A world without subduction

The greatest achievement of the generation of Earth Scientists now retiring is the concept of plate tectonics. The insight that the earth’s surface is made up of rigid plates that move has shed light on all aspects of Earth Science, from palaeontology to geophysics to the study of ancient climates. What’s less well known is that the way the plates interact has changed over time. Key plate tectonic features such as subduction, didn’t happen for large periods of earth history.

Cut-away diagram showing modern convection from computer modelling by Fabio Crameri. White is hot rising plumes, black cold sinking plates.

Cut-away diagram showing modern convection from computer modelling. White is hot rising plumes, black cold sinking plates .  Image used with permission of Fabio Crameri.

Earth scientists have a pretty good idea of the details of how modern plate tectonics works. This has required the integration of indirect observation of modern subduction zones (using geophysical techniques) with direct study of rocks that have been inside subduction zones (such as eclogites) plus the creation of subduction zones ‘in silico’ (with computer modelling).

Of these 3 methods of study, only the first (direct observation) cannot be used on the ancient earth. So what do the rocks and computer models tell us?

Old rocks are odd

We’ve known for a while that ancient rocks (Eoarchean–Mesoarchean, older than 2.5 Ga1are very different from modern ones. Often they consist of greenstone belts – containing an unusual lava called komatiite – surrounded by large areas of granitic gneiss. The pattern of metamorphism in these rocks shows high temperatures, even at shallow depths.

The chemistry of the igneous rocks tells a similar tale. Komatiites only melt at temperatures of around 1600°C – 400 degrees hotter than modern basalt lava. Granitic rocks have tonalite–trondhjemite–granodiorite compositions and are thought to have formed from direct melting of basaltic rock – unlike granites formed above subduction zones today.

Rocks characteristic of modern subduction – blueschists and eclogites  – are not found in rocks this age. There is a pretty good consensus, based on field evidence and model modelling, that subduction did not happen in the early earth. The earth’s mantle was much hotter and more heat was flowing up through the crust. Hot rocks are weak rocks – forcing a slab of rock into the deep mantle requires it to be cold and hard. Hotter rocks act not as rigid slabs but as soft blobs.

Computer modelling confirms the importance of temperature, both of the crust and the underlying mantle. Models are our best hope of understanding what a hot planet without subduction looked like. More like a bubbling pan of porridge perhaps, with tectonics dominated by hot upwelling plumes and lithospheric delamination, with blobs dripping-off down again. Some studies of mantle mixing suggest a ‘stagnant-lid’ model where the earth’s surface layer doesn’t move at all.

Subduction starts

At some point in time between 3.2–2.5 Ga, subduction started. The planet had cooled enough that a lithospheric plate stayed rigid enough to sink down into the mantle. Evidence for this is found in ‘paired metamorphic belts’. Rocks within the subduction zone remain cool at depth (as they are pushed down before they can get as hot as the surrounding rocks) and form eclogites or high-pressure granulite rocks. Rocks nearby in the overriding plate are much hotter and enjoyed granulite–ultrahigh temperature metamorphism.

Mathematical modelling of the earth suggests subduction started because the earth cooled below a particular threshold. As an explanation, this is a little dull. Much more excitingly, coverage of a recent paper suggests massive meteorite impacts about 3.2 Ga could have broken up the surface and somehow kickstarted plate tectonics. Scientists who study impacts are always really keen to use them to explain events or features on earth, whereas other scientists are sceptical, preferring to explain them via things that they study. We’ll need to wait to see who is right about this one (but my money is on the dull explanation).

Cut-away diagram showing modern convection from computer modelling by Fabio Crameri. Red is rising plumes, blue sinking plates.

Cut-away diagram showing modern convection from computer modelling. Red is  hot rising plumes, blue cold sinking plates. Image used with permission of Fabio Crameri.

