Man-made metamorphic rocks

Posted on 30 March, 2016 by Metageologist

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.

Source

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.

Kaolin

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

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.

Andalucia: a history of stuff

Posted on 2 February, 2014 by Metageologist

Andalucia is a province in Spain, at the far south west of Europe. Its long and varied human history has seen it linked to the middle East, north Africa and the Americas. The creation of these links brought new foods, metals, diseases: new stuff into Andalucia. Sometimes the impact of arrival created ripples that reached out far across the world.

Geological history

Some 200 million years ago Andalucia was within a massive continent called Pangea, close to both Africa and North America. Slowly Pangea broke into pieces, a new ocean basin – the Atlantic – filling the gap as the Americas drifted away. Some 50 million years ago, Africa was pushed north in Europe creating a long mountain range. Andalucia was part of this. Mountains form when the crust is thickened, pushing rock into the sky. A similar process at the base of the lithosphere 1 also forms a thick balancing ‘root’. Under Andalucia, this thick root ‘fell off’2 and sank into the hot convecting mantle beneath.

Mountain belts are surprisingly fragile. The sudden removal of the heavy root caused the over-thickened crust to collapse, flowing sideways and bringing deeply buried rocks up to the surface3. Fragments of mantle called peridotite, not often seen at the surface, form brown mountains around the town of Ronda. The collapse of the mountain belt went so far that its centre is now the very western part of the Mediterranean, the Alboran Sea. The mountain became a great hole in the ground.

Most of Andalucia is made up of sedimentary or metamorphic rocks folded and twisted by these dramatic changes. An exception is the basin of the Guadalquivir river. Here the weight of the collapsing rocks pushed down the rocks to the north, making a depression that has filled with recent sediments – a feature called a foreland basin. The edge of this flat basin makes a clear line that is easily visible from satellite views of Southern Spain.

From Wikimedia http://upload.wikimedia.org/wikipedia/commons/5/53/Andalucia_satelite.jpg

Andalucia from Space. Image from Wikimedia

People

People, at first hominins such as Neanderthals, have lived in Spain for over a million years. The little we know of prehistoric humans comes from their use of materials. First their gradually more sophisticated use of stone tools, then from about 5000 years ago the smelting of metals: first copper, then bronze (copper plus tin) and then iron. Mines in Andalucia have been been involved since the start, notably the Rio Tinto area. Here a Carboniferous massive sulphide deposit has yielded silver, gold and copper and spawned a global mining company.

Rio Tinto mines, Andalucia. Image from David Domingo on Flickr under CC

Around 3000 years ago (1100BC) the Phoenicians reached Andalucia, founding the town of Cadiz. A culture that reached across the Mediterranean they were also involved in trade with the British Isles. Tin from Cornwall in England was smelted with Spanish copper and the resulting bronze traded on. The olive tree reached Spain at this time, brought from the eastern Mediterranean.

A mere thousand years later, the Romans took control – their province of Hispania Baetica covers much of modern-day Andalucia. They introduced deep mining to the Rio Tinto, using the characteristically Roman combination of slaves and very big wheels to pump water up from the depths. Andalucia was a renowned source of many products for the wider Roman Empire, including silver, olives, emperors, philosophers, dancing girls, and garum, a sauce formed from fermented fish guts rich in umami flavours.

Yet Moor invaders

After the slow collapse of the Roman Empire, the next major influx of change in Andalucia was the Moorish invasions. The Islamic Moorish army4 conquered most of Spain between 711 and 718 AD, in time creating a kingdom of Al-Andalus with its capital in Cordoba. Once more Andalucia was part of a multi-national empire with good trade links. Valuable crops from further east, such as figs, citrus and pomegranate were introduced for the first time, as were sophisticated irrigation systems, some of which are still in use. By the 10th century, Cordoba was the most civilised city in Europe, it’s Grand Mosque was one of the wonders of the Muslim world.

The site of the Grand Mosque was originally a Christian cathedral (before that, a Roman temple). Built over 200 years and funded in part by mining proceeds, the Mosque was built with local stone and brick, but also recycled Roman stone columns. Some of these were found locally, but others were brought in from much further afield, from the rest of Spain and perhaps wider.

