A bit of Scotland in an English playground

Posted on 29 December, 2012 by Metageologist

There is a park near my home that my children like. As is the way of things, this means I stand around it a lot, ready to rub bruised knees or produce biscuits or push ‘faster!’, but otherwise redundant. My attention often wanders to the big blocks of stone in the park – they are worth looking at.

To start with, here’s some ‘granite’.

The white material is a medium to coarse grained igneous rock of ‘felsic’ composition – granite (loosely speaking). The dark area is a portion of material within the granite. It may be a xenolith, a piece of rock that fell into the magma, but it looks to me like diorite, possibly the result of magma mingling.

There are few blocks of mafic igneous rock:

This ‘gabbro’ above shows both fresh and weathered surfaces. Plagioclase feldspar is colourless and weathers white while the dark minerals (pyroxenes?) sit within it. Note the rusty iron patch at the top.

Another block of gabbro has a slight sniff of layering to it.

Mafic magma is molten from about 1200°C, whereas more normal continental rocks (sediments say) can melt from 700°C. Put the two together, therefore and you expect some melting, producing migmatites.

This block is of high-grade metamorphic rock, with a gneissic foliation. A thin granite vein cuts through and has itself been folded.

Here’s some more metamorphic rock, with a folded foliation and a mica sheen.

Our final type of rock is sedimentary, a conglomerate.

Notice the variety of clast types. We’ve some red sediments, some ‘granite’, vein quartz, quartzite and more. Here’s a closer look.

I’ve no idea where these blocks came from, but I know it’s not nearby. They are part of the park landscaping and so were brought from somewhere else in Britain, on a lorry. The blocks are rounded and weathered, so they are not blasted from a quarry. I think they are glacial blocks. Assuming they came from the same glacial deposit I suggest they are from North East Scotland. There is a series of syn-orogenic mafic intrusions in this corner of the world that sit within the high-grade parts of the Buchan and Barrow metamorphic areas. Granites are two-a-penny in Scotland and the conglomerate looks like the post-orogenic ‘Old Red Sandstone’.

These rocks are very similar to those in my PhD field area, so to have them turn up close to home is rather splendid.

All photos by me. I was hoping for a sunny day to take them, but I’ve given up waiting. The photos give you an authentically gloomy and dark view of rocks from Scotland, at least.

Cratons – old and strong

Posted on 19 December, 2012 by Metageologist

Cratons are pieces of continents that have been stable for a over a billion years. As earth’s plates drift along, mountains periodically rise and fall, plate boundaries appear and disappear. But cratons are like great-grandmothers at family gatherings, while younger crust moves excitedly around them, they sit quietly, occasionally remarking on how different things were when they were young.

Every continent has cratonic areas, notably the core of North America, Scandinavia, Siberia, India and most of Australia. They may be covered by a thin shawl of sediment, but often they expose ‘basement’ rocks such as gneiss. Economically they are very important – most of the world’s diamonds come from cratonic areas as do many other valuable deposits.

Cratons are stable because they are strong. The geology of the Himalayas illustrates this – the modern day plate boundary between Indian and Asia is at the southern edge of the Himalayas. The cratonic Indian plate is barely deformed, in great contrast to the vast pile of deformed soft young crust in the Tibetan Plateau to the north.

Cratonic crust is strong, being unusually cold and dry, but that is only part of the picture. Continental crust is the upper portion of continental lithosphere. It’s lithosphere that puts the plates into plate tectonics, it’s a rigid layer on the earth’s surface, as opposed to the hot flowing mantle that lies beneath (the asthenosphere). Lithosphere consists of the crust and an underlying layer of mantle that has become ‘stuck on’. This layer of lithospheric mantle can be 2 to 3 times thicker than the crust above and so contributes a great deal to the strength and stability of continental lithosphere.

