The Geology of Mount Everest

Posted on 10 June, 2012 by Metageologist

Growing up, I was mildly obsessed with Mount Everest. Even now I marvel at its wonderful geology.

Looking at that, who can blame me?

My youthful obsession was fuelled by books of British expeditions in the 1970s climbing it by various routes with varying levels of success. The photos were the best; an image by Doug Scott showing Everest, Lhotse and Makalu, taken by from Kangchenjunga was particularly mesmerising. <aside> Makalu (left-hand of 3 tall peaks) in particular is intriguing as it looks like it had a glacial ‘U-shaped’ valley that has now mostly fallen away in landslides. If you know more, please tell me! </aside>. I started my teenage years wanting to climb mountains like this, but by the end of my teens this had waned. It was too apparent that many people in the early books had been killed on the mountains. Now that Everest seems full to the seams with the rich and the foolish – littered with their corpses – the glamour has worn off somewhat.

It was still a great thrill to go on a walking holiday to the Everest region, particularly as by then the geology of the area was properly understood. I shall use my pictures to illustrate the geology, as described in a 2003 paper by Mike Searle and colleagues, available on line via his Everest map website.

Let’s revisit that first picture.

This shows the main features, north is to the left. The summit area is made of sedimentary rocks, now far from the sea. The contact with the metamorphic rocks is not an unconformity, but an extensional detachment: a structure – here brittle fault, there shear zone- that is usually associated with thinning of the crust. The Qomolongma detachment, together with the Lhotse detachment (in my picture, hidden in the Western Cwm, the valley between Everest and Nuptse) together are part of the South Tibetan Detachment System (STDS) which can be traced along the entire Himalayan chain

The rocks labelled as metamorphic rocks are greenschist facies, but the Nuptse ridge (right hand side of picture) contains high-grade sillimanite gneisses and many granite intrusions. These are part of a package of high-grade metamorphic rocks, the High Himalaya Crystalline series that are everywhere found below the STDS. Searle and colleagues explain these rocks in terms of channel flow. This amazing theory says that between 21 and 16 million years ago, a thick channel of soft hot rocks flowed out from under the Tibetan Plateau towards the Himalayas.

The Qomolangma detachment is part of the top of this channel. The high-grade rocks below flowed 200km south into their current position, moving at a shallow angle parallel to the top of the channel. The driving force for all of this? The snow and rain falling on the mountains and eroding the surface.

Let’s look at the rocks in more detail. Above is a picture of Changtse in Tibet, taken from near the head of the Khumbu valley in Nepal. It is looking up and north at the Qomolangma detachment. The prominent yellow band, often mentioned by mountaineers, is marble and lies just below the detachment. Above are unmetamorphosed sediments. Left in the foreground is mostly granite.

Turning to the metamorphic ‘channel’ rocks lets look at the South face of Nuptse, from a more direct angle.

The picture is around 3 kilometres from top to bottom. The dark rocks are high-grade gneisses, that flowed 200km towards the camera. A clue as to how they did that is found in the pale areas – granite. Rocks containing melt have a much lower viscosity. The many dykes/veins in above the granite are, according to Searle, an “explosive network of dykes emanating out of the top of the Nuptse–Everest leucogranite” caused by late-stage volatile rich magma. In other words, the fluid left behind as the main intrusion cooled and solidified.

Searle interpret the right-hand granite body as a sill that ballooned into a much thicker body. It passes under the summit of Everest and an equivalent body is found in the east Kangshung face of Everest. They speculate that the presence of this 3km thick granite body might explain the particularly high mountains.

Another view there, in different light with some moody clouds.

This is a view of an unnamed mountain on the side of the Khumbu valley. It shows a more typical view of the granite intrusions in the area, plus a glimpse of how it was intruded.

Granite sills are typical of the area, but the dyke was the only one I saw. The intrusion process was of sills making space via hydraulic fracturing along bedding/foliation planes in the dark metamorphic rock. Dykes allow flow between sills.

