Paths across the Cheshire Peak

Driving west across the edge of the English Peak District is a good way to see how geology shapes landscape. Tracing the routes that cross it – feeling their shapes with a finger on a map or with your body as the car swings round bends – hints at how they are shaped by the landscape beneath, but also the intentions of the people who first made them. Paths across the Cheshire peak were shaped by dramatic changes across both human and geological history.

http://www.geograph.org.uk/photo/1093601

The new Buxton road winds below Shining Tor. © Copyright Jonathan Wakefield and licensed for reuse under this Creative Commons Licence

Roads in Derbyshire’s ‘White Peak’ are shaped by the limestone beneath; they sit in the bottom of ‘dales’ – steep gorges etched into rock – or wind across a bucolic landscape of green fields tessellated by white stone walls. But drive out of Buxton on the Macclesfield (“Cat and Fiddle”1) road and you suddenly enter a wilder world. Within a few feet, the stone walls at the side of the road turn from pale grey to a buff beige.  The landscape is brown and open, empty under a sky that is rarely entirely blue, mantled by peat bog and growing little but heather. You are entering one of the ‘Dark Peak’, one of the wild moors of northern England, the wuthering heights where Heathcliff roamed and Ted Hughes’ hawk roosts.

Further west, at the edge of the moors everything changes again – the Cheshire Plain appears laid out for inspection. On a clear day – or better still night – the view takes the breath away. The homes and lights of 3 million people twinkle and beguile. The depth of detail invites you to study, to pick out Jodrell Bank, flights descending into Manchester airport, Alderley Edge….

The Buxton road climbs up out of Macclesfield. Taken near Toll Bar Avenue.

The Buxton road climbs up out of Macclesfield. Taken near Toll Bar Avenue.

Drivers shouldn’t enjoy the spectacle: this road needs your full attention. It’s popular with motorcyclists for its many bends. Sadly some are total idiots, making the A537 one of Britain’s most dangerous roads. Their attitude to the area is not much different from many other modern travellers – this is a place to enjoy yourself in. Older generations – those who made this and other routes – had other motivations.

 

 

Early trade-routes

Some old routes over the high moors of northern England are know as the Saltways. The ‘wiches’ of Cheshire: Nantwich, Northwich and Middlewich, are towns based on salt. Thick layers sit deep within the Cheshire Basin, formed as a shallow sea was repeatedly evaporated under the Permian desert sun.

Salt has been produced in Cheshire since at least Roman times. An important commodity essential to food preservation (cheese! bacon!) it was transported across the country by salt traders (“salters”) who name attached to their routes. Below is my inference as to the route taken by salters through this area, passing through Saltersford Hall.

Trace of Salters way

Trace of Salters way in green. Macclesfield is the town on the left, Buxton on the right. The brown area in the middle is the moor

Buxton was a Roman town  and a direct line from the salt towns to there passes this way, but there is not good evidence it is that old. These routes are pre-industrial though, used by men and horses walking through the landscape, at the mercy of the elements. A reminder of how perilous this could be – hard to remember when speeding in a warm car – comes from an odd memorial stone on the route, that reads: “Here John Turner was cast away in a heavy snow storm in the night in or about the  year 1755. The print of a womans shoe was found by his side in the snow where he lay dead”

http://www.carlscam.com/rainow/turner.htm

The front of the memorial stone. Source

 The modern age approaches

Even as John Turner grew cold in the snow,  the epoch-making2 Industrial Revolution was hotting up. Using new technology to centralise production in factories only make sense if you can then get your goods to the people who buy them – new forms of transport were an important factor.

The 18th Century – early on in the Industrial Revolution – innovation came in the form of many new roads, called turnpikes. These were independently financed toll roads, sanctioned by Acts of Parliament.

The first Macclesfield-Buxton toll road followed an old route, was ‘engineered by a blind man, John Metcalfe’, and opened in 1759. Initially controversial, it was opposed by some (local coal producers) but championed by the new industrialists.

Old Buxton road in blue, new road in red. Route over Shining Tor in brown.

