen someone who, of my teachers, is the most important figure in the wider world.

Professor John Frederick Dewey is an major figure in geology. He’s a Fellow of the Royal Society, a member of the US National Academy of Science and winner of numerous awards and accolades. He’s held professorships at prestigious universities both sides of the Atlantic and has taught over 40 PhD students and countless undergraduates. Most interestingly, he was there at the beginning. He’s part of the generation for whom plate tectonics was a fabulous novelty, a bolt from the blue that transformed the study of the earth in so many ways.

John made his career by being one of the first geologists to apply plate tectonic theory to specific mountain belts. This involves the impressive trick of linking field-based studies to high-level plate tectonic concepts (e.g. “closure of ancient ocean, then collision of oceanic island arc then closure of ancient ocean”). Throughout his career he’s covered pretty much every orogeny on the planet and many types of concept (orogenic collapse, transpression/transtension to name but two). To give a flavour of his depth of geographical coverage, the hat in the picture above was a Chinese Army one from a trip across Tibet and I’ve see him have a good natured debate over which is the best fish-restaurant in South America. He is currently based in California but his home-base as regards Geology is the West of Ireland, where he did his PhD research.

As you can see from the above, John is a bit of a showman. I know him  best from 5 years of Irish undergraduate field trips. Sitting in a wrecked bog-car to amuse the students is entirely in character, as is eating raw mussels, freshly ripped from the outcrop, to show how clean the Atlantic sea-water is. Every year in Ireland, he would allow an hour or so to make everyone play cricket, often on a road. Western Irish roads are often an arena of oddness (I was overtaken by a hearse once) but 40 young people in rain gear and one Fellow of the Royal Society knocking a ball around is still a rather unusual sight, even there.

What did I learn from him? Firstly, a love of tectonics and the grand intellectual game of understanding how mountains form. It’s exhilarating to hear him talk on this subject. Take my own research on Irish rocks (which he co-supervised). For John this is a tiny brick in a grand edifice, helping him correlate the geology of Ireland with that of Scotland to understand the Grampian orogeny. This is then a part of the grander picture of the closing of the Iapetus ocean, which formed the Caledonidian (British Isles, Greenland), Scandian (Norway) and Appalachian (US and Canada) orogenies. The short duration of the Grampian orogeny allows comparisons with New Guinea and contrasts with the Alps and Himalayas. To stand on a single Irish outcrop and link it into an intellectual scheme that spans and seeks to explain the entire world, this is real science.

Another, broader thing I learn from him is the importance of mastering the detail, but also lifting up your head to the broader things. To succeed in many professions you need to master the detail, to learn a craft and have specific skills. But to really get on you need to link this to a bigger picture. Detailed careful work is important, but maybe its worth spending less time on dotting every i and more time on telling a great story, making a splash and creating research that is seen to be interesting and important.

On Twitter earlier today, Ron Schott of the Geology Home Companion Blog asked for volunteers to host future Accretionary Wedges. I’ve long been a fan of this geologically-themed blog carnival so I jumped at the chance.

An interest in Geology, perhaps more than other subjects, is something that is often nurtured by good teaching. Geological learning is often about someone passing you a hand-lens and rock sample and telling you about the wonderful things you can see. None of us are  starting from scratch; we all have access to a tremendous body of knowledge that enriches our understanding and enjoyment of Geology. So how did you get access to that knowledge?

The theme for this month is to tell us about your ‘most important teacher’. For those of us with a formal training an obvious route is to write about a person who taught us, talking about them and what we learnt from them. This is the route I shall be following, but   my purpose here is not to constrain but to inspire you. We don’t just learn from people face-to-face. Or from people: sometimes it seems that rocks can speak. They can certainly teach us things.

Whatever your circumstances, if you’re still reading this post you are someone who loves Geology and loves to write about it. You know more now than you did when you started out. Tell us about that journey and the most important thing or person that helped you on your way.

Send in your links in comments below, or via Twitter. Since we are starting late, the closing date is the 13th April. I’ll collate all your links in the usual fashion.

This is my contribution to the Accretionary Wedge geoblog festival, number 43: My Favourite Geological Illustration. You can read all about it at In the Company of Plants and Rocks. 

I was struggling for inspiration on this latest Accretionary Wedge, but this was solved by Matt Hall’s post over at Agile Geoscience, where he talks about a map he produced as part of his degree. Geological maps have captured my imagination since a very early age. I’ve spend many an hour poring over the geological map of the British Isles (particularly the Northern sheet) but of course I had a modern copy, not the William Smith original.

I have hidden away somewhere a very similar undergraduate map to Matt’s but I won’t steal his idea outright. I have a bigger map produced as part of my PhD thesis somewhere as well, but instead I want to talk about someone else’s map.

My PhD field area was in Connemara in the west of Ireland, which is an extremely well-known piece of Geology. A fantastic detailed map was published in the 1990s but is based on decades of study by geologists from Glasgow University, Bernard Leake and Geoff Tanner. It is of course in copyright and for sale, but I include an impressionistic photo. I’m sure the Royal Irish Academy will not disapprove.