In the 30’s and 50’s mountaineers on Everest were also explorers and they collected samples One geologist, Lawrence Wager, got to with 300m of the summit in 1933 and later became Professor of Geology at Oxford (my alma mater) . One paper on Everest geology (Jessup et al 2006) is a detailed study of microstructures. It involved studying Wager’s samples, plus others collected on the 1953 expedition. One was collected by Edmund Hilary only 12 metres below the (snow-covered) summit. Either just before or just after he became the first man on the Earth’s highest point, he took the time to collect a rock sample. Mount Everest and geology are closely intertwined.

References and further reading

SEARLE, M., SIMPSON, R., LAW, R., PARRISH, R., & WATERS, D. (2003). The structural geometry, metamorphic and magmatic evolution of the Everest massif, High Himalaya of Nepal-South Tibet Journal of the Geological Society, 160 (3), 345-366 DOI: 10.1144/0016-764902-126

The Searle paper came out of joint research between Oxford and Virginia Tech universities. Both have websites with more gorgeous pictures. The Oxford one has a low-res version of a map made of the area and includes a copy of the Searle paper. The Virginia Tech one has pictures linked to a map.

Ron Schott has compiled a list of Gigapans from the Himalayas, including a few from the Everest region.

You can find more on the Wager / Everest connection online. If you want to see a photomicrograph of the highest rock sample ever collected, figure 6 of the Jessup et al paper is the place to go.

This post is the summit of my journey into the Geology of mountains. If you want more detail on channel flow and associated concepts, that’s the place to go.

Channel flow – hot rocks, big glaciers and the world’s tallest mountains

Posted on 5 June, 2012 by Metageologist

Leonardo da Vinci, famed artist and Renaissance “Renaissance Man” made some interesting remarks about Geology. When he looked at rocks in the Alps containing fossil molluscs, it was clear to his trained eye that the fossils were near identical to shells formed by creatures in the sea. That fossils are the remains of ancient creatures seems obvious now, but was a controversial idea at the time. This makes it even more impressive that he made the further step of thinking about how things that formed in the sea ended up on the top of mountains.

We now know that the highest peaks in the world – such as Mount Everest – are formed from ancient marine sediments. When we think about a coral – that grew safe and snug in a warm ocean that no longer exists – now lifted up 9 km to form the top of Mount Everest, the only proper response is awe. The rocks just below are even more amazing. They started in the sea but they’ve been buried 15 kilometres below the surface, partly melted and then drawn up to the surface by snow and rain.

Channel flow

If you bury the right sort of rocks in the right sort of way, they form a hot soft layer in the middle of the crust. In Asia, a soft layer is formed by the collision between the Indian and Asian plates stuff sediments (rich in heat-forming radioactive elements) down under the Tibetan Plateau. Stuck between a cold rigid layer (the upper crust) and a strong rigid layer (lithospheric mantle) this soft layer is like jelly (jam) in a sandwich. In special places on either end of the Himalayas, this ‘jam’ squeezes to the surface via a process of extrusion in a ‘tectonic aneurysm’. Extrusion is a particular case where the middle crust ‘jam’ reaches the surface, the more general concept is called channel flow.

The idea of channel flow was born on the INDEPTH geophysical surveys of the Tibetan plateau, led by Doug Nelson. These suggested that there is molten rock beneath Tibet right now. At the same time, geologists across the the Himalayas were puzzling over a thick layer of metamorphic and igneous rocks (High Himalayan Crystalline Series, below) that looked as if it had been squeezed out from underneath Tibet. Channel flow puts these the two observations together – hot rocks generated beneath Tibet flowed, lubricated by molten granite, out into the high Himalaya.