Old Buxton road in blue, new road in red. Route over Shining Tor in brown.

This road and the salters way are both direct but steep. This is ideal when moving goods with pack horses, but horse-drawn wagons work better with more gradual slopes, even if the route is longer. By the dawn of the 19th Century, new road-building techniques had emerged that cut into the hillside to make wider carriage-ways that avoided steep slopes even over hilly terrain.

New and old Buxton roads cross the far hillside. Cat and Fiddle pub right hand skyline

New and old Buxton roads converge on the far hillside. The Cat and Fiddle pub is on the right hand skyline

In 1808, a new Eddisbury bypass just above Macclesfield was built by the famous engineer Thomas Telford, the “Colossus of Roads”. In 1821 the rest of the Macclesfield-Buxton road was modified with new winding flatter routes and a pub for the weary. The new road is wider than the old and climbs more gradually. To achieve this is has many bends, which attract the loonies in leather on their motorcycles.

A new ancient road

The transition from White to Dark peak, as you go east from Buxton is dramatic to us, but the incoming darkness would have been felt much more keenly by the Carboniferous inhabitants. The sparkling tropical seas where trilobites frolicked in crinoid forests were suddenly snuffed out by the arrival of massive amounts of sand and mud. Rocks made from the remains of life are replaced by those where fossils have to be sought out – traces in sand, crushed shells in rare marine muds or eventually, coal.

The most modern path across the Peak is also the most ancient. Walking across these hills for pleasure is extremely popular and paths easily cut into the soft peat. The most popular routes are now paved with big slabs of the local sandstone – along Shining Tor there are hundreds of them. Covered in ripples and the traces of burrowing bivalves, walking along these makes you feel like you are on a sandy shore 300 million years ago.

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Slabs of sandstone along Shining Tor

 

Ancient ripples

Ancient ripples

"Lockeia" - traces of burrows from bivalves

“Lockeia” – traces of burrows from bivalves

Categories: England, landscape, sediments

A new paradigm for Barrovian metamorphism?

George Barrow

George Barrow (image via BGS)

The phrase ‘new paradigm’ is a little shop-worn but it still catches the eye. To see it used in a “discussion and reply” on a dry-looking metamorphic petrology paper is really something unusual. Tracing through these articles really shows how metamorphic petrology can get to the heart of understanding what happens in the core of mountain belts.

The example of “discussion and reply”1 I’m discussing here is truly remarkable. Everyone is terribly polite and there is lots of new data and ideas contained that shed light on a classic area for the understanding of metamorphism. It also illustrates the perils and challenges of interpreting complex rocks that formed in the heart of an ancient mountain.

A paper worth discussing

The original paper, entitled ‘Metamorphic P–T and retrograde path of high-pressure Barrovian metamorphic zones near Cairn Leuchan, Caledonian orogen, Scotland’ is by Kasumaza Aoki of Tokyo, Brian Windley of Leicester and others. It studies metamorphic rocks from the Scottish Highlands. You’ve heard of Barrovian metamorphism, (if not, get thee hence), this is the type locality, where George Barrow first identified zones of rock defined by particular ‘index’ minerals.

Sample of Cairn Leuchan Gneiss, collect by George Barrow himself. Garnet, plagioclase, Hornblende (brown), pyroxene (green).

Sample of Cairn Leuchan Gneiss, collected by George Barrow himself! Garnet, plagioclase, hornblende (brown), pyroxene (green). From BGS, sample S8146

Barrovian metamorphism is typical of mountain belts. It’s thought to be caused  by thrusting and stacking of rock slices within a growing mountain belt, that buries and heats up the rocks within it – causing metamorphism. This model is simple, classic and perhaps wrong. Our original paper looks at a slice of high grade rocks from the hottest, sillimanite zone. Detailed metamorphic petrology shows that “the rocks underwent high-pressure granulite facies metamorphism at P = c. 1.2–1.4 GPa and T = c. 770–800 °C followed by amphibolite facies metamorphism at P = c. 0.5–0.8 GPa and T = c. 580–700 °C”.