Subduction as a cure for boredom

When subduction first started, mantle temperatures were still 175–250 °C hotter than today. Hotter, softer slabs are more likely to break off, perhaps making subduction something that stopped and started.

Blueschists and low-temperature eclogites, high-pressure & low-temperature rocks that are found in modern subduction zones are not found until the the Neoproterozoic at 600–800 Ma. Mantle temperatures by then were less than 100 °C greater than today – this marks the wide spread development of modern-style (cold) subduction on Earth. Cold slabs of oceanic lithosphere break-off deep, allowing large volumes of dense oceanic crust to pull continental lithosphere down, creating the first ultra-high pressure metamorphic complexes.

The Neoproterozoic is the end of what is known as the ‘boring billion’ – a time of tedious environmental and evolutionary stability. A recent open acess paper in Geology suggests a link between the exciting changes that followed (glaciations! Cambrian explosion!) and the onset of subduction. The boring billion was stable in part because most continental crust was part of a supercontinent called Rodinia. The paper argues that the disruptive effects of the onset of cold subduction broke Rodinia apart, setting off a chain of events that transformed the world.

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The early earth was a very different planet. Understanding it better informs the general subject of planetology. As we get more and more data about other planets (both within and beyond our solar system) it’s natural to speculate on their tectonic activity. Why does Venus not have subduction? Does subduction here exist because of life and its role in moderating climate and creating the earth’s oceans? Ancient rocks and computer models may help us answer these questions as much as probes and telescopes.

REFERENCES

Brown M. (2014). The contribution of metamorphic petrology to understanding lithosphere evolution and geodynamics, Geoscience Frontiers, DOI:
Available here

Cawood P.A. & Hawkesworth C.J. Earth’s middle age, Geology, DOI:
Available here

Gerya T. (2014). Precambrian geodynamics: Concepts and models, Gondwana Research, 25 (2) 442-463. DOI:
Available here

Categories: eclogites, impacts, metamorphism, subduction, tectonics

#thinsectionThursday – what Twitter was made for

One of the great privileges of studying geology at university is spending time looking at thin sections. It may not feel like it at the time – learning to identify minerals down the microscope is hard work – but peering into the secrets of the earth is deeply satisfying, both intellectually and aesthetically.

For those of us who don’t have access to the kit (thin slices of rock, specialised microscopes) we have to make do with photographs. So I was very pleased to discover that rock-whisperer Chris Jennings (@chrsphr) has invented the Twitter hashtag #thinsectionThursday.

Twitter is a great place to share images, and Earth Scientists have long made use of this, posting pictures of long-dead animals (#fossilFriday) and dangerous piles of clinker (#volcanoMonday) on particular days. The potential of #thinsectionThursday is enormous. Thinsection images are often visually stunning – with varied colours and textures – plus the educational potential is vast. They can show crystals that grew in the heart of a volcano, a detailed cross-section of a fossil or the jumbled joyful chaos of a metamorphic rock. Archaeologists, meteorologists and others can play too.

I’ll be contributing to #thinsectionThursday, please do join me. If you use thin sections day to day, it’s a no-brainer: take pictures from your course-work or research and get tweeting. If you’re not on Twitter, you can still view the images. If you don’t have your own photos, there are other options….

If you use other peoples images on Twitter, you’re benefiting from their hard work, so it is very important to use proper attribution. Lot’s of famous accounts don’t bother, but you are better than them, aren’t you?

One resource I’ve made use of is the British Geological Survey. They have digitised thousands of thin section images as part of their GeoScenic image database *and* within their rock collections (search for S% in the registration number field). They are available for non-commerical use, provided you say it’s their image and provide a link back. It’s always good manners to ask, of course, but I’ve done this on your behalf:

So, what are you waiting for?

Thursday, obviously.

And the opportunity to make #thinsectionThursday the success it deserves to be.