In 2004, there was a conference hosted by the Geological Society of London on the subject. This brought a fantastic range of techniques to bear on the problem – geophysics, mathematical modelling and a wide range of geological studies. The ‘special publication’ based on this conference is a fantastic resource; it mixes maths-rich papers – modelling the patterns of flow that are possible, with papers full of pictures of rocks – inferring how they flowed in the past.

It’s a big thick book, but in a sentence it goes as follows. “Hot rocks in the middle crust of Tibet have formed a weak ‘channel’ that flows laterally out from areas of over-thickened crust (probably, but more research etc…)”. There are two main ways this lateral flow of ‘the jam’ can happen.

Something moving deep inside

Imagine you are standing in eastern China looking west at the Tibetan mountains – what does the future hold? Channel flow predicts that hot middle crustal rocks are flowing into the crust below your feet. Deep below, a continuous slow shuffling of atoms in lattices is changing the shape of mineral grains. With enough time, small things make big changes and solid rock can creep and flow and squeeze itself along. The ground you are standing on is ever so slowing being jacked up. In time it will be several kilometres higher and part of a wider Tibetan Plateau. The pandas will be gone, replaced by yaks.

This process would take millions of years longer than a human lifespan, or course and there is little solid evidence it will happen in the future. It almost certainly happened in the past though – there is evidence from river gorges in eastern Tibetan of 1.5km uplift in the last 5 million years. Channel flow is a convincing mechanism to explain how this happened.

Hot rocks, big glaciers and the world’s tallest mountain

It is quite hard to know what is going on 20km below your feet. Places where the channel reaches the surface are easier to understand as you can do Geology at the surface, studying the cooled channel rocks (the ‘fossil jam’). A 2007 JGS review paper by Nigel Harris of Britain’s Open University (available free online) looks in particular at the rocks found on the southern edge of the Tibetan plateau – the Himalayas.

One of the key observations leading to the proposal of channel flow was the observation that metamorphic rocks of the High Himalayan Crystalline Series – “HHCS” are bounded between two fault systems. Underneath there are thrust faults, above there are extensional faults, notably the South Tibetan Detachment System – “STDS” (not STDs, oh no). The combined sense of movement of these faults is to push the metamorphic rocks out. These metamorphic rocks acted in the past as a hot flowing channel, moving from under Tibet out and along to form the world’s tallest mountains.

This cross-section shows the channel flow model. Underneath the Tibetan Plateau, a hot (red) channel of soft rocks forms in the middle crust (the scale is very large so the topography is not very apparent). The stippled grey/brown material is the Indian Crust moving underneath. The Tibetan Plateau heats up in Spring and pulls moist air off the Indian Ocean up north over the Indian sub-continent to form the monsoon. Little of it gets beyond the southern edge of the plateau (cloud in the diagram) as the mountains cause precipitation. The enhanced erosion, from rain, rivers and glaciers causes exhumation (like at Nanga Parbat) and starts to draw the channel to the surface. The lower diagram shows a slightly later stage, showing specific tectonic features. Enhanced erosion on the Himalaya front initiates a flow of material that reaches deep into the crust and 100s km laterally under the Tibetan Plateau.

This is such a beautiful idea that it ought to be true. But is it?

After extensively reviewing other models and all available evidence, Harris makes some interesting conclusions. First, “There can be little doubt that the high-grade rocks of the Himalaya were extruded southward, bounded by thrusting below and normal faults and shear zones below” so the metamorphic rocks were squeezed out as a package. Further, “evidence is emerging that is largely consistent with the hypothesis that southward extrusion during the Early to Mid-Miocene was facilitated by channel ﬂow”, so it has happened, if only for a while 20-15 million years ago.

There is much less evidence that channel flow in the Himalayas has been happening since. Current day zones of high exhumation and high rainfall are found further south where they are associated with brittle thrusting in lower grade rocks. This can be explained by viscous wedge models rather than by channel flow.

