Papers and pot-shots: when geologists attack

Posted on 21 September, 2011 by Metageologist

One of my most valuable learning experiences as a Geology undergraduate was a first-year tutorial with the legendary Harold Reading. We were give an academic paper to write about (concerning alluvial fans) and we dutifully did so, showing we understood it, but simply taking it at face value, applying no critical analysis.

Discussing what we’d written, it became clear that my tutor thought the paper was rubbish. He proceeded to explain the many ways in which it got things wrong and finished up with a brief and funny attack on the characters of the authors.

This taught me not only a valuable lesson about critical thinking and scepticism, something any university course should convey, but also an insight into how science works. Getting a scientific paper published is not easy, but it is only the beginning of the process of creating science. If a paper is read, appreciated, cited and generally built upon then it becomes a brick embedded in one of humanity’s greatest achievements: the great structure of Science. Some papers are read, disagreed with and generally ignored, they end up as mere forgotten words.

With his remarks about the authors my tutor taught me something else: this noble calling of finding out what is ‘objectively true’ and what is not is performed by people, who are after all, only human. Academia is famously riven by jealousy, feuds, rivalry and outright bitchiness. This happens mostly in coffee lounges, conferences and field-trips but sometimes it seeps out into academic papers. I’m rather a fan of this sort of thing (I am very shallow) and I have a few examples to share.

There is a persuasive case to be made that, at least in the English-speaking world, modern science started in the Royal Society in Seventeenth Century England. It started with bitchiness, no doubt. Sir Isaac Newton’s famous remark “If I have seen a little further it is by standing on the shoulders of Giants“ in a letter to Robert Boyle is interpreted by some as a snipe at Boyle himself, who was somewhat short. What is certain is that the two fell out on a personal level over scientific matters.**

In academic papers, the best source of vitriol is in “Discussion” and “Reply to Discussion” papers. This is the formal mechanism for group A to raise concerns with a paper published by group B and then for group B to reply and address those concerns. Sometimes this is all that happens and everyone is very polite and thanks everyone for their useful feedback. This is dull. The best ones are where the raw emotions are barely concealed beneath superficially polite formal language.

Let’s start with an old school example and a nice turn of phrase: “The authors fail to reply to the point raised by Professor Wood… that field mapping and petrographic evidence show the northern contact to be of a ‘purely hallucinatory nature‘ “doi: 10.1144/gsjgs.137.4.0513 August 1980 Journal of the Geological Society, 137, 513-514.

An up-to-date one: “Westaway also suggests … however a glimpse of the map shows that … “ Michael P. Searle, Sun-Lin Chung, and Ching-Hua Lo Reply to Discussion on ‘Geological offsets and age constraints along the northern Dead Sea fault, Syria’ Journal, 167, 1001–1008 Journal of the Geological Society March 2011, v. 168:623-624; doi:10.1144/0016-76492010-166

The use of ‘glimpse’ is a nice example of English as spoken by the English: forms of words that are superficially neutral but in fact conceal far stronger meanings. The comedy “Yes (Prime) Minister” has some nice examples plus there is a useful guide in The Economist magazine. In my example, using ‘glimpse’ instead of say ‘careful study’ sends a clear message of disdain.

Some stronger language: “Owen refers to the interpretation of the Silverpit structure as an impact crater as a ‘speculative hypothesis’. This is consistent with an earlier statement he has made: ‘This feature, termed the Silverpit Crater, has earlier been interpreted as a meteor impact structure (Stewart & Allen 2002), without a shred of scientific justification’ (Thomson et al. 2005). The inference that the hypothesis lacks any observational basis disregards a wealth of 3D seismic analysis by Stewart & Allen (2002, 2005). The statement is at best disingenuous, as Owen chooses to ignore the thorough work of earlier workers. ” M.L.T. Wall, J. Cartwright, and R.J. Davies Reply to discussion on ‘An Eocene age for the proposed Silverpit Impact Crater’ Journal, Vol. 165, 781–794 Journal of the Geological Society December 2009, v. 166:1159-1160; doi:10.1144/0016-76492009-061

The emotion in that is clear. See here for some background to the interesting geology being debated.