The later metamorphism is pretty standard for the area, but the earlier high pressure phase is unusual, suggesting these rocks were buried much deeper than previously thought. Our authors conclude that high-grade Barrovian metamorphism is retrograde, the metamorphic minerals formed as the rocks moved back towards the surface, masking an earlier deeper phase. They also suggest that the measured high-pressure metamorphism was also formed *on the way up* and that these rocks (perhaps all rocks in the area, there’s evidence nearby at Tomatin) had previously been down to eclogite or blue-schist depths.

If this paper were science journalism (or course it isn’t) you could accuse it of ‘burying the lede’ – the abstract focuses purely on what they proved. The major implications of their results are made much more explicit in the Discussion by Daniel Viete and others. S98121XPL

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BGS sample S98121. Garnet amphibolite from Tomatin. First XPL then PPL. Relict high-pressure eclogitic garnet breaking down to plagioclase and amphibole as it moved towards the surface.

“Interesting, but maybe it’s actually….”

The discussion (by Daniel Viete, whose research I’ve written about before and others) argues against the original papers conclusions, using two lines of attack. Firstly they focus on how hard it would be to get rocks so hot, so early in the orogeny2. Other high pressure rocks in the Grampian Orogeny are much colder (e.g. blueschists in Clew Bay, Ireland). From dating, we know that the Scottish rocks were metamorphosed relatively quickly (18 million years or less). Indeed Viete has already written about the difficulty of getting rocks in this area hot enough, quick enough. This was for the well known temperatures. The problem is even greater for Aoki’s earlier deeper and hotter phase. Listing all the ways these rocks could have been heated up (mantle melts, radioactive heating, mechanical heating, already hot from rifting) they conclude that the early measured temperatures are not possible.

Secondly they describe the local geology in terms of networks of long-lived shear zones, that interleave Dalradian sediments with older basement rocks. They also provide a date of 1 billion years (much than the Ordovician age of Barrovian metamorphism) from the Cowhythe Gneiss at Portsoy, a nearby patch of high grade rocks. In conclusion, the early high-pressure metamorphism that Aoki and others describes is from an entirely different orogeny and found only in odd slices of older rock.

S94156APPL

BGS sample S94156.  Garnet amphibolite from Tomatin. Intergrowths of plagioclase and amphibole – formed by decompression of an eclogite?

“With respect, no it’s not. Plus we’ve got a brand new paradigm!”

In their reply, Aoki and others defend their conclusions. Their rocks are not basement and are totally different from the Cowhythe Gneiss. Plus, old ages can often be ‘inherited’ – dated zircons may contain old ages since the crystal was a ‘detrital’ grain within the sediment that was later metamorphosed. They then turn to their explanation for their results. Acknowledging that “a reply to a comment is not the correct place to propose an entirely new paradigm for such a classic orogen” they nevertheless provide a brief overview, promising to “present our model more fully in a future publication“.

They start with the well-known enigma of how to provide the heat for Barrovian metamorphism. Modelling suggests that stacking rocks and waiting for them to heat up (the classic England & Houseman model) actually takes 50 million years. Viete has previously proposed the heat came in via advection via hot fluids. Aoki’s model proposes “the extrusion of a major wedge of hot deep eclogite which was exhumed up a subduction channel several tens of kilometres thick“.

Wow.

What they are proposing is that hot rocks within  the subduction zone, perhaps 40 km below the slowly heating orogenic wedge, broke free and was squeezed up into it, heating the wedge and causing rapid Barrovian metamorphism. This would be an extremely dramatic thing and is a radical departure from existing models for this orogeny, which is itself the ‘type locality’ for all instances of Barrovian metamorphism. Aoki refer to one earlier paper by a group from Cambridge that propose a similar mechanism for the Alps. The Alpine eclogite wedge is still clearly eclogite forming a discrete unit within other nappes. It’s not (yet) clear how Aoki’s traces of earlier high pressure minerals in an relatively homogenous Dalradian correspond to this.

What does it all mean?