Rain and snow is amazing stuff. For a period of time it was able to influence rocks deep under the surface and 100s kms away, enticing metamorphic rocks to flow towards the surface where they can be broken into pieces and washed away back to the sea. The precipitation patterns are controlled by topography which is controlled by tectonics which is influenced by precipitation patterns… We make stories about the world, as that is the way our minds work, but a world where everything is so deeply connected can’t be reduced to our simple linear narratives.

In his paper, Nigel Harris discusses further links between precipitation and tectonics; he considers the ways in which higher rates of precipitation in the eastern Himalaya affect tectonic features. Granites associated with channel flow are younger in (rainier) Bhutan than further west. The evidence is equivocal, but the idea that climate and tectonics have been linked for the past 20 million years and over thousands of kilometres may yet be proved right. That’s (even) more interesting than a few high-altitude fossils. If only Leonardo da Vinci were here to know it.

References and image sources

The Harris paper is available free via the Open University’s open research archive.

Harris, N. (2007). Channel flow and the Himalayan-Tibetan orogen: a critical review Journal of the Geological Society, 164 (3), 511-523 DOI: 10.1144/0016-76492006-133

Godin, L., Grujic, D., Law, R., & Searle, M. (2006). Channel flow, ductile extrusion and exhumation in continental collision zones: an introduction Geological Society, London, Special Publications, 268 (1), 1-23 DOI: 10.1144/GSL.SP.2006.268.01.01

Everest picture my own, diagrams from Harris (2007) with permission of author, rainy Himalayan picture from nandadevieast on Flickr under Creative Commons.

Geology and life in the English ‘Coal Measures’

Posted on 23 May, 2012 by Metageologist

The geology of the North of England is where our modern industrial civilisation was born, based on the burning of fossil life. I’ve wanted to write about the fascinating geology I grew up with for a while. I’ve been spurred into action by Accretionary Wedge #46 where Cat asks us to write about “Geology, Life and Civilization”.

The spine of northern England is the Pennines, ending in the south with the Peak District, where I come from. This area is almost entirely made up of Carboniferous sediments that have shaped the landscape and the culture of the area.

True Grit

One of the most characteristic landforms of the Pennines is the gritstone crag.

These thick resistant layers of sandstone form prominent lines of outcrops (crags), with gentle dipslopes in between. Generations of rock climbers have been trained on them. The cliffs are low, but make for great climbing. You can battle with gravity all day and be in the pub half an hour later.

The sandstone is often coarse (gritty) and was traditionally used to make Millstones, which are now the emblem of the Peak District national park. ‘Millstone Grit’ is great building stone and the area is dotted with quarries. In some areas it forms easily into regular slabs, perfect for building drystone walls or making cobbled streets.

The crags of sandstone have fabulous names, like Froggat Edge, Stanage Edge and ‘the Roaches’ and these names enrich the stratigraphy and therefore the geological maps of the area.

Gritstone is poetic as well. The great English poet Ted Hughes grew up in these landscapes and wrote about them. In “Still Life” he writes that “Outcrop stone is miserly” and is “Warted with quartz pebbles from the sea’s womb”. The outcrop marks the ‘fly-like dance of the planets’ and thinks itself eternal, but only because it is ignorant of what water will do it, over time. Elsewhere in Wodwo he writes of walkers escaping the valley onto the moors above. The Pennines and Peak District have long been a means of release for inhabitants of the industrial cities and valleys that sit below. The mass trespass of Kinder Scout (type locality of local stage “Kinderscoutian”, 318 to 317 million years ago) in the 1930s was in favour of the ‘right to roam’ and is seen as a milestone in English social history.

Mud, glorious mud

The gritstone crags are only prominent because they are surrounded by softer rocks. Mudstones and shales are common in this area, they form the plain backdrop on which the gritstones can perform. A humble, elusive rock type, best found in stream beds the shales are not devoid of interest. Far from it – they were teeming with life.

Below is a section of an early armoured fish.

I don’t know what this is, maybe nothing.

