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

Wooden layers through time

As I chopped my first tree down it was wonderful to realise that – of course – counting the rings would tell me how old it was. Traversing through the layers of wood and so through time is one of the ways in which trees stimulate the imagination. As with wood, so with woodland.


Woodland in the Summer

Layers of life

The leaves lying on the ground fell only a few days ago, transforming the appearance of the woodland. The tops of the high beech trees are no longer shrouded in deep green mystery, their bare crowns are suddenly visible. By contrast the evergreen conifers seem almost luminous, dangerously catching the eye of a new woodland owner keen to make his axe a bit less shiny.

You always know what time of year it is in an English wood. The annual cycle of deciduous trees – winter starkness alternating with the lush privacy of summer – is matched on the woodland floor. Beech woodlands in the Chiltern hills of southern England are renowned for the spring flush of bluebells, as these and other spring-flowering plants make the most of the returning warmth in the brief window before the trees come into leaf and plunge the ground below into a dappled gloom.

Moving our sights up to the layer above we enter the world of saplings and small trees. Here they grew during the last 30 years, when very little happened to this woodland1. Some areas have a spread of native species: beech, oak, wild cherry, birch, rowan and doubtless others I’ve not spotted yet. Elsewhere flocks of fluffy evergreen western hemlock spruce saplings have spread wide beyond the base of their mother trees.

IMG_20150808_131817 (1)

Western Hemlock Spruce

Many woodland owners see these as alien intruders, to be quickly exterminated. The bad reputation non-native conifers now have has a reasonable basis – shed needles acidify the soil and native wildlife can be baffled by the unfamiliar habitat. Four legged invaders such as grey squirrel and edible dormouse also roam the area, stripping bark and damaging trees. But arguments framed in terms of ‘alien invaders’ swamping the ‘natives’ who truly belong here are obviously bogus and repellent when applied to people. Is it really that different for trees? Why not view these saplings as second generation immigrants, adding variety and interest to their new home?


Nature versus nurture

Trees aren’t people of course, which is why it’s OK for me to attack them with an axe. In doing so, I’m part of a long tradition. These woods have been used and managed by humans for hundreds, probably thousands, of years. The large evergreens (larch, spruce and western red cedar) were planted during the ‘locust years’2 the period after the second world war when ‘scientific’ management was applied to British woodland. During this time ancient trees were cleared or even poisoned and great numbers of fast growing softwood trees planted, to be managed on an industrial scale. This caused great damage to wildlife, not purely because of the species chosen. Old gnarled half-dead trees are a great habitat, and the new trees were planted for timber, meaning close together so they grow straight. In the gloom below, little grew.

So did this invasion of aliens destroy a primeval forest, in tune with nature? Not at all. The North American concept of old growth forest simply doesn’t apply on this busy little island. For several hundred years the traditional beech woods of the Chilterns were managed for the furniture trade, especially here, near to High Wycombe. The ‘last bodger’ Owen Dean worked close nearby. Bodgers were craftsmen3 using traditional tools to make furniture within the woodland itself. The grand beech trees towering up in the canopy above me now were grown for timber and so lack side branches for many metres. These same trees are seen as slender youths in pictures of Mr Dean from the 1950s.

Down into the past

To go further back in time we need to look to the layers below. The soil will contain some trace of older vanished trees, even if we cannot read it directly. The top humus-rich layer is the result of hundreds of years of leaf-fall, never ploughed and rarely dug. The mushrooms that dot the forest floor in Autumn are just minor decorations on top of the mycelium, the fine fungus filaments that thread round roots and through rotting matter. Here is a potential continuity, providing a link – even if only an imaginative one –   to the time of Shakespeare when the Chilterns were a major source of wood fuel for the nearby city of London.

This is an ‘ancient woodland’ site since we know if has been constantly wooded since at least 1600. Before this time we can only guess what grew and how it was used. Before oak trees were selectively removed to build the sinews of empire (bark for tanning leather, beams for warships) and beech favoured for furniture this was likely a mixed deciduous woodland made up of species that crept north into Britain after the end of the last Ice Age.

Humans were here too, for thousands of years. At some point a person sat on a stump to knap flint – banging pieces of this glassy stone together to make a sharp edge. I’ve yet to find a tool – an arrow head to kill deer perhaps, or an axe to chop firewood – but some of the useless discarded chips were in the first upturned tree I found.


