Hurled from the sky

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Since ancient times in China, special glass stones have been recognised. Called Lei-gong-mo, or “Ink sticks hurled from the sky by the thunder god” these are now known as tektites. These remarkable stones are found on the surface in a small number of areas (called strewn fields) around the world.

Two splash-form tektites (from http://en.wikipedia.org/wiki/File:Two_tektites.JPG)

Two splash-form tektites (from http://en.wikipedia.org/wiki/File:Two_tektites.JPG)

There are various types of tektite such as splash-form, ablated and layered. They are all lumps of extremely dry glass, often with distinctive shapes.

There is a trade in tektites and you can buy them on eBay. I’m tempted, but I gather fraud is a problem and I can never get over a feeling that they look like cat-poo.

How are they formed?

Tektites have been recognised for thousands of years. Geologist Charles Darwin* thought they were volcanic in origin, not unreasonable since volcanoes produce lots of glass. By the mid-Twentieth Century they were thought by many to be blebs of glass that had fallen through the earth’s atmosphere. Ablated tektites in particular suggest this. Look at the picture below- the central top area is the top of an originally spherical blob. The base was then melted again by friction as it fell through the earth’s atmosphere, the material being pushed up and around to form the distinctive shape. One idea was that the material originally came from the moon before falling to earth, but this was ruled out once actual lunar samples were collected: the geochemistry just didn’t match.

Aerodynamically shaped / ablated tektite (http://en.wikipedia.org/wiki/File:Australite_back_obl.jpg)

Tektites are now seen as resulting from large impacts on the earth’s surface that throw molten rock into space where it falls back, covering large areas. They are found in distinctive areas, strewn fields, nearly all of which have been associated with an impact crater. Within a strewn field, the age of the glass and the impact match, plus there is a size distribution, with large chunks near the impact and smaller tektites further away. Micro-tektites, also known as spherules (a purely descriptive name) may be found enormous distances away. Layers of micro-tektites are now being found in sedimentary rocks, particularly in the Archean (when impacts were much more common).

A recent paper by Kieren Howard in the Proceedings of the Geologists’ Association (doi:10.1016/j.pgeola.2010.11.006) provides a nice overview of modern thinking on tektites, drawing on numerical modelling of impacts. Melting of rocks within an impact crater is no surprise, the energies, and therefore temperatures and pressures produced by the impact are extremely high. What is less obvious is that melting happens in different ways. As the shock wave from the impact passes through the ground it can produce large volumes of melt. Most of this is mixed up with the fractured country rock to form melt breccias (pseudotachylites) and some is mixed up with rock fragments and thrown out to form a layer near to the crater (suevites). The rocks closest to the impact reach temperatures >5000°C, producing a mixed melt+vapour phase. This is ejected in high velocity jets travelling at ‘cosmic velocities’ (>11 km/s) that push away our planet’s puny atmosphere in a process called atmospheric blow-out. This means that small volumes of melt can effectively fly into space and then fall to earth – tektites!

Tektites are very very dry (< 0.05 wt % water) which is attributed to a process called “bubble-stripping”, where the water is lost as vapour under decompression. The dryness therefore is not related to the conditions under which the melt formed. Indeed Kieren Howard’s main argument is that tektites are especially common in impacts where the surface layers are particularly moist. He believes that the presence of water near the surface means that large volumes of especially high velocity melt can be produced.

Case of the missing crater

The biggest strewn field on the planet is the Australasian. This stretches from SE Asia through to Antarctica and is robustly dated at 0.803 Ma. The only problem is nobody knows where the crater is! An impact spread 10 million tons of glass across a huge area, but the only trace found to date is the tektites themselves. The usual pattern of size distribution in the strewn field points to SE Asia – there are lots of large (>1kg) tektites in Thailand, Cambodia and Vietnam but the expected big circular hole in the ground is not to be found. It is likely therefore that the impact was offshore and the crater has since been buried by more recent sediments. Another possibility is that the impact was oblique, at a shallow angle, and that there is a structure to be found that isn’t a classic circular crater. As you can imagine, the hunt is on, and Kieren Howard is part of it. Once he, or someone else, finds the source of these tektites, expect to hear about it. This is surely a story to capture the imagination of non-specialists (like journalists).