The Columbian exchange

Eventually Moorish Spain came to an end as Christian rulers conquered the Muslim lands. The final Moorish kingdom of Granada fell to Ferdinand and Isabella in 1492, the same year that they sponsored Christopher Columbus to mount an exhibition across the Atlantic. As the Spanish kingdoms turned into a global Spanish empire, lots of incredible things started flowing back into Andalucia. First Seville and later Cadiz were the main ports for the Atlantic trade. Gold and silver beyond imagining passed through Andalucia – enough to create a century of inflation across Europe. Some of this stuff ended up in the the Grand Mosque in Cordoba, which is now a cathedral again. Sitting within the vast pillared area of the mosque is a Christian church full of beautiful things made of American gold and silver. The choir stalls are made of American mahogany – lots of plant material crossed the Atlantic too.

Ingot of South American silver as brought over by Spanish treasure ships. Cadiz Museum.

A popular Spanish dish is called ‘patatas bravas’ and consists of potato, tomato and chili – all foodstuffs that spread across the world following the ‘discovery’ of the Americas. Andalucia was the first stop for many of these vegetable treasures. Botanical gardens turned seeds into plants, to be studied and propagated. A fine building in Seville is the old tobacco factory, dedicated to processing another new crop. The setting of Bizet’s opera Carmen, its walls are mottled with yellow and brown, like a smoker’s fingers. More immediately bad for the health, syphilis was first recognised in Europe in 1494, most likely brought back by Columbus’ sailors.

The British dimension

As an Englishman who likes history, I often visit other countries in an apologetic mood. A dimly remembered story about Francis Drake daringly ‘singeing the king of Spain’s beard’ is rather less jolly when you are sitting in Cadiz, the town that was attacked. Drake was engaged in warfare on behalf of his Queen, but also behaving like a pirate, raiding Spanish treasure ships. Still, no one seems to mind any more; it was in 1587 after all.

The British drink everything and anything. Not content with home-grown beer, gin and whisky we also crave grape-based booze. When Francis Drake returned from attacking Cadiz, he brought 2,900 barrels of sherry, a type of wine made only near Jerez in Andalucia. This went down rather well – we’ve been drinking it ever since, even getting involved in its manufacture.

Tonic water is sweet fizzy water flavoured with quinine, best taken with gin. Quinine, an extract from a South American tree was for centuries the only effective way of countering the effects of malaria. First popularised by Spaniards returning from Peru it was introduced to gin by the British in India. In the early 18th century the Royal Navy had a number of bases on Spanish soil, including in Andalucia, and passed the habit on. It remains popular in Spain to this day.

Image from Amanda Slater on Flickr under cc

A final anglo-Spanish connection is marmalade. Seville is full of orange trees, of a particular kind, bitter and rich in pectin. They are pretty inedible raw, but for some reason 5A Scottish tradition and industry grew up of preserving them as jam, with pieces of the skin floating in jars of pungent and yielding orange delight. Paddington Bear, James Bond and Alice in Wonderland all eat marmalade, along with many real people, such as me. The bitterness gives it a very grown-up feel, with some of the grimy delight of cigarettes and whisky, but in a healthy breakfast-friendly form.

The future

What of the future? Recent research suggests that soon6The oceanic crust beneath the Atlantic will start to plunge down under Spain into the earth’s mantle. The collapse of the mountain belt I mentioned at the beginning left a tear in the crust and this may grow and extend into a full-blown subduction zone. This will bringing volcanoes to fertilise the soil with ash and earthquakes to shake the buildings (if any remain). As slowly as finger-nails grown the Atlantic will vanish and Spain and the Americas will be reunited once again.

Winds of change

Posted on 13 January, 2014 by Metageologist

There’s a fabulous new site that shows wind patterns – it gives you a whole new perspective on the globe. One of the most striking things is the regular patterns across the oceans. Until quite recently long-distance travel was dependent on sailing boats, at the mercy of the wind and regular patterns of wind were needed to support regular sailing routes. The goods and people moved by these winds in turn drove dramatic historical changes that still resonate. Let’s look at a few, illustrated by the Earth Wind Map.

Turn of the sea

In the early 15th Century, Portuguese sailors began exploring the Atlantic ocean. Initial footholds were made on groups of volcanic islands: the Azores and the Canaries. Regular travel between these islands and Portugal taught sailors an odd trick: the best route back home was not a straight line but instead a big loop. This ‘volta do mar’ or ‘turn of the sea’ relied on working with the prevailing winds and ocean currents.

Atlantic winds (green), currents (blue) and approximate Portuguese sailing routes (red). Image from Wikipedia.

Here’s how the Earth Wind map looked for this area recently: the same pattern can be seen even in this snapshot of actual conditions.

Image captured from Earth Wind Map. With permission.

