Like talking about a stone wall

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I grew up surrounded by stone walls, something which certainly nurtured the geologist in me. I certainly didn’t view them in any negative Pink Floyd kind of way. These were outdoors walls, field boundaries tracing across the hills for miles. They were easily scaled, fun to walk on top of and a place to store contraband, should the need arise.

The walls of my youth were top-class, I now realise. My English mill-town home was surrounded by Carboniferous sandstone that produces beautiful regular rectangular slabs. Turning such regular stones into a beautiful stone wall is easy. A dry-stone wall mind, building a wall like this with cement would be like putting Coke into a gin and tonic: both barbaric and unnecessary.

Dry-stone walls are common on British uplands, but are found in many other countries. They are an attempt to solve two problems: a field full of stones and the roving instinct of livestock. I’ve seen Irish fields incredibly finely divided by walls, also with a pile of stones in the middle. Here the need to get stones off the ground is paramount. Some walls in Ireland are known as ‘famine walls’, they were built by starving men during the Great Hunger of the 1840s. These men were fed by the state, but the ideology of the time meant they had to work for earn the food. Sometimes stone walls dividing nothing from nothing were the result.

To a geologist, a stone wall can present a temptation. By their very nature, they are often built from stones picked off the ground, possibly moved from a distance away. And yet, they often seem to reflect changes in the geology quite precisely. Driving into the town of Buxton, for example, the bedrock change from sandstone to limestone is marked with a short (>3m) change in the rock-type of the stone walls. A geologist should ignore such evidence when mapping and look only at outcrops, but it can be very tempting, believe me….

Stone walls are a reflection of the underlying geology. This is clearly demonstrated at the millennium wall, to be found at the National Stone Centre, in Derbyshire, England. This is a series of dry-stone walls, representative of types from across Britain.

First, here’s a handsome gritstone wall, similar to the ones I grew up with. Its made from Carboniferous sandstone as found over most of Northern England.

Note how the regular bedding makes for blocks that are brick-like and easy to lay. You can get a similar but rougher effect from slates, like this Ordovician slate from the English Lake District.

If you have only igneous rocks to hand, life is harder as there are no parallel surfaces to work with. Here with this Scottish dolerite, they’ve had to work hard to get a rough looking wall.

If you’ve only got tough metamorphic rocks to work with, things are getting pretty desperate. This Scottish wall of meta-quartzite really is just a pile of stones.

Stone walls are not much made, these days. If a wall falls into disrepair, it is far easier for a landowner to put up a barbed-wire fence instead. A shame I think.

Where On Google Earth 324

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It took a while to get Matthew’s WoGE 323. He put up a view of some sort of volcanic feature and highlighted that there was a vineyard in the area. I’d spent some time in Germany, France and New Zealand before swinging over to California. With the Google Maps volcano layer on it didn’t take long to zoom into the right area.

So, on with the game! 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.

Here it is. North is straight up, in the usual fashion. Bonus points* to anyone who can link the area to Johnny Depp.

*Bonus points have no monetary value. They can’t be exchanged for any good or services at all.

 

AW #41 – why nothing is significant

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In Accretionary Wedge #41 – “Most Memorable/Significant Geologic Event That You’ve Directly Experienced” Ron Schott asked us to relate the story of the most memorable or significant geological event that you’ve directly experienced.

Living far from a plate boundary, I have a problem. There are no volcanoes in the UK. We have feeble earthquakes every now and then, but I always seem to be away or asleep when they happen (that’s right, these are earthquakes you can sleep through). I’ve witnessed rock-falls and erosion, which are important, but really I’ve not experienced any significant geological events at all. Which, come to think of it, makes me rather like a lot of the geological record.

Consider a nunatak in Antarctica. This is a lump of rock sticking out of the ice-cap. It is surrounded by ice for miles around and nothing happens. Cosmogenic nucleide studies show that the rocks surfaces are millions of years old. So little has happened that blobs of glass that fell from the sky the best of a million years ago (tektites), can be easily found on them.

On a bigger scale, Australia has had a quiet time of it since the dinosaurs. Many land surfaces on that continent are dated to be 10s of millions of years old. Weathering and soil formation has happened, but little else of geological significance.