At times the shales are clearly marine (mostly, like the sandstones, they are not). In distinct ‘marine bands’, goniatites (ammonoids) and crushed shelly debris are common.

The clearest goniatite above is middle right, where you can see the parallel lines of the external ornamentation. Here’s a close-up where the spiral shape of the goniatite is more obvious.

Goniatites are marine creatures, but here’s a clear sign of land, a piece of fossil bark. Carboniferous forests were dominated by primitive plants called Lycopods which have leaves growing direct from the stems, leaving the scars you see below.

King Coal

Cat herself mentions Iain Stewart talking about the importance of Carboniferous coal deposits in the history of the Industrial Revolution. Just how important coal was is an area of vigorous historical debate but no-one would argue that industrialisation started in the North of England and that burning of local coal deposits was important.

Let’s start with the roots of the matter. Coal is of course compressed plant material found in seams. At the base of these seams is usually a layer of very pure quartz sandstone. This is a fossil soil, a palaeosol which locally may be known as seatearth, or ganister. Appropriately enough these fossil soils often contain fossil roots, often Stigmaria, with a distinct ‘holey’ appearance.

Coal is relatively rare in the Peak District, where all of these photos and samples were taken (most on a single afternoon in the Goyt Valley). The area contains the transition between ‘Millstone Grit’ deposits and the ‘Coal Measures’ proper. Coal seams are thin and extraction was via shallow ‘bell-pits’ for local use only. The main coal fields were further north in Lancashire and Yorkshire, where whole communities were built up around the ‘pits’.

These coals put the ‘carbon’ into ‘Carboniferous’ – there are massive deposits worldwide but the name was coined in Britain. This was an odd period in earth history, associated with high levels of oxygen (perhaps up to 35% compared with 21% today). One of the lines of evidence for high oxygen are the massive insect fossils found at this time (no photos sadly). These animals depend on oxygen diffusing into their bodies, so the more oxygen, the bigger they could grow.

Coal is fossil plant material, so no surprise that it contains impressions of plants within it.

This weathered piece, from a stream bed, looks rather like shale until you break the end and see it shine. I could have set fire to it and taken a picture, I suppose, but that would just be showing off.

Cycles

Together, these rock types make up most of northern England. What is intriguing is that they often occur in a regular pattern. This is an interesting thing and I shall return to it.

Another form of cycling concerns the carbon locked up in the coal. What was locked below the surface is now floating above it in the form of carbon dioxide. Releasing the power of this buried carbon kicked off our industrial civilisation. How we deal with the powerful effects of the atmospheric version will determine how our civilisation fares in the future.

The first image, of the Roaches, image from Plbmak on Flickr under Creative Commons.

All others mine, scale bars are centimetres.

If you want to know more about English Carboniferous Geology, this open access book chapter is where to start.

Hot rocks, big rivers and the world’s tallest mountain face

Posted on 16 May, 2012 by Metageologist

In areas of active mountain-building the middle crust can get hot and weak, like a soft jam/jelly filling in a sandwich. These squishy rocks are hidden from us by the cold rigid upper crust, so we wouldn’t expect to see them reach the surface, would we? Well, what happens if you overfill a sandwich and there’s a break in the upper layer? Nanga Parbat in the Karakoram Himalaya tells us what.

Nanga Parbat is one of only 14 mountains over 8,000m in height. It’s summit is a mere 25km from the Indus, a major river flowing off the Tibetan Plateau and then along the length of Pakistan. The mountain face between the two (the Rakhiot Face, above) is often called the world’s highest as there is a drop of 7,000m, from peak to river. Nanga Parbat sits at the Western syntaxis of the Himalaya, this is the point where the structural grain of the Himalaya changes from East-West to North-South. It corresponds roughly to the corner of the Indian plate (buried somewhere below), where the plate boundary swings around.