Drifting slightly off-topic, a related paper in Geology in 2008 made me the most jealous I’ve been for a long time. It documents finding micro-tektites in Antarctica, extending the Australasian strewn field even further (>400km). The authors sampled isolated mountains, surrounded by the Antarctic ice-sheet (nunataks). Basically they picked-up whatever sand was lying on the surface or tucked into joints and found little beads of glass – micro-tektites – in it. These landed 800,000 years ago and yet are just lying there waiting to found. Tektites in other areas, Australia say, have been buried and then uncovered, pitted by weathering and such-like. But in Antarctica nothing has happened for nearly a million years (or longer). Erosion is low because of very low precipitation and there are no other sources of sediment to bury the micro-tektites. They fell out of the sky and then nothing happened, until some Geologists came by. Other studies, using cosmogenic nuclide dating have shown that some rock surfaces in Antarctica are 2 or 3 million years old. This astonishes me every time I think about it, places like this just don’t feel like they belong on earth. Just to make that clear, the surface is 3 million years old, not the rock itself but the surface you can put your hand against. What’s more, scientists going to such places are the first human beings ever to go there (unless other scientists have been there before). I’m used to places that were scraped clean by glaciers 12,000 years ago, and continuously inhabited ever since. Visiting these other-worldly places on the very bottom on the world must be a very special experience indeed.

References

“Tektites in the Geological Record, Joe McCall, Geological Society of London, 2001” is a little old now but contains lots of details on tektites around the world, including all the detailed evidence supporting the impact origin of tektites.

http://tektites.co.uk contains lots of information and lovely pictures and is the best place to go next if you want more tektite fun.

Notes

* I know, I know, he is rather better known for his role in biology, but his training was as much geological as biological. Also the concept of Deep Time was vital to his development of his theory of natural selection. Without a knowledge of the geological evidence that the earth is rather older than 6,000 years, Darwin’s insight that species evolve due to small incremental changes doesn’t really make sense. Plus he was the first to explain the formation of coral atolls.

Sediments and shiny shoes

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I’ve come across a most remarkable field area. I think I’m probably the first Geologist to study it. The samples are an extreme case of ‘float’ – they are very detached from bedrock. Exposure is excellent. Samples are nicely polished and form neat pavements, but blocks are never large and randomly arranged.

Field work is a little odd. In order to get access to the samples I have to wear some very unusual field-gear – shiny shoes, suit and tie. The samples are concentrated in special areas – the names used by the local people are ‘the lobby’ and ‘the gents’. I’ve heard that samples are also to be found in ‘the ladies’ but I am not allowed in this area due to local taboo. This is a frustration, as is the fact that taking samples is not allowed. Photography is fraught with hazard also, as I am doing fieldwork ‘undercover’. The locals are unlikely to understand why I am photographing the floor. Photography in ‘the gents’, while not explicitly taboo, is highly unorthodox. My smart phone is my friend.

I’ve battled with these hazardous conditions and returned with some photos of rather nice rocks, just for you.

Here is a typical sample:

The field of view is about 50cm top to bottom. The rock is a poorly sorted sandstone. Clasts are irregular in size and volcanic in origin, some are intraclasts, samples of the sediment reworked as big pebbles. Note the minor faults.

Here’s a look at an unbedded sample with large clasts, ‘shiny’ shoe for scale (for those of you who like precision, I take size 9 shoes which is 10 in US size, 44 European).

A more typical sample has lots of sedimentary features and shinier shoes.

You can see there is lots of fine bedding with large variation in grain size. Note the wavy shape of the lowest bed. Once the bottom gray layer was sitting there quite happily when something came along and scooped out three big bites. Later sediment filled these in until eventually the layers on top are flat again. What did this? A clue is in the big white clast sitting bottom right. In order to move something that big, you need fast moving water. I’m thinking turbidity currents and my guess is that the ‘scoops’ are cross sections through flute marks or other types of gouges that these big flows of sediment-rich water produce.