The subsoil here is called ‘clay with flints’ which while descriptive is a bit of a cop-out of a name. It is a thin layer found on the very top of the chalk hills that make up the Chilterns. It’s basically the geology of southern England in minature: after the chalk it’s just about different ways of rearranging piles of flint. Soft soluble chalk contains a lot of hard flint – at first some thought clay with flints was simply the residue of the chalk dissolving over millions of years. But it’s likely there some trace of the younger marine sediments in there as well, all mixed up by the churning of the frozen ground during the Ice Ages (the scouring ice sheets never quite got this far).

The layers below the chalk are hidden, but will include Jurassic rocks – which further east are the source of North Sea oil deposits – and perhaps Carboniferous rocks – which to the north are rich in coal. Man’s interactions with these deeper layers brings us back to surface and to the future.

Back to the future

Think about woodlands and you keep coming back to cycles – the diurnal creeping of birds and animals, the annual dance of leaves and buds, changing fashions of woodland management over the generations. The biggest and most important is the carbon cycle. Across the world, ancient forests are being dug-up and burnt, releasing fossil carbon into the air. For the moment this is giving my trees a little boost, the extra carbon dioxide having a fertilising effect. In time though – probably in my life time – changing climate will make some of my trees deeply unhappy, through drought or storms or flooding or new pests or diseases.

One of the main joys of owning this little patch of woodland – other than hitting things with an axe – is the opportunity to plan what it will look like in 30 years time and then see it come to pass (touch wood). I’m still thinking the details through. The main goals – encourage wildlife, a source of firewood, a place my family can play outdoors – are clear.  But the details – plant this, cut these down – are not. For me getting from one to the other means understanding all the layers of the wood, its past, its present and its likely future. It’s a process I’m enjoying hugely and the chances are I’ll write some more about it here. I hope you’ll be interested in what I have to say.

Categories: England, woodland

North American Arctic – icy beauty

Look at this. As an abstract pattern, it looks like something Gustav Klimt might paint.

But drill down into it in more detail and it changes into an uncomfortably close view of a reptiles skin.

All images in this post come from the North American Arctic – a place made beautiful and strange by ice. Conditions are so cold that the soil layers are almost permanently frozen. The rare occasions when it melts warps the ground in various distinctive ways.

Take the first image – the striking elongated light-blue lozenges are lakes. Lakes may form in the Arctic in areas called thermokarst. This a landscape full of hollows formed when patches of permafrost melt, causing hollows in the ground. These elongated examples are unusual. No-one knows for sure why these “oriented lakes” form, but they are often aligned with the prevailing wind, suggesting it has a role to play.

Here are some other examples, which seem to have edges with two sets of directions, making some of them look like badly-drawn hearts.

The lizard skin pattern above is known as “patterned ground” forms where soils regularly freeze and thaw. The periodic expansion of ice rearranges the soil and in the case of these polygons, wedges of ice may form in a regular pattern.

These processes occur in ‘peri-glacial’ environments. The term literally means “around glaciers” but it occurs over vast areas of the Arctic that are too near sea-level for glacial ice to build up. It can also occur at height in milder climates such as the Cairngorms in Britain or on top of African volcanoes (or even on Mars). During the colder parts of the current Ice Age, when large areas of the world were covered by ice sheets, the areas to the south of the ice were often peri-glacial. Ice-wedges and other peri-glacial features are relatively common in southern England where I live.

Pingoes are classic peri-glacial feature. In England these are round lakes formed when plugs of ice melted thousands of years ago as the climate warmed. In the Arctic they are 30-50m high hills with a core of ice – the name is Inuit for ‘small hill’. He is an example that is still a hills in Canada (note the patterned ground to its south).

These peri-glacial features are best seen near to rivers that flow into the Arctic. Further south, vast areas have very little soil, having been scraped clean by vast ice-sheets. One advantage of this, from my point of view, is that the geology is extremely well-exposed.

Here on Bathurst island in the Canadian Arctic, open folds in the some Devonian sediments are beautifully clear, complete with thickening of layers in the fold hinges.

These crazily-shaped islands in Hudson Bay are relics of folding deep within the earth. Imagine walking along those thin islands!

I’ll end with my favourite trace of ice in Canada.

The ghostly marks hiding under productive land in southern Canada were produced in a vast lake that formed as an ice-sheet melted. Ice-bergs floating in the shallow lake scraped along the lake-bed, leaving these ‘keel-marks’.

Categories: Glacial, Great Geology in Google earth, landscape