Absence of evidence is not evidence of absence, but I can’t talk about nothing and Geology without mentioning unconformities. James Hutton’s realisation that these surfaces between rock packages can represent gigantic periods of time led us to the recognition of Deep Time, one of the most profound insights we have.

Unconformities are the most dramatic reminder of the amount of geological time for which we have no record, but a simple outcrop of sediments can do the same. Most sediments don’t represent a continuous record of sedimentation. Turbidites tend to record only occasional dramatic episodes of sedimentation. The steady drip-drip (or is that drop-drop?) of pelagic sedimentation is only intermittently preserved in such rocks. Sometimes we admire only the products of dramatic events and forget the huge periods of time in between. Even a calm-looking sandstone might in fact be mostly made up of storm deposits. A package of conformable sediments can contain huge gaps, but only subtle hints such as beds with intense bioturbation give a sense that for great periods of time, nothing happened, for all we can tell.

We should think about nothing more often. Thinking of Geology as a series of dramatic events is all very well, but its the enormous chunks of nothing that are truly remarkable. The human brain isn’t equipped to understand quite how insignificant we are in terms of space or time. Perhaps we should think about this more often, while staring at nothing.

Geospeedometry – how fast is metamorphism?

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Timescales in Geology are rather variable, to say the least. The earth has been around for billions of years so things can go sloooow and still be significant, like the build-up of oxygen produced by life. On the other hand, some things happen quick, like the hour it takes diamond-bearing kimberlite magma to get from the mantle to the surface.

How fast did these garnets grow?

What about metamorphic rocks – how fast do they form? A pair of recent papers use contrasting lines of evidence to suggest that it is faster than we thought.

Barrovian metamorphism is formed in rocks buried in continental collision zones – mountain belts. It is generally explained in terms of crustal thickening followed by thermal relaxation. In other words, if you pile up lots of rocks to thicken the crust they will then slowly heat up. Doing the maths on this (Thompson & England 1984) suggests it takes at least 40Ma (40 million years) to form Barrovian assemblages.

Two recent papers with lead author of Daniel Viete of ANU in Oz suggest things happen a lot faster than this (refs at end). Both study the classic Barrovian sequence of Scotland but in different ways. Both aim to perform geospeedometry*, to directly estimate the absolute rate at which metamorphism occurs.

The first uses 40Ar/39Ar dating of mica, but not in the way you’re thinking. As with all radiometric dating techniques 40Ar/39Ar or Argon-Argon dating is based on the decay of one isotope into another (here 40K to 40Ar). Measure the amount of parent and daughter isotope and you can calculate the age. Viete don’t calculate the speed directly from the age – the quirk they use is that Argon is a gas and mica has an open mineral structure. Above a certain ‘closure’ temperature the daughter Argon just diffuses away. The calculated age is therefore often the time a which the mineral cooled below the ‘closure temperature’. The technique is often used to find out when metamorphic rocks cooled down, but here Viete does something different.

Viete et al studied a transect through Barrovian rocks, from low to high-grade. For the low grade rocks (e.g. chlorite zone – greenschists) the ages are long before the Grampian orogeny. The white mica is detrital – it has not been recrystallised by the metamorphism we are looking at but instead it was formed in an earlier metamorphic or igneous event affecting rocks that were eroded long ago. (These little glimpses into older orogenies  stored in detrital micas can be very informative, but are not what we are looking at here).  Moving up temperature into the middle of the biotite zone there is a change in the pattern of ages – they start showing a date consistent with the age of the orogeny. This change of pattern is interpreted as being controlled by diffusion of Argon rather than by recrystallisation of the mica, indeed in one sample large grains hold old ages but small grains are reset. Diffusion depends on Temperature and time as shown by the Arrhenius equation. The authors estimate the temperature using geothermometry and model the diffusion to estimate how long the heating must have lasted to explain the patterns they see (small grains reset, large not). This gives an estimate of 200 ka to 2 Ma.

The second paper looks at manganese profiles in garnets in the same rocks but follows a very similar line of reasoning.