Categories: Twitter

The Himalaya: mountains made from mountains

Good building stones get reused. Sometimes the only traces of very old buildings are their stones, built into more modern ones. It’s the same with rocks and mountain belts. Stone that now forms parts of the Himalaya was once part of a now-vanished mountain range.

The Himalaya were formed by the collision between the Indian and Asian plates. For 50 million years, the Indian plate has been pushed down into the Himalayas where it is squashed, mangled and changed by heat and pressure. Working out the details of this process of mountain building has taken decades of careful study. Modern isotopic techniques are now so powerful that researchers studying Himalayan rocks can peer through beyond the effects of the recent mountain building to see traces of older events.

A recent open access paper by Catherine Mottram, Tom Argles and others looks at rocks in the Sikkim Himalaya, around the Main Central Thrust (MCT). As you can guess from the name (and the Use Of Capitals) this is an important structure; it can be traced over 1000km across the Himalaya and separates two distinct packages of rock known as the Lesser and Greater Himalayan Series.

Figure 2c. Cross section of MCT in the Sikkim Himalaya

Figure 2c. Cross section of MCT in the Sikkim Himalaya

As the rocks of the Indian plate were stuffed into the moutain belt, much of the movement of rock was along near-flat faults, known as thrusts. These stack up layers of rock, shortening and thickening the crust. Thrusts near the surface may be a single fault plane, but at greater depths rocks flow rather than snap and a thick thrust zone of deformed rocks is formed. This makes drawing a line on a map and calling it the Main Central Thrust rather difficult. Should the line be placed where the rock types change, or where they are most deformed, or where there is a break in metamorphism? Each approach has its advocates.

Our authors took an isotopic approach, measuring Neodymium isotopes for the whole rock and Uranium-Lead in useful crystals called Zircon. Their analysis shows that the two packages of rock separated by the MCT can be distinguished using isotopes. The actual boundary is not sharp: they prove interlayering of the two rock packages within the thrust zone, rather than a single boundary. This is not surprising given that thrusting is a gradual process and thrust surfaces are not flat.  Deformation seems to have started at the boundary between the Lesser and Greater Himalaya and gradually moved down over time.

The patterns of isotope measurements that can be used to distinguish between the Greater and Lesser Himalayan Series also tell us about what happened before India met Asia.

The zircons whose isotopes were measured are of two types, detrital and igneous. The first are grains that were eroded from old rocks and settled into a sedimentary basin. The second crystallised from molten rock: their ages record significant events. Together these sets of dates give a view of a long and complicated pre-Himalayan history.

Our authors attempt to reconstruct the leading edge of the Indian plate, as it might have looked before it crashed into Asia.

Figure 10.

Figure 10. “Schematic illustration showing the pre-Himalayan architecture of the Sikkim rocks, during the mid-Palaeozoic. The Lesser Himalayan Sequence lithologies were once separated from the Greater Himalayan Sequence rocks by a Neoproterozoic rift. The Bhimpedian orogeny was responsible for closing the rift and thickened the Greater Himalayan Sequence, causing metamorphism and intrusion of granites. The failed closed rift may represent a weak structure later exploited by the Main Central Thrust. Lithologies are the same as in the legend in Figures 1 and 2.”

The Greater Himalayan Sequence had already been heated and deformed in the roots of a mountain belt long before the Himalayas existed. This a relatively common situation. Polyorogenic rocks such as these1 need to be treated with care, otherwise we might mix up events separated by millions of years. A single garnet crystal may contain different areas that formed in totally separate mountain building events

One of the detrital zircon grains dated in this study was 3,600,000,000 years old. We can only guess how many cycles of erosion and burial, how many splittings and couplings of continents this mineral has ‘seen’. As it was buried and heated once again maybe, like the bowl of petunias in The Hitchhiker’s Guide to the Galaxy it thought to itself: “Oh no, not again”.

