The deceptive simplicity of a metamorphic rock

I’d like to introduce you to a rock.


Pretty isn’t it? The white crystals caught my eye, as they did that of three different geologists of the British Geological Survey, who between them collected 5 different samples from the same small area of Scotland.

When did these crystals grow? How old are they? These rocks here are part of the Moine supergroup which started as a pile of sediments a billion years ago and the last geological event in this part of Scotland was a mere 60 million years ago, so there’s a wide possible range.

The first and easiest tool available to a geologist is to establish the age of something relative to other events. The white spots are potassium feldspar that grew when the rock was metamorphosed – changed from a muddy sand into something (even) more interesting. Metamorphosis is most often associated with geological structures. Minerals most often form because rocks are buried deep and heated and this squashes them,  flattening or folding the sedimentary layers and metamorphic minerals alike.


Yes the sea is azure blue and the beach empty. It was a fabulous week’s holiday.

In this picture we see a set of lines folded round which are original sedimentary layers. Our rock comes from the darker layer to the right. The white spots within it are roughly flattened along a plane that passes through the middle of the fold. This suggests they formed at the same time as the rocks were folded.

There is a little more detail to be seen in thin sections made by the state-funded geologists who have passed here before me1.

Image taken from BGS thin section image archive

Here we are looking down a microscope at the light that has passed through a thin slice of the rock – we are peering into its soul. The plain white areas are the feldspar crystals which we can call porphyroblasts if we are feeling fancy (meaning they grew as big crystals during metamorphism). Notice also the patterns made by the long and thin red-brown and grey mica crystals. There are little folds.


With more magnification (and in cross polarised light, so the minerals look different) you can see that the feldspars contain grains of other minerals that a strung out in lines. These are minerals that were swallowed by the growing feldspar and give a glimpse of earlier structures.

Here’s one interpretation of what is going on. An early alignment of the minerals was parallel to the sedimentary bedding. This was horizontal in our field of view. The feldspar grains grew over this fabric. Later the rock was squashed in a different direction, causing the folding we see in the outcrop and in the thin section. The mica grains are now mostly vertical with only a few areas staying flat. Some feldspar grains stayed put, but most have been rotated so that their long axes are vertical.

I’ve deliberately gone for the most simple explanation, but it’s a plausible one based on what we see in the outcrop. Two sets of squashing and one phase of mineral growth. Nevertheless its likely that we are not seeing the full picture here. The same package of rocks looks very different depending on where you are. About five kilometres west of where my rock came from, similar Moine sediments have vertical layers, but so little deformed that sedimentary cross bedding is visible.

Cross bedding in vertical Moine sediments

Cross bedding in vertical Moine sediments

Nearby there are folds, but these were formed when it was still sediment, the layers folding due to slipping of sand. You can tell this because the layers either side of the folds are flat.

Soft sediment deformation in vertically bedded Moine sediments

Soft sediment deformation in vertically bedded Moine sediments

Go ten kilometres east and the you are still in Moine sediments, but they are rather more intensely deformed and metamorphosed. Here the original sedimentary layers are stretched out into layers as thin as centimetre.

Intense folding in Moine sediments, near the Sgurr Beag thrust.

Intense folding in Moine sediments, near the Sgurr Beag thrust.

Clearly, it’s important not just to look at a single outcrop – which is where geological mapping comes in. This shows that these metamorphic rocks are part of a wide area over northern Scotland. This is unconformably overlain by undeformed sediments of Devonian age. So sometime between 1000 and 416 million years ago these sediments were heated and folded – that’s when the white crystals grew.

These are old techniques and technology marches on. Modern earth scientists, armed with sophisticated machines, scary acid and an understanding of radioactive decay are able to date the age of metamorphic events and even directly date the age of individual metamorphic minerals.

The Moine rocks of Scotland are well studied. Bring together hundreds of radiometric dates, highly detailed mapping and the study of thousands of outcrops and thin sections and you get a picture of almost terrifying complexity.

