What drives plate tectonics?

Posted on 30 October, 2020 by Metageologist

In the previous section we described how plate tectonics controls many things on the earth both today and in the past. Here we’ll describe the forces that drive the movements of plates, the patterns of how plates have moved and how computer modelling lets us understand the links between the deep mantle and the surface plates.

The earth is affected by gravitational forces from the sun and the moon. These move huge volumes of water every day, causing the oceanic tides. They even cause small (but measurable) changes to the shape of the earth, the surface moving up and down by a few centimetres. As the earth and moon rotate quickly, these movements are quick and elastic, meaning the earth returns back to its original shape. It’s like a tall building swaying in the wind or during an earthquake. Elastic changes are where atoms move apart from each other, but the bonds within the material are not broken. Plate tectonics is like the entire building is moving, a permanent change caused by rocks slowly flowing or breaking.

Forces caused by plates

There are broadly two sets of forces that move the earth’s plates: those created by the plates themselves and those involving interactions with the mantle below.

Plate tectonics requires plates to be rigid and deform at the edges, they are strong enough that a force acting on one part of the plate pushes the entire plate. There are two sets of forces created at the edge of oceanic plates, one where they are created and another when they destroyed.

diagram of ridge push http://www.columbia.edu/~vjd1/driving_forces_basic.htm>

Ridge push is a force created at mid-ocean ridges where the flowing mantle (asthenosphere) rises up towards the surface. This occurs because the reduction in pressure allows melting and reduces the density of the material. As oceanic lithosphere (oceanic crust plus the stiff mantle material fixed to it) moves away from the ridge it cools and sinks as its density increases. This causes a slope and gravity acting on the higher ridge causes a horizontal force that pushes the entire plate horizontally. Some scientists calculate that a lot of the force pushing India into the Eurasian plate (creating the Himalayas) comes from the many ridges in the Indian ocean pushing on the plate.

Slab pull is a force associated with subducting oceanic lithosphere. Old cold oceanic lithosphere subducts because it’s denser than the surrounding mantle, therefore this negative buoyancy causes a force pulling on the edge of the plate. As it sinks it heats up, but also it is put under increasing pressure from the rock above it. This starts to drive metamorphic reactions that change minerals in the rock into different ones, more stable under the new conditions. Generally minerals with more compact, denser mineral lattices are stable and so the density of the rock is increased. The first transformation is called eclogitisation, but some oceanic plates reach the middle and lower mantle and so will undergo multiple transformations.

We get a sense of the strength of this force by considering how these transformed subducted rocks – eclogites – reach the surface again. Eventually any subduction zone runs out of oceanic lithosphere, and the thin leading edge of the attached continent is pulled into the subduction zone where it is transformed at depth into eclogite. Contintental crust is much more buoyant and thicker than oceanic and resists subduction, meaning that subduction eventually stops. The deeper subducted oceanic lithosphere is pulling the other way and eventually it breaks in two. Once the force of the sinking oceanic lithosphere is removed, the buried edge of the continent, together with a stub of oceanic lithosphere, is quickly pulled back to the surface – bobbing back up like a balloon under-water.

The flowing mantle

Slab pull and ridge push are together one set of forces that act once plates are moving. In addition forces will push onto plates from the convecting mantle below.

Convection is a physical property of bodies that can flow and are hotter below than above. Hotter material is less dense than cool, so rises up and is replaced by cooler sinking material. You can see this sometimes in cooling soup, where patterns of flow affect the surface.

Within the earth, sinking oceanic slabs will drag mantle material down with it and so become the downward flow part of convection. Similarly mid-ocean ridges are places where heat is released from the mantle and may correspond to an upward flow. However evidence from volcanic islands such as Hawaii suggests that mantle flow is more complicated than that. Most earth scientists believe in the existence of mantle plumes, long-lived flows of hotter mantle up towards the surface. The track of a mantle plume across the Pacific explains the pattern of the Hawaiian Islands. A mantle plume that caused volcanic activity in Greenland and the British Isles when the North Atlantic Ocean opened 60 million years ago is still active under Iceland, making that portion of the mid-Atlantic ridge above the surface.

These plumes appear to be unaffected by the passage of plates above them, and some scientists regard them as being fixed in location within the earth, being deep-seated structures.