References

Mottram C.M., Argles T.W., Harris N.B.W., Parrish R.R., Horstwood M.S.A., Warren C.J. & Gupta S. (2014). Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya, Journal of the Geological Society, 171 (2) 255-268. DOI:

Argles T.W., Prince C.I., Foster G.L. & Vance D. (1999). New garnets for old? Cautionary tales from young mountain belts, Earth and Planetary Science Letters, 172 (3-4) 301-309. DOI:

Categories: geochemistry, Himalaya, metamorphism, mountains, open access, tectonics

BRITICE-CHRONO: death of an ice sheet

Using many different techniques, dozens of scientists are studying the death of an ice sheet that once covered Britain and Ireland. They want to understand the future fate of modern-day ice.

The phrase “ice sheet” doesn’t do justice to our subject: this is not something you shatter when stepping on a frozen puddle. Covering over a million square kilometres, this sheet is also kilometres thick. As it grew it pulled enough water out of the world’s oceans to lower them by metres, affecting tropical coastlines as well as the land entombed beneath the ice. The vast bulk even pushed down the crust beneath, slowly moving the underlying mantle aside.

Melt pond on icesheet. Photo by Leif Taurer used under Creative Commons.

Melt pond on ice sheet. Photo by Leif Taurer used under Creative Commons.

The ice is constantly in motion. Snow falling on the ice sheet will eventually make its way to the sea, slowly flowing down and along.  Most is channelled into fast moving ice-streams.  This ice sheet is ‘marine-influenced‘, it sits partly on land, partly on the sea – most of its ice will end its days as an iceberg. The edges of the sheet can become undercut by the oceans, turning the edge into delicate ice shelves.

In the way it grows and flows, this ice sheet can seem almost alive. It will surely die, one day. Changing climate tips the balance between snow build-up and melting, the unstable ice shelves collapse and the ice-streams send ice to melt in the sea. In time the sheet thins to nothing and the world is transformed again.

BRITICE-CHRONO

My description of an ice sheet applies to the modern West Antarctic sheet. Scientists who study it worry about how, in the face of a rapidly changing climate, it might collapse, flooding cities across the globe. The IPCC identified this risk and highlighted how little we know about it.

27,000yearsago (2)

The British & Irish ice sheet, 27,000 years ago. Image courtesy of Chris Clarke.

My description also applies to the ice sheet that sat over Britain & Ireland 25,000 years ago.. A multi-disciplinary consortium, called BRITICE-CHRONO will greatly improve our understanding of the death of this ice sheet. This will be of great local interest, but will also help us predict the potentially troubled and troubling future of both the West Antarctic and Greenland ice sheets. The ancient climate change that killed the ice sheet was natural, but modern human-made warming melts ice just the same.

BRITICE

The physical traces of the death of the British ice sheet are easy to find: erratics, moraines and glacial lake deposits are just a few of the subtle but distinctive features to found over much of Britain. A now complete project called BRITICE, led by Professor Chris Clark of Sheffield University, mapped them all, focussing on traces of the final retreat of the ice sheet. Similar work in Ireland allows the pattern of retreat for the entire ice sheet to be inferred.

Maps showing the evolution of the British & Irish icesheet over time. Image from Chris Clark.

Maps showing the evolution of the British & Irish ice sheet over time. “19 ka BP” means 19,000 years before present. Image from Chris Clark.

CHRONO

BRITICE-CHRONO involves nearly 50 researchers from 8 universities plus the British Antarctic and Geological Surveys. A big part of the work of BRITICE-CHRONO is working out the age of various features. Familiar techniques such as radiocarbon dating are useful, but a new generation of dating techniques can do things that seem almost magical.

Optical stimulated luminescence (OSL) dates the last exposure of sunlight for individual quartz grains. Natural radioactivity traps electrons within defects in the crystal lattice of the quartz grains. If light comes through it frees them again and produces more light (the luminescence). Quartz exposed to sunlight at the surface does not show luminescence, but grains that have been buried in a sand bank for thousands of years do. Measuring luminescence in the lab allows an estimate how long they have been buried for and therefore when the sand was deposited.