Nanga Parbat contains granites at the surface that cooled only a million years ago. Not lava but granite that was intruded at depth and yet is now at the surface. A active major thrust puts gneisses on top of river gravels. It’s estimated that 25km of crust has been eroded away in the last 10 million years (or 12-15km over the last 3Ma, take your pick).

A hard rain falling

In order to remove 25km of crust, you need a lot of erosion. That sort of erosion requires high rainfall and a major river to take all the bits away. Both of these things are present at Nanga Parbat. What is mind-expanding is the idea that the erosion and the tectonics are linked.

The “tectonic aneurysm” model (defined in Zeitler et. al 2001) suggests that the tectonics of the area are caused by the high rainfall and presence of the Indus river valley. Consider a river valley cutting deep into a mountainous area – it’s made a kilometres deep hole in the crust and this weakens the rocks below and either side of it.

To state the obvious, remove rock from the surface and the material below moves closer to the surface. This does two interesting things – firstly the reduction in pressure makes brittle rocks weaker. Secondly, the valley will change the thermal structure of the crust. Cutting a hole into the crust will bring hotter rocks closer to the surface (because the surface is lower). Then if the rocks beneath the valley start moving up (a process called advection) then they may move up so fast that they can’t cool down on the way. There’s more. Releasing pressure in very hot rocks (decompression) can trigger melting and granite production. The presence of melt reduces the strength of the crust by an order of magnitude. This means that the crustal flow will become easier as the rocks become weaker.

Erosion causes uplift which causes crustal flow which causes more uplift which causes more erosion which….. This positive feedback loop turns the initial weakness into a much bigger structure that affects the entire crust. The term ‘tectonic aneurysm’ refers to a medical condition where a weakness in an artery wall can cause serious medical problems as it gets bigger. My analogy of the overfilled sandwich with a cut in the top is more cheerful, but ‘tectonic aneurysm’ sounds much more sciency.

Egg or chicken?

The tectonic aneurysm model has also been applied to the Eastern syntaxis, at Namche Barwa in Tibet, where the Tsangpo river forms a deep gorge. These syntaxes are tectonic features, related to the corners of the Indian indentor, yet the aneurysm model regards a river gorge as thing that initiates extrusion. Why is there a connection between the two?

For me this is the most satisfying part of the model – the rivers flow across the syntaxes because of the geometry of the mountains – tectonics controls the location of the rivers which in turn influences the tectonics. Processes that affect the surface and those involving the entire crust are intertwined in a dance that lasts millions of years.

Consider the growth of the Himalayas. Rivers that used to drain off proto-Tibet into the Tethys ocean are now blocked by a mountain range, where India is pushing into Asia. These rivers start to flow parallel to and behind the mountains. Over time syntaxes develop and news rivers start to cut into them. Eventually these new rivers reach the old mountain-parallel ones and ‘capture’ them. The precipitation that has been stuck to the North of the Himalayas can now get through and the major river systems of the Indus and the Tsangpo-Bhramaputra can finally reach the Indian Ocean. At the ‘knick-point’ where these rivers drop sharply in altitude major gorges are formed. These ‘cut the sandwich’ and allow the weak hot middle crust to extrude out to the surface, forming some of the most fantastic scenery on earth.

The idea that patterns of erosion affect tectonics is a lovely illustration of the interconnectedness of the geosciences and is a current topic. A very recent paper in GSA Today by Paul Kapp and co-workers looks at wind erosion in the North of Tibet. They show significant rates of erosion and speculate a link with the (small-scale) tectonics of the area. It seems you don’t need big rivers (or indeed water) for the atmosphere to influence the way rocks deform.

The processes of crustal thickening that created the hot middle crust now extruding to the surface at Nanga Parbat affects all of Tibet. What happens to this soft material where it can’t reach the surface? Also are the syntaxes the only places the sandwich has leaked? Interesting questions. Someone should write a blog-post about them…

This post is part of my journey into the geology of mountains.