Seeing the effects of mantle convection on the surface movement of the plates is difficult. There are places on the earth, such as southern Africa which are much higher than we would expect. It seems that this is an area of upward mantle flow and this force is raising up the African continent, forming the high plateau that covers much of South Africa.

Supercontinents

Plate tectonic movements in the past show patterns where continents joined together into supercontinents, only to split apart again. The most famous supercontinent is called Pangea and it existed 270-200 million years ago. It contained all modern continents joined together. It’s breakup led to the creation of the continent shapes we are familiar with today. The Atlantic split apart the Americas from Europe and Africa. India, Antarctica and Australia were split apart by the creation of the Indian ocean. Also the Tethys ocean closed sending India colliding into Eurasia.

Map of Pangea showing modern plate boundaries on it. Like https://www.worldatlas.com/articles/what-is-pangea.html. There are some wonderful examples on http://www.earthdynamics.org/earthhistory/Learn%20About%20Palaeogeography.html

Pangea formed late in the earth’s history. Before it the continents were separated from each other, but by different oceans that are now totally lost (the oceanic plates are now down in the mantle somewhere). We can trace the lines of the lost oceans by the traces of the collision when they formed or by patterns of fossils. Also slices of them called ophiolites may be found, lost within the centre of continents.

The line marking an ancient ocean can be found in countries around the North Atlantic. Called Iapetus, the join where it once was is often close to the modern Atlantic. By painstakingly tracing traces of ancient oceans, combined with computer modelling, scientists have discovered other supercontinents older than Pangea. These are from times so far back that the shapes and names of continents are unfamiliar, but the processes are the same. Up to 13 supercontinents have been identified and named, going right back to earth’s earliest rocks. They appear to form and break-up at intervals of a few hundreds of millions of years.

Whether or not the earth’s continents are all joined together or split into different parts affects many things. Continental shelves are great places for life, creating large areas of shallow water. If all continents are joined together, there is less continental shelf for creatures to live on. Cycles of supercontinent creation and break-up have been linked to changes in climate, patterns of the evolution of life, creation of continental crust, formation of ore deposits and many other things.

Breaking up continents is not easy as continental lithosphere is stable and strong. The break-up of supercontinents seems to be linked to mantle plumes. A plume rising above a continent will heat it and push it up. Mantle flow away from the plume will start pulling the plate apart and the heat and volcanic activity make it easier to break. Some theories suggest that a supercontinent insulates the mantle below and eventually causes a hot plume to rise beneath it. This would explain why supercontinents form and are destroyed again and again in earth history.

Computer modelling goes “beyond plate tectonics”

The only real way to understand the complicated patterns of flow in the earth is by using computer modelling. Only computers than track the different types of force and the fact that this is all happening on a spherical earth. They can be used to try and bring together the theory and the real-life observations to reproduce patterns of plate tectonics over the history of the earth.

In building the models, scientists can use a lot of equations that describe the physics of how hot rocks flow – how they deform and how convection works. They also ensure that all the forces balance, that the earth remains the same size and that the surface plates are not spinning around the world.

Into this theoretical model they add all the observations that led scientists to produce the theory of plate tectonics in the first place. Geological evidence of continental drift shows when continents were joined or moved apart. Magnetic stripes show how ocean basins opened, seismic tomography can see ancient subducted plates and help calculate where subduction zones were in the past. Techniques like palaeomagnetism show what latitudes rocks were at in the past.

Computer models now include all of this information and link it together in a consistent way. These models can describe both the movements of tectonic plates and patterns of ancient mantle plumes. Some say this is moving beyond plate tectonics and into a deeper understanding of how the entire earth works, not just the surface.

One example of the power of these models comes from studies of the distribution of diamonds at the surface of the earth. Diamonds form deep within the mantle, potentially over much of the earth, but they only come to the surface in particular places. Diamonds reach the surface within special types of volcanic eruptions called kimberlites. These are only found in very old parts of continents in Africa, North America, Australia and Asia. Deep under old continents, there is a thick and stable layer of cold and strong mantle attached to the crust. Kimberlites form when this old material is heated and molten rock rich in carbon dioxide is formed. This super-light material quickly shoots to the surface containing fragments of mantle rock within it, sometimes with diamonds. Computer modelling of past plate movements suggested that kimberlite eruptions occur when old continental lithosphere is heated by mantle plumes rising from below. Furthermore, mantle plumes tend to rise from the edges of mysterious structures at the core-mantle boundary called LLSVPs.