So I’m visualising somewhere near volcanoes, steep underwater slopes down which big piles of sediment-rich water periodically sweep. What do you often get associated with volcanoes? Earthquakes. Shaking up piles of sediment can do some very interesting things.

First the faulting. You saw some in the first picture and you probably visualised this as something that happened to the sediments long after they were lithified. Maybe. Have a look at this.


I know its not the world’s best photo, any advice on discretely taking photos of shiny surfaces in dark corners gratefully received. The short dimension is 8 centimetres. Here’s an annotated version to make things clearer.

Green lines are sedimentary boundaries. The bottom one is clearly cut by a single fault. The second one is cut by a few minor faults and seems a little folded next to the faults. The top one is unfaulted. Did the faults simply peter out and not reach the top layer? I’m not so sure, look at the layer immediately beneath the top boundary. It is thicker above the downthrown side (the right). It looks to me like this faulting happened while the sediments were forming. The top layer isn’t faulted as it wasn’t there when the faulting happened. I’ve not traced the faults thorough the thick layer, as there the disruption seems a bit more diffuse, which I guess is also consistent with fracturing of unlithified sediments.

This is all on a tiny scale, but a similar process happens on a km scale, where faulting in sedimentary basins can be the main control over sediment thickness.

What happens if you pile coarse sand on top of web mud and shake it? Here’s what:

Firstly, did you assume that this photo is the right way up? Looking at the irregular top boundary I think it is ‘upside down’. I thought of rotating the photo to match the orientation when the sediments were laid down, but that is only my interpretation as I’ve no way of knowing for certain. If you work in structurally exciting areas, the ability to tell a sedimentary rock is upside-down is a useful skill.

Look a the layers about a third of the way down. The coarse sand was sitting on the finer-grained mud. When the sediment was shaken by an earthquake, the heavier sand started to sink as lobes into the mud. Displaced mud moved to fill the space, the earthquake stopped, eventually the rock lithified and froze this rather nice blobby-wobbly structure in place. I believe convolute bedding is a more generally accepted term.

Here’s a more dramatic example.

The rocks are rather green. They may have been metamorphosed slightly, just enough to grown green minerals such as epidote or chlorite, but not enough to recrystallise so much that sedimentary features are lost. A few samples look as if they might have some spaced cleavage in them.

My attitude to sedimentary rocks is usually summed up by the word ‘protolith’, but these are so nice I’m almost converted. I still don’t where these rocks are from originally as the records of the people who placed them here are lost. Any ideas? Also, any other features I should be looking out for?

I’ll end on a mystery. It looks like boudinage, but the folded layers are cut by higher ones which seems to rule out deformation. What is it?

Inside the mind of a singer

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So here I am in the middle of the tenor section singing my heart out.

What am I looking at? Well, there is the music. My eyes are never far from the copy I’m holding and I’m managing to keep them slightly in advance of where the notes I’m currently singing are on the page. This means keeping track of which stave (line) I’m on, ignoring the 5 or more others. Oh, and for goodness sake don’t forget to turn the page over before you need to sing the notes on the *next* page, or you’ll look a right idiot. Extra marks for style if you turn the page ever so slightly ahead of everyone else. I’m also keeping an eye on the conductor, some distance away in front of me. They get sad and lonely if you ignore them, sometimes grumpy with it too. When I’m not singing I might cast a critical eye over the audience. How many of them are there? What’s the average age, are they enjoying themselves?

What am I listening to? My fellow singers, of course. Mostly my own tenor section, often others, for cues. A choir should be more than just a bunch of soloists singing at the same time, we listen to each other to make sure we are singing the same notes, in tune, at the same time. On a good day I listen to be inspired, to ensure the blend is perfect and the phrasing consistent. On a bad day I’m desperately listening to the person behind me who has learnt the music better and who I can get the notes from. Today is a good day. As well as listening to help me sing better, I’m also a part of the audience and I am singing some of the best music ever written. Shivers are never far from my spine, my goose is bumped.