Manganese (Mn) likes garnets even more than I do. A classic ‘prograde’ profile through a garnet has high levels of Mn at the core, tailing off to the edge. This is interpreted as high levels of Mn being incorporated into the garnet early on, as it moves out of mineral phases where it is less happy into the new garnet. As we move out from the core, we trace the garnet growing bigger through time, and we see the amount of Mn dropping off as there is less and less available in other phases. Eventually garnet holds nearly all the Mn in the rock. This is a classic growth profile and is consistent with closed system behaviour on the scale of the hand-specimen: there is a finite amount of Mn available and the garnet has it all. Growth profiles are typically not preserved in higher grade rocks, which instead show a flat profile, maybe with a change at the rim. This is due to diffusion kicking in at high temperatures. In the hotter garnets the Mn can move around, which flattens the profile, as Mn is equally happy anywhere in the garnet.

Garnet samples studied by Viete show this pattern, a bell-shape, at lower temperatures  but at higher grades, the profile is flattened. Based on some neat reasoning involving patterns around Mn-free inclusions, they identify length scales of Mn diffusion for samples with a particular temperature. Sadly the experimental data on Mn-diffusion isn’t good enough to perform geospeedometry directly, but it does allow relative timings to be inferred. Taking their garnet and Argon work together and looking at other Scottish garnet work (a thin rim of further diffusion in higher-grade rocks) they argue for a later local metamorphic event of even shorter duration (order of 10,000 years), in addition to the relatively brief regional event.

Geospeedometry based on studies of diffusion in natural minerals is an exciting area, combine it with the ability to directly date portions of garnet and we have the exciting prospect of creating P-T-t paths that contain estimates for all three variables.

In their conclusions, Viete and their colleagues make some interesting points. The first is that this work supports a body of evidence that the Grampian orogeny (and by inference perhaps other Barrovian type mountain belts) was short-lived (a few million years). My own PhD work has a small walk-on part in this regional story, so I will surely write more about it one day.

More generally, if a Barrovian metamorphic event lasted only a few million years, what about Thompson and England and their 40Ma estimate? They calculated this based on conductive heat transfer only. Viete suggest that advective heat transfer is actually dominant, where the heat is brought in from elsewhere, not through the rocks themselves. Noting an association of high grade rocks and shear zones they suggest that hot fluids and magmas, plus mechanical heating in the shear zone is the most likely source of (advective) heat. The rapid (100,000 year) event they have evidence for could be explained by such a mechanism (both heating and cooling would be rapid as it occurs in small areas). The regional event might itself the cumulative effect of lots of little heating events.

Two questions are raised by this research. The first is the question of whether mountain belts can form quicker than we thought. There are other recent studies based on direct data of minerals that suggest slow rates of heating so if Viete’s results are correct, they are unlikely to apply to all metamorphic events. The second question relates to estimates of conditions from metamorphic rocks. If heating events are short-lived then the likelihood that chemical equilibrium was achieved is reduced, making thermobarometry a lot trickier. Finally, its nice to know that even a classic area of geology, well know for over a century, can still provide new insights.

References

Thompson, A.B. & England, P.C. 1984. Pressure–temperature–time-paths ofregional metamorphism II. Their inference and interpretation using mineralassemblages in metamorphic rocks. Journal of Petrology, 25, 929–955.

Viete, D.R., Forster, M.A. & Lister, G.S. 2010b. The nature and origin of theBarrovian metamorphism, Scotland: 40Ar/39Ar age patterns and the durationof metamorphism in the biotite zone. Journal of the Geological Society,London, 168, 133–146. doi: 10.1144/0016-76492009-164.

Daniel R. Viete, Jörg Hermann, Gordon S. Lister, et al. The nature and origin of the Barrovian metamorphism, Scotland: diffusion length scales in garnet and inferred thermal time scales Journal of the Geological Society 2011; v. 168; p. 115-132 Geological Society, London doi: 10.1144/0016-76492009-087

* geospeedometry is used in the Viete papers and is my new favourite word. I think it was coined here:

Lasaga, A.C. 1983. Geospeedometry: an extension of geothermometry. In: Saxena, S.K. (ed.) Kinetics and Equilibrium in Mineral Reactions. Springer, New York, 81–114.