<<example of results of this modelling. first diagram in https://www.pnas.org/content/111/24/8735/tab-figures-data

This particular research is fairly recent and like most new studies is not accepted by all scientists. But it illustrates the power of these computer models. If it correctly explains where and when kimberlite eruptions occur, it could help mining companies find new kimberlites and so find new diamond mines.

These computer models are never complete. Scientists using computer modelling of complex systems like climate or the earth’s interior joke that all models are wrong, but the best ones are useful. They understand that new research will improve on existing models, but ones today can increase our understanding and suggest new areas of research.

First publication by Xiaoduo Media in Front Vision. Front Vision is a Chinese online science magazine for children. My original English text produced with permission.

Ultrafast eclogitisation through overpressure

Posted on 4 October, 2017 by Metageologist

My blogging torpor has been ended by a super-interesting new paper that links together many of my favourite topics. It includes eclogites, metamorphic petrology, ultra-fast metamorphism, determining timescales via diffusion profiles, tectonic overpressure and even the Grampian-Taconic orogeny and opens up new avenues of research. What more could I ask for from a scientific paper?

The Rocks

The paper, “Ultrafast eclogite formation via melting-induced overpressure” by Chu et al. has just been published. It’s based on a very close study of a set of rocks from Connecticut in the USA. They are part of the Taconic orogeny, (also seen in Connemara, Ireland, my old field area) which was caused by an Arc colliding with a continent.

The particular location is within a tectonic slice and consists of felsic paragneisses (metamorphosed muds and sandstones) containing lenses of mafic and ultramafic rock (metamorphosed gabbro). The gneisses are migmatitic – they were partially melted. The whole area of high-grade rocks is bounded by mica schists, within 50m, suggesting the heating of the rocks was localised.

The mafic lenses are eclogitic – they contain an eclogite facies assemblage of minerals that formed under high pressure. As is common in eclogites, some of the peak minerals have retrogressed (changed) back into fine mixtures of lower pressure minerals called symplectites. The phengite and omphacite are now symplectite pseudomorphs but garnet (Twitter’s 5th most popular mineral) remains itself and contains some invaluable information.

The chemical analysis

The paper describes estimates of the conditions of metamorphism, derived from various forms of analysis. The garnets are the key to this, as they show a wide range of different compositions. Since they grow outwards, the change of compositions from the core captures changing metamorphic conditions as the garnet grew. The image below is a map of the amount of particular chemical elements within the garnet. The different colours represent different amounts and since the chemistry of the garnet depends on the pressure and temperature while it grew, this gives us a lot of information.

Part of figure 4 showing sharp and complex chemical zoning in garnet

As marked on the image above, parts A-C record heating at around 8 kbar (typical crustal depths) from 660 to 790°C, then D represents eclogitic conditions of 14 kbar and E a return back to 8 kbar.

Speed

Garnets that have spent a long time at high temperatures lose their chemical zoning due to diffusion. Since we understand the speed at which diffusion works, we can use chemical zoning in minerals to estimate the speed of metamorphic processes. Look at the images above and you’ll notice that the boundaries between the different domains are quite sharp. Using two sets of forward modelling (one of the diffusion profiles and another of the garnet growth) they conclude that the pressure increase (C-D above) took only 500 years. The pressure decrease (D-E above) also took only 500 years. These are not the sort of timescales geologists are used to dealing with. I’ve been to educational establishments that have lasted longer than 500 years – it’s no time at all.

These dates cannot be explained by normal models of eclogite formation, which involve subduction of material and eventual return to shallower depths. Rates of subduction are measured in cm per year, but to explain the rate of pressure increase this way would require rates of 30m per year.

How does pressure change so fast?