My favourite example of all is found in a pair of papers concerning porphyroblasts. First some background; Professor Tim Bell, self-described on his own university web-site as a maverick structural geologist has spent the last 20 years promoting a startling idea. The many research papers produced by him and his group seek to prove that during orogensis, porphyroblasts do not rotate with respect to geographical coordinates and that the fabrics preserved in them contain extremely valuable information about past deformation. Put simply, in the case where curved fabrics are seen in porphyroblasts, the fabric (and therefore the entire rock) has rotated, but the mineral grain itself hasn’t. This is not a mainstream view and the debate is ongoing (my very English use of ‘startling’ above is a guide to my feelings on the matter, for what its worth).

Prof. Bell’s ideas were first published in the Journal of Metamorphic Geology in the early 90’s. From gossip at the time, I gather that they ended up there as the structural geology journals wouldn’t publish it. In 1992 a critical response was published in the same journal: “Porphyroblast rotation: eppur si muove?” (DOI: 10.1111/j.1525-1314.1992.tb00083.x).

The ‘eppur si muove’ bit is explained as “Italian: ‘but she moves nonetheless’, the words reportedly spoken by Galileo Galilei on 22 June 1633 at the offices of the Holy Inquisition in Rome, after having been forced to renounce the idea that the Earth rotates around the Sun.” The paper then proceeds to critique the original paper with lines like “Critical assessment of the assumptions and data used to support the theory of irrotational porphyroblasts reveals numerous flaws“.

So, facing a robust rebuttal of their work that uses a pretentious Italian quote, how would any Australian act? Why, with a vigorous response and a vulgar mockery of the quote, of course!

T. H. BELL, S. E. JOHNSON, B. DAVIS, A. FORDE, N. HAYWARD, C. WILKINS, Porphyroblast inclusion-trail orientation data:eppure non son girate!, Journal of Metamorphic Geology, 1992, 10, 3.

The tag explained as: “Italian: ‘and yet they have not rotated’, the words reportedly spoken, after a strenuous performance, by Santaccia, a lady informant of ill repute for the Roman poet Belli (1832).”

All my examples are rigorous scientific papers in proper journals, but I’m sure I’ve only scratched the surface of what is out there. Does anyone else have any good examples they’d like to share?

** I urge you to find out more about the Royal Society as it is fascinating stuff. Both Boyle and Newton have ‘Laws’ named after them, but that is just scratching the surface of what they achieved. There is also a tension between Newton as ‘theorist’ and Boyle as a mere ’empiricist’ which has a lot of contemporary resonance with Geology vs. Earth Sciences. Anything by Lisa Jardine is worth reading and there are some interesting podcasts about the Royal Society, both via the Royal Society itself (via ITunes) and on the awesome In Our Time on the BBC website.

What you ought to know about metamorphism

Posted on 15 September, 2011 by Metageologist

I love metamorphic rocks. In hand-specimen they are varied and mysterious, an invitation to do some mineral spotting. In thin-section they are positively gorgeous: solid garnet, foxy biotite (where you can see radiation-damage around zircons), fluffy fibrolite, elusive cordierite…

Metamorphic rocks are both attractive and interesting, a compelling combination at any party. They generally aren’t as popular as they should be though. Perhaps they are not approachable enough, a bit aloof perhaps, coming across as obscure and hard to understand?

Think of me as a matchmaker, then. Metamorphic rocks may lack the glamour of fossils, they aren’t as approachable as sediments or as exciting as volcanic rocks, but once you get to understand their mysteries, you’ll be smitten.

In a series of posts I’m going to attempt an introduction to metamorphic petrology, for the geologically aware who would like to know more. I did some full-on metamorphic petrology as part of my PhD, so I have been initiated into the Dark Arts and put in many hours looking down a microscope or aiming an electron microprobe. Many years have passed since, but I hope what remains in my memory is the interesting and the important stuff.

What is metamorphic petrology?