It turns out that the white grains in my rock sample with its apparently simple history could have formed in any one of at least five different occasions when metamorphic minerals formed in the area.  Each one of these represents a significant event – an ocean closing, an arc smashing itself into oblivion against an unyielding continent – yet somehow a single rock shows only a single part of this saga.

I’ll tell this complicated geological history, and why it’s not visible in a single outcrop in another post.

Categories: metamorphism, Scotland, tectonics

Stirring tales from the deep past.

My cup of tea is sitting nearby1, the rocket-fuel for the mind is sitting in a piece of man-made metamorphic rock and lying on the saucer is a humble object that bears mute witness to ancient, earth-changing events.


Tea in England is typically taken with milk and sometimes with sugar – lots if it’s “builders’ tea” – and a small spoon is required to add and blend the ingredients. These spoons can be made of silver or even gold. I’m not a Duke or a Prince, so mine are made of stainless steel.

Steel is an impure form of iron and has been made for thousands of years. The use of Archaeological periods (Iron Age succeeding Bronze Age) works because smelting iron tools is harder to do, but gives a better product than bronze. Strong and sharp iron tools are excellent for slicing through both fields and people. Successful Iron Age societies such as the Roman Empire were based on both swords and ploughshares, working together.

Iron-carbon phase diagram. From Wikipedia

Iron-carbon phase diagram. From Wikipedia

Much as with ceramics, small differences in the processes used can make a huge difference in quality of the end product. Molten iron easily mixes with carbon, which was originally introduced from the burning charcoal used in the furnaces. Mixing different amounts of carbon within iron results in different phases (with different atomic structures) – as the phase diagram above shows. This sort of diagram will be familiar to anyone who’s studied igneous petrology. It can be used to predict which minerals are produced in which order when a molten substance is cooled. Producing a molten mix of carbon and iron of the correct composition and cooling it at a rapid rate results in a fine-grained mix of different materials that is strong yet not brittle.

Our ancestors worked all the best forms of steel by trial and error over generations and in many different places. Getting hold of iron ore was never a major issue as iron is one of the most common elements on earth and beyond. It’s atomic nucleus is one of the most stable and a common product of stellar furnaces. Indeed the earth and other rocky planets are so rich in iron that as well as forming part of many silicate minerals in the mantle and crust, left over iron trickled down into the centre of the earth where it forms a dense magnetic core.

Banded Iron Formation. Source.

Banded Iron Formation. Source: James St. John on Flickr

Much modern iron comes from deposits that have their roots in a remarkable transformation of the earth- the “great oxidation event”. The early earth was a very different place with no oxygen roaming ‘free’ in the air – this reactive element was everywhere bound up with other elements. Iron at the surface was mostly in the reduced ferrous form of iron that easily forms compounds soluble in water. The first forms of life that lived via photosynthesis were tremendously disruptive. The Oxygen they produced never reached the atmosphere but quickly reacted with the surrounding seawater. Often it reacted with iron, changing it from ferrous into ferric iron that forms compounds that are not soluble in water. These same compounds are familiar to us as red rust.

Slowly, photosynthesising organisms turned dissolved iron into layers of iron minerals on the sea-bed. Over several billions of years, grain by grain, these bacteria produced huge volumes of iron-rich sediment. These banded iron formations are found in ancient rocks around the world and form the great iron-ore mines of Western Australia that have helped build modern industrial China.
Eventually the critters won. The earth ran out of ferric iron and the oxygen stopped reacting in the sea and started bubbling into the atmosphere, building a world, our world, that would in time support oxygen breathing animals who sip tea.

The iron in my spoon, was once transformed by the caress of ancient slime, but now it is nearly pure metal, strong and shiny. Surrounded by Oxygen, it is under threat – the Oxygen could bind with it again, undoing the smelting process and turning it back into rust. To counter this – to make it stainless steel – just over 10% of Chromium has been blended in. Chromium is not resistant to Oxygen’s charms either, but the resulting oxide does not let Oxygen pass through. This process of passivation means my spoon has a thin protective film over it, keeping it shiny even in the hostile toxic environment of my tea cup.