What am I doing? I’m singing, which means I’m vibrating air in my chest, throat and head to produce a sound with a specific pitch. To try and do this well, I’m thinking about my posture, using my diaphragm and other abdominal muscles to support the sound. I’m also trying to keep my soft palate up, the back of my tongue down and avoid tension. This is an all-body experience, from the soles of my feet (firmly planted on the ground) to my forehead (where high notes feel like they come from).

What am I thinking about? I’m translating the notes on the page into the rhythm and pitch of the notes I’m singing. Since I don’t have perfect pitch, I work out the intervals between notes so I can move between them correctly. Plus the words, which today are in German so I must try and sing them like a German would. Also I’ve written things on the copy (in pencil, I’m not a barbarian). The most useful things are signs pointing to bear-traps, like VS for a nasty page turn, or a pair of spectacles where mis-reading the music is easy. Also I’m making a particular type of music which means, at least with this conductor, not singing it like other types of ‘classical’ music but in an ‘authentic’ style. This means a dance-like lilt to it, with a gentle emphasis on the first and third beats.

I’m doing all this thinking and listening and looking at the same time. I am relying on ‘muscle memory’, musical memory, years of hard work at singing and weeks of work rehearsing this particular piece with these particular people. I am very much in the psychological territory of flow. I am totally involved in the performance, immersed in it. It is such a joyful experience, I have never felt more alive. The fugue passages are the best, as a line of music that sounds great alone is started by different sections of the choir at different times. These lines then are piled up on top of each other producing exquisite music. As the fugue starts, I know I’m going to be tested physically and mentally. Once my part comes in, its such an intense feeling, the lines are complex, leave barely any space to breath and incredibly satisfying to sing.  For me to reach of the end of these sections having done justice to this wonderful wonderful music feels like I’ve just achieved something magnificent.

These heights of experience wouldn’t be heights if they lasted. Eventually the piece is over and the audience applauds. This was a good gig and the audience are happy, clapping like mad, and I feel that they’ve shared in some of my joy, our joy. The sweating and beaming conductor certainly has. Gradually I return to reality. My diaphragm hurts, which is good, but so does my throat, which means my technique is not good enough. I barely notice though because of the drugs. Endorphins are natural opiates produced when you do important things, like sing, exercise, fall in love, have/make babies or eat curry. They are pumping through my veins and it feels goood.

Backstage now and full of a room of my fellow musicians, all revved up, excited, talkative, full of joy. It is essential now to drink alcohol. This prolongs the joy. The best is a visit to a pub where you all talk fast and happily about the experience you’ve all just shared. Having a few audience members there is good too, as while we pretend otherwise we are all desperate to be told how good we were.

Eventually bed, sleep and waking up with a smile on my lips, counting the days until I can do this again.

Metamorphism: evolving ideas

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‘Ontogeny recapitulates phylogeny’ is such a lovely phrase. It was coined as a biological concept, and is now somewhat discredited. The idea was that as an organism develops as an embryo it passes through stages of growth corresponding to stages of its evolution as an species. My excuse for typing ‘ontogeny recapitulates phylogeny’ (twice!) is that is also a useful concept for talking about ideas.

Often the best way of introducing a body of knowledge is to work through the history of the study of that subject. Scientific ideas build on what came before – build and modify.
Take the atom. In my mind it is like a solar system. A tiny nucleus, made up of coloured lumps fused together. A long way off even tinier dots of electrons orbiting around. This model is useful, it worked for Rutherford firing alpha particles through gold in Manchester (it’s his model). Add in the concept of multiple orbits and you get valence and can explain patterns in the periodic table.
But it’s 100 years old, too simple and out of date. Quantum mechanics replaces the image of little electrons whizzing around with probability clouds, the nucleus is a liquid drop and there are cats in boxes somewhere…  Umm. Ultimately I’ll never really understand what’s really going on without grappling with the maths. But thank goodness for the Rutherford model, it marks the limits of my physics education. If I’d been taught nuclear physics by starting with the most modern models, chances are I’d have ending up knowing nothing.

What does this have to do with metamorphic rocks? Well, my previous posts about metamorphism, have taken us up to the mid-1990s. I’ve painted a picture of metamorphism where rocks are closed systems, that achieve chemical equilibrium and go on nice smooth paths through pressure temperature space. This model is simple, useful and demonstrably wrong.