Petrology is the study of rocks and the conditions in which they form. Metamorphic rocks are rocks changed by elevated heat and/or pressure. Putting the two together means inferring how deep a rock was buried and how much it was heated. This is interesting in itself, but also very useful in interpreting the geological history of the area.

What are metamorphic facies?

Metamorphic facies is a key concept. In a sentence, they are groups of minerals that are representative of particular areas of pressure-temperature space. Here’s a picture to show what that means.

Metamorphic facies, from http://www.tulane.edu

The pressure axis can be thought of as depth, with each 100MPa being roughly 2.5 km of depth, the temperature axis is more intuitive, but note that your oven would only get about a quarter of the way along it.

The concept of geothermal gradient is a useful way in: visualise yourself at top left (you are, I hope, reading this under conditions of atmospheric pressure and low temperature); tracing one of the dotted geothermal gradient lines through the picture shows current conditions under your feet. Depending on where you are, a different geothermal gradient applies. For me, sitting in the middle of plate that is geologically inactive, geothermal gradient line B applies, with a sequence of zeolite, greenschist, amphibolite and then granulite facies until by 1000 MPa we are likely running out of crust. Incidentally, pressures of 1000MPa occur at about 40km depth, so the nearest granulite facies rock to me is directly under my feet. We don’t normally think about geology like this, but perhaps we should.

If you are sitting above a subduction zone, then rocks are being stuffed down below you so fast they are cooler at deeper levels (they’ve not had time to heat up), so you can expect a cooler gradient, C which is associated with blueschist and eclogite facies. Finally if you are in a volcanically active area then shallow rocks may be heated a lot, and you can expect hornfels facies rocks.

I’ve started linking descriptions with tectonic conditions, but facies were originally defined as characteristic groups of mineral assemblages based on description of (mafic) rocks in the field. So greenschist facies rocks are often green, being rich in epidote/chlorite group minerals and blueschists contain blue glaucophane. Amphibolite facies rocks may often contain amphibole but by definition granulite facies ones do not.

A note on rock-type

Gorgeous eclogitic facies metamorphosed igneous rock, the sort of rock I won’t be talking about. Image courtesy of www.thinktank.ac.uk

The exact minerals you get in a metamorphic rock depend on the rock chemistry. A metamorphosed pure quartz sand will always contain just quartz (except in really extreme eclogite rocks). Give a metamorphic petrologist a sample of quartzite and you’ll get a funny look. Give them a metamorphosed limestone or a mudstone and you’ll get a much warmer reception. These rocks have a more varied chemical make-up that means a wider set of minerals can grow in them.

Igneous rocks can be metamorphosed, but usually in less interesting ways. Gabbro forms at granulite conditions (solidus of 1200°C), and as it cools is often cut by alteration veins where incoming water metamorphoses igneous minerals into lower temperature equivalents, processes such as serpentinisation of olivines, or amphibole replacing pyroxenes. Eclogite facies igneous rocks can be lovely, with greens and purples and…. But let’s not get distracted, from now on my examples with generally be metamorphosed sediments rich in mud, also known as pelites.

Metamorphic rock types refer to texture, facies to mineral assemblages, but the two map together quite well. Slates are low temperature/pressure facies, schists are blue or green and by the time you get into eclogite or granulite facies, you are likely to have a gneiss on your hands.

How do we know the conditions of metamorphism?

Individual minerals are stable under particular conditions. Diamonds form very deep underground and are only metastable under surface conditions. Pure carbon on the surface is much happier as graphite or soot. Happily, diamond bearing rocks were brought to the surface and cooled quickly, so they still preserve a record of the high pressure conditions they formed under. This is how we infer the conditions under which a rock was metamorphosed. We look for minerals or groups of minerals that only form under particular conditions and that still remain in a rock sample we have collected from the surface.

A less glamorous example but more useful example than diamond are the aluminosilicates, andalusite/sillimanite/kyanite. The diagram shows that if a rock contains an aluminosilicate, then the type of mineral tells you something about the metamorphic conditions: sillimanites are associated with hot rocks (e.g. migmatites), kyanites with cool, deeply-buried rocks (e.g. blueschists) and andalusite with shallow metamorphic rocks (e.g. metamorphic aureoles and hornfels).