Chromite in Serpentinite. Source: James St. John on Flickr

Chromite in Serpentinite. Source: James St. John on Flickr

The Chromium in my spoon probably came from South Africa. It is a relatively uncommon element and places where it is concentrated enough to mine are not easy to find. The best place is in a large igneous intrusion formed from the melting of the earth’s mantle. The magma itself isn’t very rich in Chromium, but the biggest intrusions cool very slowly, which allows layers of different minerals to form. We’re still not exactly sure what the exact processes are, whether the minerals sink to the bottom of a pool of molten rock or form later at the slushy mushy semi-solid stage (probably it’s a bit of both). As the owner of a spoon we don’t need to know, just to be grateful that some of the layers are so rich in a mineral called Chromite they form a rock called chromitite.

There is a massive frozen magma chamber where this has happened in South Africa. Called the Bushveld intrusion it is rich in Chromite, and also most of the earth’s known reserves of Platinum group elements. It was formed from a million km3 of magma that was intruded into the crust 2 billion years ago.  To put that in context, that would fill the Baltic Sea fives times over. Four Bushveld’s worth of magma would fill the Mediterranean Sea with some left to slop over the side, making a hell of a mess. The Bushveld is an amazing thing, if a little overshadowed by the gold and diamond mines and the massive eroded meteorite crater that sit nearby.

Science and history isn’t just something that sits out there somewhere, in museums, or distant galaxies. It can be found within the most ordinary of objects. These stories aren’t just stories. We can’t identify which ones, but some of the iron atoms in my teaspoon were affected by the great oxidation event. The same atoms really were there billions of years ago. My choice of tea as a drink is not a simple act of will; I’m guided to it by global events hundreds of years ago. Look deeply into anything around you and there are amazing stories to tell.

Categories: geochemistry, Getting under the surface

Man-made metamorphic rocks

There’s a cup of tea next to me, steaming gently. I’ve already written about the history of the drink, how a Chinese herb ended up defining Englishness and having the power to create riots in Ireland. But what of the cup? It’s a posh one – not a thick heavy earthenware mug but a slightly translucent piece of porcelain, strong enough to be made into thin delicate shapes. Like the tea within it’s on my desk thanks to early modern globalisation but it is also a type of metamorphic rock, of anthropogenic facies1 as you shall see.

The cup comes from my granny’s ‘China tea set’. In England, fine elegant pieces of porcelain are associated with China, as that’s where sometime between the 7th and 9th Centuries porcelain was invented. It soon made it’s way along the Silk Road through Central Asia to the sophisticated cities of the Islamic world2. Poor and backward Europe didn’t see much porcelain until we worked out how to bypass the Silk Road by ship.

The ships that started the trade in tea with China in the 16th Century, sailed East laden with silver bullion as this was the only thing the Chinese wanted from the West. The silver itself was mostly mined from the South and Central American mines run by the Spanish, but that’s another story.

Ships that swapped silver for tea and silks had a problem. These Eastern luxuries are light and the ships were designed to sail best when sitting low in the water. Needing to add weight – ballast – they shipped back something heavy that they could sell in Europe – Chinese porcelain. Starting with royalty a taste for ‘fine China’ spread across Europe from the 16th Century. In a pleasing symmetry the finest examples were, in Europe, as valuable as silver. This beautiful strong translucent material was used to make plates, bowls, cups, tulip holders….

Porcelain is not the same as ‘earthenware’ or ‘stoneware’ that Europeans were already familiar with.  Take most forms of clay and bake them at high temperatures and they form some sort of pottery. Clay minerals have a platy layered structure dominated by a lattice of aluminium and silicon oxides. Water is always to found between these sheets3 but also a wide variety of other elements. Clay minerals form during weathering of other rocks – where water and time break up tidier mineral structures that formed deep in the earth but are less suited to existing at the surface.