The usual assumption in metamorphic petrology is that the pressure recording by the minerals is lithostatic pressure – that caused by the weight of rocks above. This is useful as it allows you to track the rocks passage into and out of a subduction zone or mountain belt and so link metamorphic P-T estimates to tectonic models. A number of recent papers have started to challenge this assumption. Their approach varies, but all suggest that rocks can experience high pressures without being buried to great depth.

What’s so great about Chu et al. is that is provides direct evidence of this happening and offers an explanation as to how it happened.

Burying rocks at 30m a year is not credible, so if we accept their estimates for the speed and amount of pressure increase (it persuades me, but more knowledge people than me may disagree) we must look for ways of generating high pressure without piling up more rocks on top.

Chu et al.’s explanation takes us back to the unusual location of these rocks, within a narrow active shear zone surrounded by colder rocks. They see rocks as being rapidly heated due to strain heating along the shear zone. This one of the mechanisms invoked by those arguing that localised and rapid metamorphism is more common that we thought. The narrow zone of hot rocks is surrounded by colder more rigid walls. As the rocks start to melt, the sudden volume increase causes an ‘autoclave’ effect, a sudden local deviation above lithostatic pressure. This causes the rapid growth of eclogitic minerals with the mafic pods. Soon the melt escapes or crystallises, reducing the pressure back to normal.

Part of figure 9, showing the model of transient overpressure due to partial melting

What does it all mean?

This paper adds a fascinating petrological argument supporting the theoretical studies suggest that overpressure within metamorphic rocks is possible. No longer can the mere presence of high-pressure minerals necessarily indicate the rocks was buried to great depths.

As the authors say, it does not invalidate most of what we know about eclogites. The mechanism described does not apply to the many ultra-high pressure terranes like Western Norway where large volumes of eclogitic rocks exist – these surely did get stuffed deep into a subduction zone.

My first thought was of the Glenelg eclogites in Scotland. This is a small area of eclogite within a high strain zone which makes very little tectonic sense. Perhaps this is a candidate for being formed by local overpressure? As far as I can tell they aren’t associated with migmatites, which makes them not directly analogous. But the point is that there is a new possible interpretation for them, new avenues of research. That’s what makes this paper so interesting to me.

Please read it, and if you think it’s wrong, let me know.

Chu, Xu, et al. “Ultrafast eclogite formation via melting-induced overpressure.” Earth and Planetary Science Letters 479 (2017): 1-17.

Update: another possible candidate for this effect in Scottish rocks is within the Central Highlands Migmatite Complex. See brief description and my write-up of a discussion of the significance of these rocks.

The Himalayan mountains: flow and fracture

Posted on 2 August, 2017 by Metageologist

Earth science departments are home to three styles of working, each of which tries to answer similar questions, but from very different perspectives. First we have the field geologists. Armed with field gear and a hammer, they gather data from actual rocks in the form of photos, diagrams and above all maps. Next we have those who live in the lab studying rock samples using mysterious machines to gain startling insights. Finally those who seek to recreate the world in a computer, building models to understand the underlying equations that can explain how the earth works.

In some departments the very different cultures and mental models that these approaches require can lead to conflict. But the best science is done when they are combined, in a department, research group or even a single brain. A recent paper by lead-author Andrew Parsons weaves together different techniques to provide a coherent model of how rocks flow to build huge ranges of mountains.

Different ways to build a mountain range

The Himalayan mountain range is the best place to study how mountains form. Sixty million years ago the Tethys ocean sat between the Indian plate and the Asian. Soon the Tethys oceanic crust vanished down into the mantle beneath Asia and the Indian plate collided with Asia. At the site of that collision now sits the highest mountains on earth, plus the huge and high Tibetan Plateau.

Geologically the Himalaya are long (over 2000km), thin and consistent. From Kashmir and Ladakh on the Indo-Pakistan border, through all of Nepal and further East through the Buddhist kingdom of Bhutan the same sequence of rocks is seen. Starting from the undeformed Indian Plain and walking up, we pass through the sub-Himalayan zone, through the Lesser Himalayan Sequence (LHS), the Greater Himalayan Sequence (GHS) and finally the Tethyan Himalayan Sequence (THS).

These distinctive packages of rock are separated by major faults. Sudden movements on these faults cause the major earthquakes that regularly affect the people who live in this beautiful part of the world. Early models for how the mountains formed focused on these thrust faults. We saw mountains as thick piles of slabs of rock, separated by brittle faults.