Stability of andalusite/kyanite/sillimanite in P-T space. Image courtesy of serc.carleton.edu. NB diagram has pressure axis inverted compared with the facies diagram, which is a sneaky trick metamorphic petrologists are fond of

So, spot some sillimanite in your rock, and you know its been very hot at some point in its history. Congratulations! You have inferred something about the formation of a rock by studying it; you are now a metamorphic petrologist.

Metamorphic facies are groups of minerals, but the same principles apply. Spotting garnet alone doesn’t tell you much, but spotting it with orthopyroxene and cordierite strongly suggests you are in the granulite facies. In the same way as as for kyanite/andalusite/sillimanite, the groups of minerals found in a metamorphosed mudstone can be used to infer the conditions under which it formed. With aluminosilicates you only have three areas on the P-T diagram but with mineral assemblages you have lots of minerals and lots of combinations and lots of areas and therefore more accurate estimates of conditions.

Why do mineral assemblages depend on P-T conditions?

The reasons for this lie in mineral chemistry and thermodynamics, in the minimisation of Gibbs free energy, Margules parameters and other interesting things. However I’ve forgotten the detail of all of this, so I shall fall back on the pathetic fallacy, and talk about minerals or atoms liking particular conditions. I think we’ll all be happier that way.

As minerals get hotter, the atoms in them vibrate more and more. As temperatures increase, atoms that are happy in a fluid phase, such as water or carbon dioxide increasingly prefer to be in the interstitial fluid that sits between the minerals. So at greenschist conditions, water happily sits within the widely spaced mineral lattice of chlorite (lovely grass-green colour, spooky grey in crossed-polars). As temperatures increase water increasingly prefers to sit in the fluid phase in the rock. Driven by this preference, a chemical reaction occurs: chlorite breaks down into other minerals and water. The chlorite begins to vanish and the atoms that made it up go into other minerals: metamorphism in action! The other minerals may already exist, in which case they get bigger, or they appear for the first time.

Under greater pressure, atoms prefer to belong in tightly packed mineral lattices (diamond is nearly twice as dense as graphite). This can mean that particular minerals become unstable under increasing pressure.

So pressure and temperature affects the stability of minerals, which determines which mineral assemblages are happy under particular conditions. A subtler affect is also possible, where reactions can be continuous.

Many rock-forming minerals allow different atoms to sit in the same type of location within the crystal lattice (this is called solid solution). Classic examples are Sodium and Calcium within plagioclase feldspar, or Iron and Magnesium in many minerals. As pressures or temperatures change particular elements will start to prefer one mineral above another. This process often drives continuous metamorphic reactions where no new minerals are formed, but their composition changes. For example, a piece of biotite sits next to a piece of garnet. They both contain Fe and Mg and grew at the same time so the ratio of their contents Fe and Mg reflects the temperature at which they grew and we say they are in chemical equilibrium. If the temperature changes, this equilibrium is disturbed. The minerals themselves remain stable, but Fe and Mg atoms slowly swap themselves, some moving into the garnet, some moving the other way taking their place. Given enough time, the composition of the minerals will change, but no new minerals will have grown.

To summarise, metamorphism is driven by metamorphic reactions, which in turn are driven by mineral stability and the preferences of elements, which in turn are driven by changes in both pressure and temperature. One way of visualising this is to think of the reactions as a line in Pressure-temperature space. On each side you expect a different set of minerals. Here’s an example diagram:

Phase diagram showing metamorphic reactions in P-T space. Taken from serc.carlton.edu

There’s a lot going on here. Note the Kyanite/Andalusite/Sillimanite triangle again (labelled Ky/And/Sil). Black lines represent metamorphic reactions, with the minerals taking part on either side of the line. Look at the black line with a little f on it, near the middle. This represents a reaction where Biotite (Bt) and Sillimanite (Sil) break down into Garnet, Cordierite, K-Feldspar and a liquid phase. These are granulite facies conditions. A rock might cross this line either by being heated (moving left to right) or by being decompressed (moving down).