It turns out that the lithospheric mantle beneath cratons is unusual. In oceanic crust, as it ages it cools and eventually it sinks down into the mantle again. It is thought that continental lithospheric mantle can fall off as well, but in cratons this doesn’t happen. Finding out why is an interesting challenge but before we start that journey, a brief interlude….

Geochemical interlude

My feelings about geochemistry have changed over time. As a doctoral student I was exiled out into an annex 10 minutes from the main department because a new piece of geochemical apparatus required a lab to be expanded, swallowing our little rock-filled office. I could relate to metamorphic petrology and analysis of mineral chemistry, but the isotope stuff, seemingly requiring months of juggling Hydrofluoric acid to get a couple of data points, left me cold. Years later, with a broader, wiser perspective I find myself marvelling at the way isotope geochemistry leads to so much awesome science.

There are two flavours of isotope geochemistry, stable and radiogenic. Many chemical isotopes are unstable and over time spontaneously turn into different elements, the bits left over being thrown out as matter and energy – radiation. For geologists, this real-life alchemy, known as radioactive decay, is mostly used to tell how old something is. The rate of radioactive decay is constant, so measuring the current abundance of isotopes and working backwards tells you the time elapsed since an event happened. Getting from measurements to age involves assumptions about the sample which may be incorrect – my own research successfully predicted a published age was incorrect. Nevertheless modern geochemists have a wide range of extremely robust and accurate techniques at their disposal. Datable events include a crystal forming from a magma or growing in a metamorphic rock, a surface being exposed to the atmosphere, a critter growing, a mineral cooling below a particular temperature, when magma was extracted from melting rock, even the age of ground water.

Stable isotope geochemistry relies on the fact that two isotopes of the same element, for example Hydrogen and Deuterium, are not precisely the same. Most physical and chemical processes treat them identically, but some do not, the slight different in mass means slight differences between isotopes. For example during evaporation of water the lighter H₂¹⁶O will vaporise first, creating a difference in isotopic composition between the water vapour and the remaining liquid. This phenomena of isotopic fractionation affects water in other ways, such that there is a relationship between fossil oxygen isotopes (in ice or fossil sea shells) and the temperature of the sea they came from. This tremendously useful technique is just one example of the seemingly magical way tiny differences in the physics of atoms can shed light on diverse geological problems. There are many other applications, but the technique shines most when we have very little other evidence to go on. Research into the formation of the earth and moon relies heavily on studies of arcane isotopes. We have only limited information about lithospheric mantle from beneath cratons (remember them?) and isotope geochemistry tells us many interesting things…

Back to the cratons

A recent review by Cin-Ty A. Lee, Peter Lufﬁ, and Emily J. Chin of Rice University focuses on the creation and destruction of continental mantle (I’ll mostly discuss creation).

If lithospheric mantle beneath cratons was the same composition as the rest of the mantle then over time it would cool and become denser and unstable. A consensus has emerged that it does not because at some point in the past it was involved in melting and up to half of its volume was removed. The rock left behind after the melt flowed away is known as depleted peridotite and it is stronger and more buoyant than normal mantle, so it remains stable for billions of years.

Fragments of mantle peridotite in lava.

Pieces of continental lithosphere do reach the surface, sometimes as huge slabs such as at Ronda in Spain. Some weird volcanic deposits start at mantle depths and bring up pieces of the surrounding rocks. Sometimes they contain diamonds, but also pieces of the lithospheric mantle. Armed with these samples, geochemists have applied a massive array of techniques.

The geochemistry of melting rock is well understood and it allows us to infer many things about the conditions under which it happens. For example comparing ancient with modern samples, we can tell that older samples melted at higher temperatures (1,500 to 1,700◦C 1,300 to 1,500◦C) and saw more melt extraction (50 to 30% versus <30%). This is consistent with the fact that the early earth was hotter.

Melting is affected by pressure as well as temperature – deeper mantle rocks contain different minerals (at these pressures, garnet at depth, spinel above) which melt in subtly different ways. Cratonic peridotites melted at shallow depths (90km) but were later moved deeper (180km).