Thrust faults stack up layers of rock. Image from Wikipedia

In these models deformation is focused on the thrust surfaces and the rocks in between are relatively strong and rigid. This is a reasonably good description of the rocks of the LHS. However studies of the GHS rocks showed them to have been strongly deformed at high temperatures. Intrusions of granite are common and appeared to form while the GHS rocks were deforming and moving rapidly towards the surface. Also the fault between the GHS and the barely deformed sediments of the THS is a normal fault, one with an opposite sense of movement to the thrust faults.

View of Mount Everest, with sediments of the Tetyhan Himalayan Sequence forming the summit and metamorphic rocks of the Greater Himalayan Sequence below

To explain these features, the concept of ‘channel flow‘ was borrowed from fluid dynamics. This describes how viscous fluids flow when trapped in a channel between two rigid surfaces (think of jam/jelly squeezed out of an overfilled sandwich). We know that hot rocks deep in the earth can flow (slowly!) even while remaining solid. The channel flow concept saw the GHS rocks as flowing out from underneath the Tibetan Plateau towards the surface, perhaps moving into the space created by rapid erosion of the high Himalayan mountains.

An alternative model for explaining the hot and deformed GHS rocks is called wedge extrusion, where deformation on thrust faults brought it to the surface, rather than channel flow.

These three models are often seen as being mutually exclusive. But what if all were true?

Going with the flow

Andrew Parsons work is based upon his PhD studies at the University of Leeds in England. He’s published a map of the Annapurna region of Nepal, covering all the main sequences of rock and so is at home in the fieldwork tradition of the Earth sciences. No doubt he has pictures of himself in field gear, looking sun-burnt and grinning in front of some amazing scenery. At the core of his prize-winning paper is hours and hours of lab-work. After wielding his hammer to collect over 100 samples in a transect across the Himalayas, he had them cut by a diamond saw into thin sections to have their secrets probed.

His studies used an optical microscope (to get a sense of this, check out #thinsectionThursday on Twitter) and also a scanning electron microscope. The goal was to identify precisely how the minerals in the sample were squashed.

Rocks are made of minerals and the way they flow is by deforming those minerals. We know a great deal about the many ways common rock forming minerals (like quartz, feldspar, calcite) deform. There are a range of different ways in which the minerals deform. Minerals are crystalline; the atoms within are aligned in consistent repeated patterns. Slow movement of atoms along particular slip-planes or the propagation of defects change the shape of the mineral and so deform the rock. The mineral structure is complicated and different planes of slip are favoured depending on the temperature. Careful study of subtle patterns in the minerals, plus measurement of the average orientation of the atomic structure in each grain tells a great deal.

A portion of figure 6, showing different microscope images of rock samples, with different features highlighted.

For each sample, the authors were able to estimate the temperature at which the rocks were deformed. Also they got a sense of the style of deformation. Imagine a perfect sphere in a rock that is then deformed. Rocks can be flattened turning the sphere into a cow-pat or M&M shape or maybe stretched into a cigar shape. As well as this, the deforming sphere can also be rotated. A combination of field observations, thin section studies and the scanning electron microscope work together gives a view of how the rocks were squashed.

Bringing it all together

The core achievement of this award-winning paper is linking the mass of data to the predictions of the various tectonic models. Channel flow is predicted in the GHS rocks while they were hot and rapidly flowing. The entire channel should flow and rotation only be seen at the edges. This is what the samples from the upper section of the GHS show, plus an indication that this style of deformation ended while they were still at 550°C and changed from being hot and soft to being more rigid.

At this point channel flow ceased and another mechanism, rigid wedge extrusion, came into play. Here the lower portion of the GHS was deforming at a lower temperature. As the model would predict, deformation involves a lot of rotation, consistent with the whole of this sequence of rocks acting as a broad shear zone, bringing the upper GHS rocks closer to the surface.

The lowest temperature deformation is found in a thin zone of rocks within the lower GHS that were acting as a thrust fault, stacking up different slices of rock.