Later sailors showed this pattern to be of global significance. It stretches across the entire Atlantic – Columbus used it on his return from the Americas. Eventually Spanish sailors correctly guessed that it applies to the Pacific, allowing the Spanish empire to connect Central America and the Philippines.

These patterns are caused by global circulation of the atmosphere. Warm air rises at the Equator and flows up and towards the poles. At around 30° latitude (north or south) it descends again into a region of high pressure called the Horse latitudes. Further north a different circulation cell is found, forever whirling in a different direction. These flows of air, on a spinning globe, create patterns of prevailing winds: ‘trade winds’. These patterns are fundamental features of the earth and have been recognised in ancient climates.

Spice

The Portuguese weren’t the first people to sail long distances, of course. Trade across the Indian Ocean between Africa, the Middle East and India was established long before; it supplied the Romans with spices as well as lions and tigers for their circuses.

The northern Indian, like the Atlantic at the same latitude, has trade winds that move towards the south west. This is convenient if you want to speed your cargo of hungry tigers from India over into the Red Sea and via Egypt to Rome. But how do you get a cargo of gold coins to India to buy the animals in the first place? Unlike the Atlantic, the horse latitudes are on-land: there is no turn of the sea to get you east.

The winter pattern of winds, away from India

In the Indian Ocean something remarkable happens. The billion year old trade winds are switched, flipped completely round every year. From May until October winds blow from the south and west, towards India. This pattern allows repeat journeys and supports trade. In later centuries these winds sculpted a maritime ‘Dhow culture’ that stretched from the East African coast via Arabia into India.

What has the power to overturn a global pattern of wind? The Himalayas and the Tibetan Plateau do. This massive area of high altitude land was formed and is kept aloft by the ongoing collision between Indian and Asia. During the summer, heating of the high land lowers air pressure, drawing in air from the ocean. This reverses the pattern of winds and brings massive rainfall to India, feeding crops that feed millions.

Sugar, gold and blood

European exploration of the Atlantic of course led to the ‘discovery’ of the Americas, a new land full of Silver and Gold and indigenous civilisations collapsing under the onslaught of vicious diseases to which they had no immunity.

For many, the question was: how to make money from this new land? For the Spanish in central and south America, silver and gold was the obvious answer. The Portuguese in Brazil and the English and others in North America and the Caribbean turned to farming, preferably of addictive substances.

Sugar, cotton and tobacco could all be grown in the Americas and shipped west to newly addicted populations in Europe. But who was going to grow the stuff? Shamefully, the answer was slaves from Africa, sold in exchange for rum or textiles made in Europe (from ingredients grown in the Americas). Part of the reason this system worked was to do with the winds. The ‘turn of the sea’ was scaled up into the ‘triangular trade’ that took ships from Europe, to Africa, to America and back again.

In time Europeans realised the full horror of this system. The world’s first consumer boycott was of sugar, first organised in Britain in 1792. They realised that not a cask of this slave-grown sugar came into Europe “to which blood is not sticking”.

Tea

What the British mostly did with sugar was put it in their tea. Until the mid-19th Century, drinking unboiled water in a British city was a good way to die young. The traditional solution to this was to drink beer or gin – both sterile, if not completely healthy. In time, powered by growing commercial power in Asia, tea replaced booze as the British drink of choice (at least during working hours). This thirst drove the last great flowering of commercial sailing ships: the tea clippers.

Tea is a seasonal crop and the best tea is fresh tea. For these reasons fabulous beautiful ships were built to speed it across the world. The great tea race of 1866 saw four sailing ships race from China to Britain packed with tea. The first ship to arrive would command the best prices. The route they followed was the fastest possible, drawing on hundreds of years of knowledge of the trade winds. Let’s trace their winding route on a windy globe.

From China, down the South China Sea through the Sunda Straight

Across the Indian Ocean and round the Cape

Back to Blighty, past St Helena and the Azores

What’s in a (geological) name?

Posted on 8 October, 2013 by Metageologist

Devil’s toenail

The Earth Sciences are often about bringing order to the wonderful overflowing complexity of the natural world. The geological way of doing this is often to classify. A hill-side of rocks, often a bewildering mess to an amateur (or first year student) can with experience be tamed. It can be mapped as a series of rock units, made up of rock types containing minerals. It may contain fossils that can be assigned a species which may reveal which period of geological history it was formed in. All of these things have names.

Some of these names shed light on the history of the science or the difficulties of classification. Some are poetic, some generate schoolboy sniggers. Let’s meet a few.

Minerals

Common minerals were first named by non specialists. Quartz comes from an old root word meaning hard, Feldspar is the German for ‘mineral of the field’ and garnet is from an old french word meaning ‘dark red’. However most are named by scientists, after mineral properties, places or people.