Not all samples are of peridotite, next common are pyroxenites, sometimes referred to as eclogites. Chemically these are similar to basalts formed at modern mid-ocean ridges. Their oxygen isotopes show wide variation, such as is seen in oceanic crust following hydrothermal alteration. Other physico-chemical processes could in theory cause similar patterns, but it seems only low-temperature processes explain the patterns seen. The fact that they were once part of oceanic crust makes these pyroxenite rocks very different from the peridotites (which have only ever been part of the mantle).

Further evidence that pyroxenites were once near the surface comes from sulphur isotopes from eclogitic inclusions within diamonds. Fractionation of these isotopes occurs in ways only explained by chemical reactions high in the atmosphere. These diamonds with eclogitic inclusions themselves have carbon isotopes that are extremely ‘light’. Light carbon is a sign of life – organisms preferentially contain light carbon. In these ancient rocks, life probably means bacteria. So we can lengthen the list of “lovely things bacteria help to make” – cheese, soy sauce, wine, yogurt, healthy digestion and now certain types of diamonds.

NB for visitors from the German wikipedia page on Diamonds, I’m delighted you’re here. Here is a better reference for the science on this.

This is a wonderful thing. A bacterial mat sitting on the sea-floor billions of years ago ends up being stuffed down into a subduction zone. It enters a world of crushing pressure and extreme temperature where it is transformed utterly, its carbon forming a diamond. Later a weird eruption happens and the diamond is squirted back up to the surface for us to marvel at. Only isotope geochemistry (and some generous assumptions on my part) allows us to tell this story.

How does continental mantle form?

Our authors discuss how continental mantle forms (spoiler: we don’t really know). One model is that a large plume of hot mantle material rises up underneath a continent. This would indeed form lots of depleted peridotite (underneath a big pile of lava). However this model is inconsistent with the evidence described above – melting would be expected at greater depths (200km) than is seen.

A plume model also fails to explain the eclogitic parts of the mantle. One mechanism to explain how portions of oceanic lithosphere end up in the sub-continental mantle is that when subducted instead dropping steeply down they remain buoyant and stack up beneath the continents. In this model, the sub-continental lithosphere grows by progressive capturing and stacking of oceanic lithosphere. Its been argued that parts of the Farallon plate were captured beneath western north America in recent times. Hotter oceanic lithosphere (common in the Archean) subducts at a shallower angle and so is more likely to end up getting stuck to the base on the contintent.

This model explains the evidence for shallow melting and subsequent burial, plus the presence of former oceanic lithosphere. Indeed it predicts a larger proportion of eclogitic pyroxenite than is observed. Our authors propose a process of “viscous drainage” whereby the crustal portion of the stacked ex-oceanic lithosphere (now dense inclined sheets of garnet pyroxenite) slowly ‘drains’ down and out, leaving the peridotite ‘framework’ behind.

Models of the creation of continental crust emphasise the importance of island arcs. An oceanic island arc (where oceanic crust, subducts under oceanic) may collide with a small continent, turn into a continental island arc and ultimately become a new piece of continental crust. This mechanism will not give thick lithosphere on its own however – so some process of ‘orogenic thickening’ (squeezing it so it’s thicker) is required.

You will perhaps have sensed that the paper drifts into speculation a little. This is appropriate in a review paper and inevitable as we are on the ragged edge of what we can know about these rocks, so distant in space and time. The paper ends with a ‘future directions’ section full of as yet unanswered questions.

“How do continents form?” is a simple question to ask but we still can’t give a complete answer to it. We still don’t know the secret to great-grandma’s longevity.

Peridotite picture from dun_deagh on Flickr under Creative Commons.

Craton map from Pearson and Wittig under Geological Society of London fair use policy.