Figure 12

This diagram emphasises the key insight – that different ways of building up mountains can be active at the same time, within different parts of the mountain belt. The mountain building (orogenic) system is composite, made up of different parts. Studying individual mineral grains helps us understand the structure of a massive mountain range. This is because that is how it grew. Countless millions of mineral grains slowly shuffling their atomic lattices over millions of years: this is what builds mountains.

The system is still active, as India pushes into Asia. We know from geophysics that there are hot rocks under the Tibet – perhaps these are right now flowing to the surface as a channel. As the Himalayas are eroded away they will get nearer the surface and start cooling, causing different styles of deformation to come into play.

The terminology of ‘superstructure/infrastructure’ used in this diagram is taken directly from computer modelling work. Numerical models of how mountain belts might work have directly informed this work. Cold rocks near the surface can be a lot stronger than the hot rocks below. By combining computer modelling with field work and lab studies, geologists are getting a good understanding of how the Himalayan mountains formed and have evolved over time. Many places are built on the roots of ancient mountain ranges and high-quality integrated studies such as this help us understand rocks across the world.

Parsons, A. J., et al. “Thermo‐kinematic evolution of the Annapurna‐Dhaulagiri Himalaya, central Nepal: The Composite Orogenic System.” Geochemistry, Geophysics, Geosystems 17.4 (2016): 1511-1539.

Speed of metamorphism: cooling down

Posted on 18 June, 2017 by Metageologist

A while ago, I asked Twitter for suggestions of topics for future posts. A great one came from Brian Romans, a Prof at Virginia Tech and a long-standing pillar of the online geoscience community:

How fast can rocks undergoing metamorphism be exhumed to the surface? How do we know (how can we measure/estimate)? https://t.co/qcsnxXKqhL

— Brian Romans 🌎🏔🌊🧊 (@clasticdetritus) December 24, 2016

This post follows on from my discussion of how quickly metamorphic rocks heat up which discussed an old conceptual model of metamorphism being caused by slow, gradual processes building mountains and heating the rocks inside. A more modern understanding shows that metamorphic events can be extremely fast, driven not by slow conduction, but rapid (even catastrophic) pulses of hot fluid or pressure.

Let’s think what happens next to the mountains in the old gradual conceptual model. They are slowly but surely brought low by erosion. Grain by grain of quartz or mica bidding farewell to their long-standing neighbours and heading off on an headlong tumbling adventure down a river to the sea. As the hills and mountains are laid low, once deeply buried metamorphic rock slowly emerges, cooling as it heads towards the surface over tens of millions of years.

Turns out, if you actually date the age of metamorphic rocks at the surface in modern mountain belts, they can be a lot younger than expected. Just as it doesn’t necessarily take millions of years to create a metamorphic rock, so they can reach the surface in double quick time.

How to date cooling

How do we know this? If you’re in an active mountain belt such as the Himalayas and you have a metamorphic sample in your hand, that’s already one piece of data – it’s at the surface and cool today. Date the age of the metamorphic event and estimate it’s temperature and you have a second data point. Those two together may be enough to demonstrate rapid cooling. But more data is always good. Luckily radiometric dating can really help us here.

Whenever geologists quote an absolute age1 for something, they are referring ultimately to an estimate derived from our understanding of radioactive decay. Certain elements (most famously Uranium) are unstable and their nucleus can fracture into pieces. Some of these pieces may be small and fast-moving – they make up the radioactivity that can be so dangerous – but there are larger fragments of nucleus too, which tend to stay put and are called ‘daughter elements’. They are the products of natural alchemy and form atoms of new different elements. The rate at which this radioactive decay occurs is steady and unchanging, perfect for a clock. Simply put2 if you measure how much there is of both the radioactive substance and its daughter, you can calculate the age of something. This is geochronology.

For us today, the key concept here is that ‘something’ we are dating the age of. Often we seek to find out the age at which a mineral grain grew, either in a metamorphic rock or crystallising from a magma. This works if the mineral acts as a closed system – once the grain is formed, nothing comes in or out. If instead some of the daughter element has left the system, then the date measured is ‘wrong’ – it doesn’t tell you the real age of the mineral. However, since the mineral becomes a closed system when it cools below a particular temperature3, careful measurements can give you the date when the mineral cooled below that temperature. This is therefore a new independent data point in the rock’s history of cooling. This is the principal behind the technique of thermochronology.