Some names are the victim of changing language use. Welsh and Dick are normal names, likewise the town of Cummington doesn’t cause surprise. However the habit of adding -ite to the end of mineral names has created much amusement for students learning the properties of welshite, dickite or cummingtonite.

James B. Thompson, Jr postulated a whole new way of stacking structures in silicate minerals. When examples were found, in his honour they were given the awesome name of jimthompsonite and even better, clinojimthompsonite: my favourite mineral names.

Dead things

There are manymany fossils and so lots of names. As a student I was powerfully of the opinion that there were too many. My favourite name? Fossil bivalves called Gryphaea have the old popular name of “Devil’s toenails“.

Time

Let’s start with the Geological Periods:

(Pre)Cambrian – latin name for Wales
Ordovician & Silurian – latin names for Celtic (Welsh) tribes
Devonian – English country of Devon. People from Devon sometimes refer to themselves (correctly) as Devonian, even when they are younger than 400 million years old.
Carboniferous – ‘coal-bearing’ in Latin. The industrial revolution was ignited by burning coals from rocks this age
Permian – ancient Russian kingdom of Permia
Triassic – tri = three. In some areas has three distinct layers
Jurassic – Jura mountains, just north of the Alps
Cretaceous – Latin for ‘chalk bearing’
Tertiary/Quaternary or Palaeogene/Neogene/Quaternary/Anthropocene – lots of Latin

Personally, the whole Latin thing leaves me cold. The names I like best are where another language comes in. Take the original British subdivisions of the Ordovician: Tremadocian, Arenig, Llanvirn, Llandeilo, Caradoc and Ashgill. These are mostly based on Welsh placenames. Even pronouncing them correctly is a challenge for a non-Welsh speaker.1

The Geological Periods were named by British Victorian gentlemen and reflect their preoccupations (latin, the Romans) and their focus on European geology2. Further subdivisions of time have been changed over the years to better reflect global geology. One major purpose of dividing geological time is to assign ages to rocks. Changing the detailed subdivisions to match specific events (the first appearance of fossil x) makes sense as does naming the new subdivision after areas that contain good sedimentary sequences of this age.

Following this practice, most of the Welsh Ordovician names above have been replaced, by names based on Swedish, Chinese, Australian and American locations. The same applies across the geological timescale. It’s hard not to see this change of names as also tracking the flow of power and scientific innovation around the globe, away from Europe.

Pieces of rock

Rocks mean different things to different people. To a kitchen fitter or an architect, marble and granite are very general terms for hard and soft polishable rocks. Geologists are more precise. Even geologists are precise to a different degree -a rock that might reasonably be mapped as a ‘granite’ might properly be described as a granodiorite.

A type of rock may have a whole series of names. Coal-bearing rocks of Carboniferous age often have a layer of pure quartz sandstone beneath the coal. To geologists an orthoquartzite or arenite this rock was known to miners as a ganister or seatearth, and was used to line furnaces. It formed as an ancient soil horizon so it could also be called a palaeosol.

My favourite rock name is pseudotachylite, which means roughly ‘fake volcanic glass’. It forms when movement along deep fault zones heats rock so much that it melts. It’s other obsolete name is ‘flinty crush rock’ which would be a good name for a musical sub-genre.

Packages of rock

On large scale, geologists identify packages of rock and give them names. Supergroups, groups, formations, units – there are a lot of names for the different types of packages – and countless examples. Packages tend to be named after the place they are best exposed – they have local character.

As a monoglot anglophone (who likes technical terms) I treasure every exposure to non-English words. As a British geologist who had an Irish field area, I’ve heard a lot of gaelic places names, both Scottish and Irish. The Bennabeola Quartzite, the Ariskaig Tillite, the An-t-sron and Ghrudaidh Formations, the Currywongaun Gabbro. Mmmm, close my eyes and I can taste the Guinness (or whisky).

South Africa is a complicated place, with place names based on English, Afrikaans and a variety of African languages. Rock package names pick up this diversity. The ancient Barberton rocks contain the Moodies, Onverwacht and Fig Tree Groups plus the Schoongezicht Formation. The Pongola Supergroup contains the Sinqeni Formation, which contains a Zulu click-sound in the middle that takes a lot of practice to master.

The most poetic set of rock unit names I know come from Yosemite Valley in California. Diorite of North America, Dikes of the Oceans, Tonalite of the Gray Bands – they are the most evocative names for a bunch of granite that I’ve ever heard.