PEARSON, D., & WITTIG, N. (2008). Formation of Archaean continental lithosphere and its diamonds: the root of the problem Journal of the Geological Society, 165 (5), 895-914 DOI: 10.1144/0016-76492008-003
Lee, C., Luffi, P., & Chin, E. (2011). Building and Destroying Continental Mantle Annual Review of Earth and Planetary Sciences, 39 (1), 59-90 DOI: 10.1146/annurev-earth-040610-133505

Erosion makes mountains beautiful

Posted on 9 December, 2012 by Metageologist

The thing that makes mountains so beautiful and fascinating,is not so much their height as their steepness. Climbers and trekkers flock to the High Himalaya, not to get altitude sickness but for the grandeur of the landscape, the experience of seeing views that require you to lift your head up. Mountains are created by deep-seated geological processes that raise the surface of the earth, but it is erosion that creates the scenery we love.

Erosion is often taught in terms of gentle everyday processes – tiny fragments of rock falling off and slowing washing downstream. Over the awesome expanse of deep time such tiny events can indeed wash away mountains. But mountainous areas can rise at rates of centimetres a year – is small-scale mechanical weathering really quick enough to keep up with such rapid uplift? Erosion in mountains also involves faster acting processes: ravine-making rivers, grinding glaciers and lots of landslides.

If you’ve ever had the pleasure of visiting high mountains such as the Himalaya, you’ll have noticed how common landslides are. A recent scientific paper seeks to quantify how their frequency relates to other factors.

Larsen & Montgomery from the University of Washington, studied the Namche Barwa area in the eastern Himalayas. This is an area where rocks are being brought to the surface extremely quickly by high rates of erosion. The mighty Tsangpo-Brahmaputra river flows through the area and its influence may extend deep into the earth, creating a tectonic aneurysm.

A river only directly erodes within a tiny area, but has a wider influence. Our authors start by describing the threshold hillslope paradigm. This is a concept three year old children understand: if you dig a hole in a sand pit, once the edge reaches a particular steepness, sand slides back into your hole, no matter how fast you dig. Within a mountainous area, if erosion rates are high enough, hill slopes are limited by the material strength of the rock.

“Vertical river incision into bedrock is thought to over-steepen hillslopes with gradients near the threshold angle, increasing relief until gravitational stress exceeds material strength and bedrock landsliding occurs.”

Somewhere like Namche Barwa, with high uplift and erosion rates, we would expect to see lots of landslides. To test this, they used an array of remote sensing data, including declassified spy satellite images, to map more than 1500 landslides over a period of 33 years. By looking at an area affected by differing erosion rates, that found a rough correlation between landslide erosion rate, stream erosion power and even cooling ages of metamorphic rocks. The study area showed an order of magnitude difference in exhumation rate which corresponded with a tiny 3 degree difference in average slope angle.

As well as providing evidence for the importance of landslides, they found insights into the processes. They see a link between large landslide dam bursts (flooding events), and landslides – flooding causes more erosion at foot of slope, destabilises the valley sides and causes a series of small landslides.

Another recent paper by James Spotila at Virginia Tech also derives useful data simply from looking at mountains in detail. He made a detailed analysis of topographical data from mountains across the world.

Browse photo galleries taken in mountains and you’ll find most pictures are of the peaks and mountain ridges – that is where the beauty lies. In contrast models of mountain erosion are (conceptually) peering into the valleys. Ridges and peaks are seen as the consequence of valley-shaping processes, in map view merely the negative image of drainage networks.

Spotila hypothesises that the highest peaks will form where two ridges meet, at ‘divide-junctions’. High peaks are often broadly shaped like pyramids, forming where three glacial or river valleys meet (and therefore where two ridges meet). Peaks of this shape may be mechnically more stable than on single ridges. Using digital topography to study 255 of the world’s most prominent peaks, Spotila finds that 91% of prominent peaks studied are found at divide junctions. Of these, all are pyramid shaped.