For what it’s worth, I’m a little sceptical – my headline follows Betteridge’s Law – but we’ll have to wait for the paper that properly presents the new model before we can judge.

One thing that strikes me is how much Wheeler’s paper on the importance of stress throws doubt on this work. Tales of packages of rock squeezing 10s of kilometres up into an orogeny puts a lot of weight on the traces of high pressure metamorphism that are the main evidence. Explaining high pressures in terms of localised stress starts to seem like a much simpler explanation.

This is a fascinating series of papers3. It highlights how vital metamorphic petrology is to understanding mountain building processes. George Barrow first identified his zones over a hundred years ago and the Scottish Highlands have been intensely studied ever since, yet we still don’t fully understand how they formed.

The fact these papers are hidden behind a paywall is in stark contrast to the pictures I’ve used. All come from the British Geological Survey who have made them free to all. The depth of coverage is fabulous – I’ve been able to find images from the key localities mentioned in the papers within minutes. This illustrates the power of open data rather nicely – if only we could all find scientific papers as easily.

References

The original paper.
AOKI K., S. MARUYAMA & S. OMORI (2013). Metamorphic P–T conditions and retrograde path of high-pressure Barrovian metamorphic zones near Cairn Leuchan, Caledonian orogen, Scotland, Geological Magazine, 151 (03) 559-571. DOI: http://dx.doi.org/10.1017/s0016756813000514

The discussion and reply.
Viete D.R. & S. A. Wilde (2014). Discussion of ‘Metamorphic P–T and retrograde path of high-pressure Barrovian metamorphic zones near Cairn Leuchan, Caledonian orogen, Scotland’, Geological Magazine, 151 (04) 755-758. DOI: http://dx.doi.org/10.1017/s001675681300099x

The Alpine paper
Smye A.J., Tim J.B. Holland, Randall R. Parrish & Dan J. Condon (2011). Rapid formation and exhumation of the youngest Alpine eclogites: A thermal conundrum to Barrovian metamorphism, Earth and Planetary Science Letters, 306 (3-4) 193-204. DOI: http://dx.doi.org/10.1016/j.epsl.2011.03.037

An old paper on the Tomatin ‘eclogites’  

Categories: eclogites, metamorphism, mountains, Scotland, tectonics

Metamorphic petrology: under pressure and getting stressed?

High pressure (HP) terranes are areas containing eclogites and other eclogite-facies rocks found within many mountains belts, including the Himalaya and the Alps. HP rocks were metamorphosed at extreme pressures, up to 3 or even 4 billion Pascals (or GPa. Atmospheric pressure is 0.0001 GPa). Based on the assumption that metamorphic pressure relates to depth of burial, these rocks have been to depths below the base of the crust. Explaining how to bury rocks so deeply, and then bring them back again, is a tricky problem.

Two parallel threads of research are challenging the assumption that high metamorphic pressures relate simply to depth of burial. Both seek to show that metamorphic reactions, and the patterns of minerals that they form, are also influenced by the squeezing and squashing rocks receive as they flow deep within the earth.

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High pressure rocks looking moody and gorgeous. Photo of the Internal zones of the Alps from John Wheeler.

A little theory (please, don’t get stressed)

Metamorphic petrology is the art and Science of calculating the conditions under which metamorphic minerals form – I’ve written extensively about it already1. It has its basis in an understanding of the thermodynamics of the reactions that form minerals, which depend (among other things) on the temperature and pressure at the time. The lithostatic pressure that affected the rock is assumed to be caused by the weight of the column of rock overhead, following Pascal’s law2. With some assumptions as to the density of that rock, pressure can be converted into depth.

Earth scientists who study the structure of deeply buried rocks look instead at the patterns that the minerals form: fabrics, lineations, folds and the like. They have a different way of looking at the way rocks are squashed. Theories of rock deformation talk about stress, often visualised as arrows in 3 dimensions. If the stress is isotropic – equal in all three dimensions – then it is conceptually the same as lithostatic pressure. In contrast, differential stress is where the arrows are different sizes. Differential stress drives deformation – the rocks change shape.