Different minerals, and different pairs of elements become closed at very different temperatures. Many common minerals are used to determine radiometric dates: biotite; muscovite and other white micas; various feldspars; garnet; calcite; apatite. The spread of closure temperatures is from around 1000°C (for Zircon) down to 70°C (for Calcite)4.

This gives us a very powerful tool kit. Different minerals in the same rock may give different ages, each dating a different stage in the rock’s cooling history.

Irish rocks are cool and once they were really hot.

Let’s move to an example of how thermochronology can give insight into how fast metamorphic rocks cool. It comes from a recent paper by Anke Friedrich and Kip Hodges focusing on Connemara in Ireland, (my old PhD field area). The paper cites my research, which immediately makes me think well of it.

Figure 7 from Friedrich and Hodges (2016), showing the range of thermochronological dates and the contrasting T-t paths across Connemara.

This figure is the core of the paper. Each box is a different thermochronlogical measurement, plotted against time and Temperature. Different coloured boxes are different types of systems. Even the same colour of box can be at a different temperature because these estimates include also a consideration of the size of the mineral grain. The data is superficially rather noisy, and could be taken to indicate a slow cooling history, but the geology of the area is extremely well known, allowing distinct 3 sets of dates to be identified.

Those defining the dark line – labelled North – show a brief period of heating and then cooling. The south of Connemara was the site of igneous activity for much longer than the north – the later cooling is clear from the data. Data points from the right hand side of the graph are associated with heating from much later granites.

Scream if you want to go faster

Prof. Romans is a sedimentary guy and interested in metamorphic rocks as something that ends up as a layer of sand or mud. Linking metamorphic rocks to sedimentary ones means tracing out one of those great big linked cycles that make earth science so mind-expanding. My favourite example – good evidence for what went on deep in the Himalayas can be found in sediments deep in the Indian Ocean. As an aside, for these Connemara rocks we can also trace the point at which they start being eroded and ending up in a sedimentary basin. As a further aside, the grains that were dated from Irish metamorphic rocks can also be found in modern sediments. These studies (detrital mineral thermochronology) can be used to find out where sediment came from or to estimate the timescales of sedimentary processes. That’s a topic for another time.

Brian Roman’s original question started with asking how fast metamorphosing rocks can be exhumed. Exhumation means the rocks reaching the surface – like digging up corpses only much much nicer. This is linked to a rock’s cooling history as in order for a metamorphic rock to cool significantly and quickly it needs to get nearer the surface. Metamorphic rocks may form in unusually hot parts of the earth but the fact that the earth gets hotter the deeper you go is the main control. Places where rocks are exhumed quickly must be places where the rock above is removed quickly and one way to do this is to erode it away.

The fastest rates of exhumation known are in the Himalayas. Specifically at the eastern and western ends: Namche Barwa in the eastern Himalaya and near Nanga Parbat in the west. Here rocks are being drawn to the surface so fast that 25km of rock has been eroded in the last 10 million years. Given that the Himalaya are around 50 million years old and on average 5 kilometres high, this is quite amazing.

The likely reason for such rapid exhumation is that these points are where massive rivers, full of glacier melt from the Tibetan plateau and Himalayas, cross the Himalayan mountain range. The rivers cut deep down into the earth and soft hot rocks flow into the space created (more detail here). The Himalayas and the Tibetan Plateau cause the Monsoon which drives rain-clouds north from the ocean to feed Himalayan glaciers. Melting glaciers fill the rivers, which cut deep into the earth and drive a flow of rock from kilometres under the ground, up to the surface where it is turned into sand and mud that flows back down into the ocean. There it forms sedimentary rocks, that one day may be buried and be turned back into a metamorphic rock. There are so many cycles in geology and so many links between them.

In the first post in this series we learned that the formation of those beautiful metamorphic minerals paraded on Twitter every #thinsectionThursday is less like the slow transformation of dough baked into bread but instead more like the sudden explosive transfiguration of a hard kernel into tasty pop-corn5. In this post we’ve learned how we know how quickly rocks cool. This can be slow, like letting a roast chicken rest before carving but other times it’s like popping some morsel straight from the fryer and onto your plate