The highest mountains of central Nepal contain most of the world’s highest peaks. The Himalayas are cut by a number of large rivers. The high peaks are preferentially located close to the divides between these rivers – the drainage pattern controls the location of the highest peaks.

Once mountains become pyramidal and have flat triangular faces, they become more resistant to erosion. In the Himalaya the flat faces tend not to be covered by glaciers, for example. This may make them more stable over time. Some workers have talked of ‘teflon peaks’ that rise above the glacial buzzsaw. Recent models of landscape evolution have emphasised the importance of the migration of drainage divides over time. However if drainage-divide peaks are stable over time, they will anchor the overall drainage patterns, making them much more stable.

These details of the way mountains erode may seem detached from tectonic studies of mountains, but in fact so many aspects of mountain geology are beautifully intertwined. Erosion of mountains influences processes deep within the crust, allowing material to flow towards the surface – patterns of crustal flow are linked to drainage patterns via erosion. That these patterns of crustal flow become fixed in space due to the way peaks and ridges form is a beautiful idea about beautiful places.

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First photo of landslide area, Annapurna region of Nepal, courtesty of Julien Lagarde on Flickr under Creative Commons

Second photo of Matterhorn, Swiss Alps, courtesy of Martin Jansen on Flickr under Creative Commons

Larsen, I., & Montgomery, D. (2012). Landslide erosion coupled to tectonics and river incision Nature Geoscience, 5 (7), 468-473 DOI: 10.1038/NGEO1479

Roering, J. (2012). Tectonic geomorphology: Landslides limit mountain relief Nature Geoscience, 5 (7), 446-447 DOI: 10.1038/ngeo1511

Spotila, J. (2012). Influence of drainage divide structure on the distribution of mountain peaks Geology, 40 (9), 855-858 DOI: 10.1130/G33338.1

Seasonal flow of geological learning

Posted on 2 December, 2012 by Metageologist

‘Big data’ is the idea that the Internet provides organisations with an unprecedented amount of data that deserves new forms of analysis. The more general idea that sophisticated analysis of big data sets is important is quite topical, just ask Nate Silver.

Google have always been good at this sort of thing. They make (some) of their internal ‘big data’ publicly available for all of us to have a geeky play with. One example is Google Trends which shows (normalised) data for the volume of searching on particular search terms. They also split data geographically.

I’ve been playing around and there are some interesting patterns that suggest to me there is a seasonal flow of geological learning. Take a look at this.

[trend w=”600″ h=”400″ q=”Subduction” geo=””]

There is a clear seasonal pattern there. Searches are lowest in the Summer (July/August) and show a clear dip in December. The pattern holds for plate tectonics and other terms too.

[trend w=”600″ h=”400″ q=”continental+crust, oceanic+crust” geo=””]

It seems pretty obvious these correspond to term time in schools and colleges and that most searching for geological terms is done by students. Further evidence comes from Google Correlate, which shows searches terms that are correlated in time. Subduction searches in France and the US correlate with other terms that students with geological assignments are likely to type in.

Some geological words have other meanings of course, look at this.

[trend w=”600″ h=”400″ q=”quartzite” geo=””]

The peak in January confused me, until I looked at the related terms and found out about the large mineral fair in the town of Quartzite, Arizona in January every year. There is a peak for searches for ‘Mars’ in March which is related to that being the French word for March.

My examples are fairly trivial perhaps, but what staggers me is the potential of this sort of data. Another post I’m writing is about recent scientific research based on publicly available ‘big data’ (remote sensing and altitude data). There must be proper scholarship to be done using this Google Trends data, but for me its just an interesting diversion. I leave you with a puzzle, why is the seasonality of ‘photosynthesis’ searches so distinctive? What happens in October and November? Do thousands of biology teachers all start the academic year with the basics of plant biology?

[trend w=”600″ h=”400″ q=”photosynthesis” geo=””]