When heated, clay minerals break down. The water hidden inside them becomes keen to escape, driving chemical reactions that build new minerals. When a clay-mineral rich rock is buried in the guts of a mountain range, it is transformed into a metamorphic rock – a schist say – where aluminium rich metamorphic minerals grow – micas, Garnet, Staurolite and many more. If you’re really lucky the rock will partially melt, resulting in an migmatite. Taking clay and baking it in a kiln is the same process – a human created (anthropogenic) form of metamorphism.

Human metamorphism is much quicker than the natural form and at much lower pressure, but occurs at a much higher temperature (>1200°C for porcelain). The minerals and textures that form are therefore different, notably the grain size is much smaller. There is also partial melting of some minerals in the clay, which helps to bind the material together. In standard pottery, the clay contains a mixture of different clay minerals so a variety of new minerals form, giving it a generic brown sort of colour.

When Europeans first encountered porcelain, it was like nothing they’d ever seen. Strong, fine and translucent, with a pure white colour. Almost immediately they attempted to copy it. Early attempts involving ground glass and ash from bones. This soft-paste porcelain had the desire translucent quality but was softer. Only in the 18th Century did Europeans manage to produce the real thing, first in Dresden in Germany and then in France (Sèvres) and Britain.

The secret was to start with the right ingredients – nearly pure quantities of a particular clay mineral called Kaolinite (found in kaolin, or “china clay”), mixed with a common mineral called feldspar. Kaolinite – Al2Si2O5(OH)4  – forms from the breakdown of feldspar under the action of hot water. For all three of the early European sites of porcelain manufacture, the kaolin came from granitic rocks4. The English deposits in Cornwall are still being mined- here soon after it formed, the cooling granite pulled in groundwater, heated it and pumped it around, rotting itself from the inside.

China Clay pits visible from Space in Cornwall. Image from Wikipedia

China Clay pits visible from Space in Cornwall. Image from Wikipedia

Just like the progression from mudrock to slate to schist to gneiss, the metamorphic process to form porcelain has various stages5. First the water is driven off leaving a disordered material called metakaolin (Al2Si2O7). Between 900 °C and 1000 °C a new mineral phase forms, with a spinel structure. Above 1050 °C the mineral Mullite (Al6Si2O13) forms. This mineral was first found in nature on the Scottish island of Mull6 where small quantities of muddy rock were engulfed in lava and so heated above 1000 °C – nature’s own failed attempt to make porcelain. Within baking porcelain, Mullite initially forms in the shape of plates (platelet Mullite) but above 1400 °C the minerals start forming in the shape of needles. Plates can slide against each other, but needles cross and interlock with other, so this final change to the shape gives porcelain its great strength.

These are tremendous temperatures – it’s rare for rocks to experience such temperatures (except for within the deep earth) and Mullite is unusual in thriving under such conditions. Hessian crucibles were containers prized by alchemists and early chemists across Europe for their ability to survive whatever flames and chemicals were inflicted on them. Made to a secret recipe, only recently was it discovered that they are rich in Mullite.

Alchemists were in the business of finding miraculous transformations. The person credited with first discovering the secret of porcelain in Europe, Johann Friedrich Böttger, the “porcelain prisoner” was an alchemist imprisoned and instructed to turn lead into gold. After 14 years, having discovered a different form of profitable transmutation and brought the Meissen porcelain factory into being, he was finally released.

What to our ancestors seemed miraculous we take for granted. Let’s take a sip and pause to thank those people – Chinese and European – whose hard work lets us transmute mud into practical elegance.

Categories: Getting under the surface, History, metamorphism

All the tea in China

“Is there any tea on this spaceship?” Arthur Dent, the Hitchhiker’s Guide to the Galaxy.