All the lines in the diagram are discontinuous reactions but its possible to add other lines of mineral composition showing continuous reactions also. The diagram is already complex enough, but its gives you a feel for how much information there is available. Creating diagrams like these is the core thing metamorphic petrologists do. Such a diagram allows us to link rocks to conditions of metamorphism and ultimately to tectonics.

Summary

I hope I’ve given you an introduction to some of the core concepts of metamorphic petrology. Heating rocks under pressure changes the stability of minerals, driving metamorphic reactions that grow new minerals that are characteristic of the new conditions. The job of the metamorphic petrologist is to study rocks found at the surface and use the minerals to infer past conditions of metamorphism. This is a job worth doing as it gives us insights into conditions within the roots of mountains, subduction zones or even impact craters.

In future posts I will be taking you deeper in, explaining how we can quantify conditions of metamorphism, how we can track the journeys rocks have been on and maybe a peek at what is going on at the cutting edge.

A continuing theme will be the interplay between insights based on chemistry and physics and evidence from traditional geological pursuits such as field-work and detailed observation of real rocks.

Blogger lifeinplanelight has published a really good series of posts, one on metamorphic reactions plus examples each of regional and contact metamorphism. If you liked this, (or perhaps especially if you didn’t) you should definitely head over there.

Some of my diagrams are stolen from an excellent page on the website of Carleton College. This talks about a specific field area in detail, including the metamorphism.

I’ve done some more, including one on metamorphic grade, zones, index minerals, and whisky.

Relict of the flood?

Posted on 29 August, 2011 by Metageologist

I’ve recently spent a lot of time with my kids in the fabulous public parks of Macclesfield. An ex-mill town in the north of England, Macclesfield expanded greatly in the Nineteenth Century and the civic leaders at the time took care to build large green spaces into the town. In a nicely manicured lawn in West Park, there is a large stone, which is rather interesting.

It is a large boulder with rough edges, but a striated smooth top. It is of course a glacial erratic, that is a large block of rock, picked up in a glacier or ice sheet, moved a long way and then dumped somewhere else. It is, in the loosest sense, granite and to anyone familiar with English geology, comes from the Lake District. Smaller blocks from the Lake District are extremely common in the East Cheshire area, and are often seen ‘sticking out’ of dry-stone walls, where their rounded shape spoils the otherwise regular pattern.

A teenie-tiny glacial erratic incorporated into a typical Macclesfield stone wall. The rounded white-weathering block of Lake District igneous rock contrasts with regular bedded local stone

At the last Ice-Age maximum, Macclesfield was near the southern edge of the glaciated area, which covered pretty much all of the British Isles to the north. An ice-dome centred on the high ground of the Lake District swept down south over the Cheshire Plain. Just east of Macclesfield is the higher ground of the Peak District, that remained unglaciated.

The West Park boulder was found in 1857 and was moved a small way by a team of 8 horses into the 3 year old park where it was put on a plinth and a brass plaque attached.

The bottom section of the text says: “This Stone is similar in composition to the Granite Rocks near Ravenglass on the coast of Cumberland; it is supposed to have been carried by an Iceberg from that district and deposited on the bottom of the sea which once covered parts of Cheshire and the adjoining Counties. Vide Buckland’s Relio. Deluvianae, pages 198 & 224″.

William Buckland, was an interesting character and important figure in Nineteenth Century science. His 1823 book, referenced on the plaque, “Reliquiæ diluvianæ: or Observations on the organic remains contained in caves, fissures, and diluvial gravel, and on other geological phenomena, attesting the action of an universal deluge” was a best-seller and evidently a copy made it as far as Macclesfield. Following the plaque reference to page 199 shows that the reference to Ravenglass is a direct lift from Buckland’s book (rocks like this are found all over the Lake District, Ravenglass is only where Buckland happened to see them). Page 224 summarises his main argument, that blocks like this are evidence of a ‘universal deluge’, such as described in the bible, and that this block is a relict of the Biblical flood.