So, building on your understanding of this simple old model, let’s see how the science has evolved since, to understand how reality is more complex.

Staying off-kilter

Thermobarometry relates metamorphic conditions to the minerals in a rock using our knowledge of the chemical equilibria affecting silicate minerals. The assumption is that every mineral, every atom in the rock is in chemical equilibria with every other one. Is this realistic? What things can prevent a rock achieving chemical equilibrium, that can stop atoms finding the place they’d most ‘like’ to be?

It turns out there a few, that can be referred to generally as “reaction kinetics” the processes by which chemical reactions occur.

Our first complication is diffusion. Can the atoms physically get to places they would rather be in time? Along grain boundaries, in the presence of a fluid, this is relatively easy, but in dry rocks, atoms have to wriggle their way through crystal lattices, slowly. In my simple (simplistic) head, diffusion is caused by atoms jiggling about randomly, swapping places and moving around. Heat is the amount of jiggling and temperature the measure of this jiggling. We can estimate rates of diffusion in minerals and it is very temperature dependent. It also depends which minerals and atoms you are talking about. Small atoms that take part in solid-solutions (e.g. Fe-Mg) can move around more easily than big atoms that form part of the mineral lattice (e.g. Al).

So, given enough time, hot rocks are likely to reach chemical equilibrium. Colder rocks, especially in the absence of water may not, which helps to explain why peak assemblages are preserved and not replaced as the rock returns to the surface. For more on this check out my post on the Vredefort impact structure. There I drew on a recent PhD thesis all about studying equilibria in metamorphic rocks.

Getting started is hard

Another obstacle to achieving to chemical equilibrium is nucleation. Minerals consist of atoms arranged into regular structures – lattices. Growing a mineral means adding atoms to its edge and is a relatively easy process. Building the initial chunk of  that lattice (the nucleus) is relatively hard, it can take a long time. You are probably familiar with nucleation in igneous rocks. Obsidian is a rock formed by lava that cooled before substantial nucleation and mineral growth could occur.

Does something similar happen in metamorphic rocks? In a situation where e.g. staurolite is stable, reactions that would create it can’t be active if nucleation hasn’t happened.  Evidence for this can be found in a study of rocks from the Bushveld aureole in South Africa (Waters, D.J. and Lovegrove, D.P. (2002). Assessing the extent of disequilibrium and overstepping of prograde metamorphic reactions in metapelites from the Bushveld aureole. Journal of Metamorphic Geology, 20, 135-149.). The Bushveld is a massive mafic igneous complex, the size of Ireland and up to 9 km thick. As you would imagine, it has a massive metamorphic aureole. There isn’t much deformation associated with the intrusion, so wonderful metamorphic textures are preserved. Also we can accurately model the heating associated with it, making it a wonderful natural laboratory. So Dan (“the man”) Lovegrove, assisted by Dave Waters, his doctoral supervisor were able to do things backwards. Starting with a set of known P-T-t paths (different paths at different distances from the intrusion) they then did a thorough thermobarometric study of the metamorphic rock and compared the two sets of results.

They showed a clear difference. The observed and predicted sequences of metamorphic reactions are different, and minerals first occur at higher temperatures than estimated based on chemical equilibria. Their conclusion is that ‘overstepping’ is an important factor. The difficulties of nucleating new phases means that they first occur at higher temperatures than expected, up to 40°C is estimated. The scale and rates of heating around the Bushveld aureole are typical of many orogenic belts (roughly 0.1 to 1°C per thousand years), so these results can be applied generally.

Dan also studied the affect of heating rates on grain size (slower heating = bigger porphyroblasts). More details can be found on Dan’s own website.

So our lovely theoretical model doesn’t apply perfectly to real rocks. No surprise there. Does this mean that this model is wrong? No. On the right rocks, thermobarometry works just fine. Also metamorphic petrologists are starting to get a handle on reaction kinetics as well, meaning that we can extract even more meaningful information from the rocks. Here’s a flavour of that from a real metamorphic petrologist, Dave Waters.