I’m tempted to rhapsodise at length at the Geology shown by this map, but I’ll be brief. It is a small area (top to bottom about 20km) but it contains such a lot! Such as: evidence of Precambrian glaciations; two overlapping phases of metamorphism, from greenschist to granulite; three major phases of folding; three suites of igneous intrusions, one syntectonic, two post; three tectonic terranes; two terrane boundaries, one a thrust the other an extensional detachment complete with syn-deformational sediments; some world famous marble.  Oh, and some sediments around the edge.

One of the greatest pleasures of my PhD research was meeting and disputing with Geoff Tanner and Bernard Leake, standing on the rocks themselves and referring to their fabulous map.

I have a busy suburban lifestyle which rather restricts opportunities for fieldwork. Consequently I make the most of the few opportunities that do arise. These might be a glacial erratic in a park or turbidites in the toilets but here it’s granites in shopping centres. These are found sometimes on counter-tops, but often as flags on the floor. So, if you’ve seen someone apparently furtively taking pictures of his shoes, while his children try to pull him into a shop, that would have been me.

Here’s a nice piece of granite (a good place to sit and eat sticky buns), green shoe for scale:

By granite, I mean granitoid or granite ‘sensu lato’ (e.g. in the loose sense). The term granite ‘sensu stricto’ is tightly constrained in terms of proportions of feldspar, quartz and so on. I’m not an igneous petrologist and when I look at ‘granites’ I don’t worry about the composition of the feldspars, I work out whether they are aligned randomly or not. Look through the eyes of a structural geologist and you see fabrics. In these blocks there is a planar fabric, largely defined by the alignment of the feldspar laths. Like so:

So this is a deformed granite, so we should call it a meta-granite? Not necessarily. It turns out that fabrics can form in granites as they cool.

Normally in deformed rocks, fabrics are formed by the deformation of the actual minerals. In intensely deformed rocks such as mylonites, the individual minerals have had their shapes dramatically changed.

Here’s a counter-top example of an igneous rock that has deformed after cooling:

Nice counter-top. Bad coffee, sadly.

Note how the original igneous minerals have all been deformed and/or recrystallised. The fabric was caused by crystal-plastic deformation. The rock was deformed after it crystallised.

In my original example, the fabric was defined by the alignment of laths of plagioclase, but these feldspar grains were themselves undeformed. It turns out that fabrics like this form in granites while they are still molten. If you put stress on a cooling magma (which is a mix of molten rock and freshly crystallised minerals) you can deform it and so align the minerals.

Also, if the magma is compositionally inhomogenous it will end up as a granite with patches of different colour. If it has been deformed, you will end up with flattened patches, which themselves form a visible fabric in the rocks.

Time for some examples, from a set of slabs that cover huge areas of my local shopping centre. Here’s my favourite:

There’s a lot going on there, so I’ll put on my annotating hat.

Talking through in sequence, we start with a fine grained granite (area with red text). This contains compositional layering that is a little bit folded. Next we have an area (blue text) of more homogeneous slightly coarser granite. It looks like the early ‘red’ block has sharp edges and so was mostly solid when the ‘blue’ magma arrived on the scene. The blue area has a faint fabric to it. Finally we have a ‘late’ granite vein (green text) that crosses everything. This probably is late in the granite’s history as its coarser grain size and the zone of alteration around suggest it was rich in fluids and so some of the magma that cooled last.

The ‘blue’ granite is the only area showing a fabric. This is a neat proof that this fabric was caused by deformation of the magma, not solid rock. The fabric was formed after the early ‘red’ block was solid and before the late ‘green’ granite was intruded. It therefore must be syn-magmatic and as such is an insight into what goes on it cooling magma chambers.

Here’s a close up of the most interesting area. Note the sharp edge between ‘blue’ and ‘red’.

Here’s an intriguing one:

Those are not my shoes.

A bit of annotation to bring out some interesting features:

Note another coarse cross-cutting vein, in an area with a clear fabric. What is intriguing is the way the fabric appears to swing into the vein, with a consistent sense of shear. What *might* be going on here is that there was a shear-zone cutting the fabrics, which the vein subsequently is intruded into.

What causes the fabrics then? It is very likely, for the examples shown, that the fabrics are caused by normal processes associated with granite intrusion. Intrusions of new magma, changes in volume on cooling or other processes can cause movements that explain these weak fabrics.

What if granite is intruded into crust that is being deformed? If the country rocks are being flattened or sheared, what effect does this have on the cooling intrusion? Its now recognised that many granites are syn-tectonic and that they contain magmatic fabrics that ultimately are caused by deformation of the surrounding rocks. More than that, there are classic areas, such as Donegal in Ireland, where it is clear that the granite was intruded into a deforming shear-zone. This is a whole new take on the classic ‘space problem’ for granites. One answer to the question of how you make space for granite magmas to fill is that shear-zones are involved. Much of the evidence for this is found in fabrics like the ones I’ve shown you.

My PhD was based around a set of syn-tectonic mafic intrusions. These had magmatic fabrics too, only in gabbro rather than granite. In this case I demonstrated a link between the fabrics in the intrusion and the fact it was actively being deformed while cooling. I’ve never seen gabbro fabrics in a counter-top though, more’s the pity.

 This story is my contribution to the Accretionary Wedge #42.