Conversely, TCN (terrestrial cosmogenic nucleides) is a technique used for dating how long a surface has been exposed. Cosmic radiation is constantly streaming down on us and within minerals at the Earth’s surface it produces radioactive elements such as 10Be and 36Cl. The more of these we find, the longer the surface has been exposed to space. Apply this technique to a boulder dropped by a glacier and we can infer when the ice was last present.

BRITICE-CHRONO's area of investigation. Image from Chris Clark

BRITICE-CHRONO’s  8 transects. Image from Chris Clark

As part of BRITICE-CHRONO people are collecting hundreds of samples from all over Britain and Ireland. Guided by the BRITICE work, they are sampling features tied into different stages of the death of the ice sheet. The goal is to build up a large and robust dataset to understand how quickly the ice sheet shrank.

To the sea

When the ice sheet was there, sea levels were much lower (because the water was in the ice) and the ice left many traces on what is now the seabed.  BRITICE-CHRONO is using geophysical techniques to understand the distribution of glacial sediment on the seabed (sometimes on land too). Collecting cores from the sediments on the seabed also provides samples for dating. Cores from far offshore contain large rock fragments. These show that floating icebergs melted overhead, dropping stones scraped from land that became entombed within the ice sheet. Marine fossils offer their own special insights.

Offshore features. Image from Chris Clark.

Ice retreat features, both offshore and on. Image from Chris Clark.

There is a lot of interest in understanding features on the sea-bed – construction of offshore wind-farms requires better knowledge of what is out there. Also we now understand the potential for archaeology under these shallow seas. The British-Irish ice sheet may be long dead, but that doesn’t mean people never saw it1.

Recreating the ice sheet ‘in silico’

We know a lot about the world in which the last British ice sheet died. Ice from this time still exists, buried deep in the central parts of the Antarctic and Greenland ice sheets. It contains bubbles of air that once blew over a colder world. From this and other evidence, we have a good record of the climate spanning the period in question.

Scientists have built up sophisticated computer models of how ice sheets grow and die, in part based on research in Antarctica. Take known parameters, such as climate and topography and its possible to recreate an ice sheet ‘in silico’, to build up layers of ice within a computer and watch them disappear as the climate warmed.

BRITICE-CHRONO will build up a robust 4-D dataset of how the ice sheet retreated over time. Combining this with computer modelling will create a positive feedback, increasing our knowledge of how ice sheets behave, both in the past and the future.

Scientific Aims

BRITICE-CHRONO will test three main hypotheses, all of which are relevant to the goal of predicting the fate of modern ice sheets:

  1. The portions on ice close to sea level  collapsed rapidly (in less than 1000 years) but the rate of decay was slower for ice on land. Just how catastrophic was the death of the British-Irish ice sheet?
  2. The main ice catchments draining the ice sheet retreated synchronously in response to climatic and sea-level change. Was the retreat of the ice controlled entirely by external factors, or did the response vary over the ice sheet? This helps us understand the significance of local rapid retreat of ice in Antarctica. Does seeing it in one place necessarily mean it is happening to the whole ice sheet?
  3. The volume of ice-rafted debris depends on changes in ice sheet mass balance. Finding large stones in layers of offshore sediment is a direct record of where melting icebergs were found in the past. How is this linked to changes in the ice sheet? Does the amount increase when ice sheets grow, or when they retreat?

BRITICE-CHRONO is less than half way through its 5 years so it is too early to draw any conclusions. The goal is to produce a robust set of data so individual dates will not be published until the full picture is know. Last year saw a massive sampling effort that will continue this year. Although the focus is dating, put experts in the field and they will find new features such as a whole new suite of moraines in Scotland.

The consortium has a blog and is active on Twitter so you can join me in following their progress as they bring an ice sheet back from the dead.

Categories: England, Glacial, Ireland, Scotland