Time for an analogy. Lithostatic pressure is like water pressure. Deep hot rocks flow like a fluid (slowly) and so the stress is equal in all directions. Rocks keep the same shape, but are put under tremendous pressure. It’s the same as putting a styrofoam cup into deep water.

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Sytrofoam cup crushed at 600m ocean depth. Image from Gina Trapani under Creative Commons.

Differential stress is like putting the cup between the grabbers of the submersible. Squeezing in only one direction will change the shape of the cup.

Now we’re ready to look at the two challenges to the depth-pressure connection.

Challenge 1 – High levels of differential stress exist in the crust

Metamorphic petrologists who argue that high pressures are caused by deep burial believe that the effects of differential stress are unimportant. These rocks are usually extremely deformed, but they were hot and soft – by flowing easily they prevented high differential stresses from building up. In a similar way, as every toddler learns, you can’t squeeze jelly hard with your hands, because it just flows out of your fist.

Some scientists, with an interest in the European Alps, disagree. Neil Mancktelow argues that the effects of differential stress (he uses the basically equivalent similar term ‘tectonic overpressure’3 ) can be dramatic. Mathematical modelling of conditions within a confined channel (like a subduction zone) gives ‘overpressure’ of “perhaps even a few GPa”. Rocks that formed under these conditions would yield high pressure minerals but may not have been buried very deep.

At this years EGU meeting Stefan Markus Schmalholz, Yuri Podladchikov, and Sergei Medvedev used computer modelling of the Alpine orogeny, and other arguments to suggest that Alpine high-pressure eclogites need not have been deeply buried, but instead their distinctive minerals formed under conditions of tectonic overpressure.

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Folded layers from eclogite facies rocks in Norway. Photo from John Wheeler

Challenge 2 – John Wheeler’s recent paper in Geology

Computer models are extremely useful, but only as good as the assumptions made in them. Alone they will always have their critics. John Wheeler, a professor at Liverpool University has fingers in many pies: metamorphism;  structural geology; field-based studies; laboratory work… He takes a different approach to also puts the assumption that we can apply Pascal’s law under intense pressure 4.

In a recent open source paper in Geology he combines 2 pieces of theory. The first is from thermodynamics as used in metamorphic petrology. The second is an equation relating stress to the phenomena of pressure solution (that can form cleavage in slaty rocks).  Both are concerned with the movement of atoms to grow new minerals and neither is controversial. He converts both into terms of chemical potential and combines them to link differential stress to standard metamorphic reactions.

Applying uncontroversial values for differential stress (e.g. 50 MPa, or 1/40th of the levels Mancktelow proposes) to a standard metamorphic reaction he shows that they shift by up to 500MPa – equivalent to 15km of lithostatic pressure.

This is an extremely dramatic result from a provocative paper, as John Wheeler himself acknowledges. Metamorphic rocks are almost always deformed, sometimes dramatically so. If both approaches are correct, then putting differential stresses of Manckletow levels into John Wheeler’s equations suggests any estimates of depth of metamorphism are open to challenge. Maybe eclogites never left the crust after all?

metagabbro deformed and recrystallizing to new minerals at the same time (from Alps).

Metagabbro deformed and recrystallizing to new minerals at the same time (sample from Alps image from John Wheeler).

Where are we now?

John Wheeler  expects some  “turbulent interactions in the next few years”.  I’m sure there are  metamorphic petrologists writing their replies to his paper right now. But assuming they can’t find some fundamental flaw he hasn’t spotted (extremely unlikely, I’d say), metamorphic petrology is facing a very big challenge.

This seems like bad news. Much work on metamorphic petrology is put to work refining tectonic models by building P-T-t paths that track metamorphic rocks as they are buried and return to the surface. The error bars on these estimates were big enough already – now a entire literature of estimates that ignore differential stress are thrown into question.

But let’s think positively. Might there be ways to disentangle the effects of lithostatic pressure and differential stress? Might there be ways to get even more information out of deformed metamorphic rocks? John Wheeler certainly thinks so and he aims to get there by integrating detailed studies of rock samples with laboratory experiments.