William Buckland is a great figure in Geology, because, like Darwin, he was one of the people who used careful observation to overturn the traditional idea that the Bible is a literal description of the Earth’s history. By 1836 he had changed his mind about the flood and by 1840 was in agreement with Louis Agassiz‘s theory that features such as erratics were the product of glacial processes.

So, by 1857, the theory expressed in Reliquae diluvianae had been rejected by its own author in favour of one we would agree with today. However nobody had told the good folk of Macclesfield. Or had they? The reference to an iceberg as the form of transport does not come from Buckland’s early book; Noah’s ark was not an ice-breaker, so where does the ice reference come from? It suggests to me that the text on the plaque is a form of compromise. The book and its idea of the block moving while Macclesfield was underwater is in there, but so is a reference to the more modern theory of ice being involved. The result is still wrong (an ice-sheet, not an iceberg was the form of transport) and it is not surprising that they started their description with the phrase “it is supposed to have been”, if they couldn’t agree amongst themselves.

So, next time you get access to most of human knowledge via your smart-phone, spare a thought for your ancestors. This plaque reminds us that only 150 years ago, in a town in one of the (then) most-developed and wealthiest countries on earth, the most definitive source of geological information available was a 30 year old book.

Accretionary Wedge – 37: Sexy Geology

Posted on 11 August, 2011 by Metageologist

Accretionary Wedge #37 called for examples of ‘Sexy Geology’. Here’s mine:

You always remember your first time.

I was a young man, freshly graduated and I’d somehow persuaded the government to give me (just) enough money to spend three years studying for a geology doctorate. Once spring arrived I eagerly set off for my field area. I’d been here before, but chaperoned on a undergraduate field trip. Now it was just me and the field-area, alone at last.

I spent most of my first field season with a small hill called Currywongaun. She’s a lovely thing: a glacially scoured lump of gabbro facing the Atlantic in Connemara, Western Ireland. The view is beautiful, the rocks are beautiful and sexy.

Gabbro is lovely stuff, with a touch of the exotic coming from its origin in the mantle. Most gabbro intrusions are petrologically interesting but structurally simple. Not my Currywongaun, it has modal layering, sure, but its orientation varies wildly and sometimes it’s folded. It is also has a fabric defined by the igneous minerals, showing that the magma itself was deformed while it was still partially molten. It is a syntectonic intrusion, emplaced not into the edge of an opening ocean basin, like the Cuillins or Skaergaard but into an actively deforming mountain belt.

Magmatic fabric in syntectonic gabbro

The base of the hill covers the edge of the intrusion and the rocks beneath. These are the sorts of rocks that get fans of structural geology all hot and bothered: mylonites, with lots of consistent shear sense indicators. Two sets of fabrics too, high temperature ones related to thrusting parallel to the intrusion edge and lower-grade cross-cutting extensional shear zones recording orogenic collapse.

Fancy a bit of metamorphism? Me too. Rocks close to gabbro get very hot indeed and metasedimentary xenoliths on Currywongaun have corderite-spinel-orthopyroxene assemblages; virtually all water (found in minerals such as muscovite) has been driven out of the rock due to extremely high temperatures. There’s a nice aureole around the intrusions as well which is large and high temperature (sillimanite isograd) since the rocks were already fairly hot (500 °C).

Cordierite-spinel-orthopyroxene granulite facies xenolith

A bit of mystery can be sexy too. There was an odd lens of an off-white rock, kind of a streak across the hillside, that I couldn’t quite identify in the field. Once back in the lab, armed with thin sections, I could piece together a story to explain the mystery.

Responding to powerful forces from deep within, the mafic magma forced its way deep into already hot metamorphic rocks. It crystallised into gabbro, became rigid and immediately the contrast in rheology with the softer reforming rocks around it created a zone of thrusting along its edge (I found sheath folds). The host rocks quickly responded, they were heated and metamorphosed with reactions that produced granitic melt and lots of hot fluids. These were forced back into the intrusion, altering it, transforming it. My mystery sample? Well, the gabbro is noritic, rich in orthopyroxene which when moistened is altered to everyone’s favourite amphibole, cummingtonite.

Sheared cummingtonite (with minor plagioclase and lichen)