I’m sitting here with a steaming cup of inspiration at my side. An Englishman drinking tea from a porcelain cup stirred by a spoon: what could be more ordinary than that? But dig carefully into what led to this scene and you can find stories of magical transformations, global trade, empires and planet-changing events billions of years ago. Let’s start with a taste of the history of tea, the enriching elixir, the healthy pause that refreshes.
The tea bush, isn’t native to Britain1; when and why did this country get into the habit of putting its leaves into hot water?

english tea

Mick Jagger and Paul McCartney discuss whether to put the milk in before or after the tea

Tea has been drunk in China for thousands of years – the plant Camellia sinensis is native there. It’s leaves are rich in caffeine, most likely to reduce damage from insects or associated fungal attacks2.

Caffeinated drinks first made a big splash in Europe in the Seventeenth Century. Coffee, sourced from the Middle East was drunk at ‘coffee houses’ alongside chocolate from the Americas and tea. Processed tea was brought direct from China by sea around Africa, a route opened up by the Portuguese the century before. Though still only available to the elite, coffee houses had an impact in many ways. As a semi-public space, typically with newspapers to hand, they were a venue for political activity: Charles II of England wanted to shut down London’s coffee houses as they contained “idle and disaffected persons” who “produced very evil and dangerous effects”. A great public outcry ensued and, not the most powerful of monarchs (his father had been executed by Parliament only 26 years earlier), Charles backed down and the seditious sipping continued. Insurance titan Lloyds of London started life as a coffee house3 around this time. Meetings of the early Royal Society, (attended by the guys who now have scientific Laws named after them: Newton, Hooke, Boyle and others) often finished up in Coffee Houses.
It’s tempting to make a link between the dramatic changes associated with this Early Modern period in history with the sudden infusion of caffeine. To drink safely in urban areas without modern sanitation you need a drink made from boiled water. Before tea and coffee became popular, this was often an alcoholic drink, wine or beer, weaker than we are used to today. It’s tempting to think of coffee and tea jolting the modern world awake one sip at a time.


For the next 150 years tea’s popularity in the west only grew. In the period leading to American independence in 1776, the British Government’s desire to monopolise (and tax) the supply of tea to its American colonies led to the Boston Tea Party and tea-drinking being seen as unpatriotic. The Tea Act, that so annoyed the patriots of Boston was passed to help the hugely influential British East India Company, who also had a monopoly on supplying tea to Britain. This, plus high taxes led to high rates of smuggling of tea (and other goods) from France via the England’s southern coast.

Tea is native to India, particularly in Assam, but it’s leaves were seen locally as a herbal medicine rather than a way to make a drink. By the 1830s the East India Company (who by now had control over most of the Indian sub-Continent) were looking for an alternative source of tea than China and recognised a suitable climate in Assam4. Initially plantings were made of transplanted Chinese plants, but the native varieties fared better. Tea plantations spread, soon taking in the area near Darjeeling and eventually spreading to Southern India and Ceylon (modern-day Sri Lanka). All these names are now also varieties of tea. By the early Twentieth Century tea drinking became popular in India itself, characteristically as milky, sweet and spicy Chai5.


By the second World War, tea was so ingrained in British culture that a regular supply of tea was seen as the second most important factor in troop morale, behind actually winning and ahead of food6. The UK government took control of world supplies of tea 7 – and assigned it first to troops, then to home front, then the rest of world. This caused problems for the Irish republic, whose tea habit was at least as strong as Britain’s and whose trade mostly passed through the Empire it had freed itself from 20 years before. According to an not entirely impartial history of the time8: “The Irish government, no doubt fearful of a mass revolt in the event of tea shortages, set up a private limited company, Tea Importers (Eire) Ltd., in order to start importing tea directly from country of origin” (emphasis mine).

Tea drinking is now in decline, with Britons seduced by the delights of espresso and cappuccino, but new delights such as green or white tea are now available. Tea’s global spread was part of the Western Imperial phase of globalisation – unequal trade enforced by gunboats. The current phase of global change is in many ways more equal: major UK brand Tetley Tea is now owned by Indian conglomerate Tata following an amicable takeover.

Those of you hoping for more Geology will be pleased by the next posts in this series, when I turn to the cup and the tea-spoon.

Categories: Getting under the surface