Big advances in Science often give us new ways to understand evidence that previously didn’t make sense. Studies of metamorphism from the classic locality of the Scottish Highlands (e.g. Viete et al 2011) have noted that the highest grades of metamorphism occur within shear zones. They tried to explain this in terms of the flow of hot fluids, but maybe the answer is simpler: stressing rocks really does cause them to grow different suites of metamorphic minerals.

References:

Mancktelow N.S. (2008). Tectonic pressure: Theoretical concepts and modelled examples, Lithos, 103 (1-2) 149-177. DOI: http://dx.doi.org/10.1016/j.lithos.2007.09.013

Wheeler J. (2014). Dramatic effects of stress on metamorphic reactions, Geology, DOI: http://dx.doi.org/10.1130/g35718.1

Viete D.R., G. S. Lister & I. R. Stenhouse (2011). The nature and origin of the Barrovian metamorphism, Scotland: diffusion length scales in garnet and inferred thermal time scales, Journal of the Geological Society, 168 (1) 115-132. DOI: http://dx.doi.org/10.1144/0016-76492009-087

Categories: eclogites, metamorphism, mountains, subduction, tectonics

Dinosaurs and the dangers of pedantism

Did you click here because of the word pedantism in the title? If so, you’ll like this (perhaps) apochryphal exchange between a married couple I know.

Person A: ” You’re always correcting me! Your pedantism is really getting me down.”
Person B: “I think you’ll find the word is pedantry….”

Most of can see both sides of this exchange. Nobody likes to be corrected, but who of us has not felt the deep urge to right wrongs.

So, often we have a choice to make: risk looking like a dick or miss an opportunity to educate people. For things like rogue apostrophes or grammar in general, this is a game we can all play. But for scientists or those involved in communicating science things are more complex.

Mastering technical terms is an important part of a scientific training. In conversations between experts, using technical terminology is a tremendously efficient way of discussing complex issues and a powerful way of signalling that one is part of the science in-group. When communicating to non-scientists, forgetting to translate these terms into plainer English is a obvious mistake.

Things are less clear-cut where the scientific term is a word that is used by non-scientists. Sometimes this is where a technical term seeps into normal speech (such as my use of ‘in-group’ earlier).  Here, for scientists to defend the original meaning is uncontroversial. People who describe a new product as ‘a paradigm shift’ deserve to mocked.

Sometimes a word has been given a more specific meaning. The distinction in Physics between mass and weight is one example, or I suggest, the words granite and marble. Here we need to be careful. It may be annoying for a geologist to see a ‘marble’ worktop with fossils in, but a little humility is required. The stonemasons’ definition of marble as a carbonate rock that takes a polish is at least as old as the geological one of ‘metamorphosed limestone’.

The English comedian David Mitchell has a habit of going off on rants about the way ‘scientists’ say that tomatoes are a fruit. The humour lies in how seriously he takes such a trivial point, but you get a sense that it genuinely annoys him. There’s a serious point: his working definition of fruit as being something that you’d put in a fruit salad is not the scientific one, but it is not incorrect. Tell someone that they are wrong for using it and they won’t thank you for it.

This leads me to dinosaurs.

The great pursuit of evolutionary biology – fitting all life into branches of a tree of immense size and age – is important and interesting. It provides a  extremely specific meaning of the term ‘dinosaur’ that includes birds, but excludes creatures such as Dimetrodon and marine reptiles such as Ichthyosaur that fit perfectly the more popular understanding of the word. Most people (myself included) are much more taken with the palaeobiological sense of dinosaurs (huge reptiles, teeth, claws, RAAAAR!) than the phylogenetic one. Biographies are more popular than family trees for a reason.

So, let’s all be very careful with the ‘not a dinosaur’ thing. Dinosaurs are a gateway drug into Earth Sciences – let’s not spoil things with pedantism. If you must tell someone that their definition of a word is wrong, you’d better do it in an interesting and light-touch way.

Categories: etymology