The west of Ireland: a geological journey

Posted on 23 January, 2013 by Metageologist

The west of Ireland is a special place. During the Celtic Revival, a literary and political movement spanning the 19th and 20th Centuries, it was seen by many as the ‘true’ Ireland. Haunted by the ghosts of the Irish potato famine, it’s gaelic-speaking communities were taken as a template for a future country freed from English interference. W. B. Yeats, poet and follower of magick, sought inspiration here. Other great figures of world literature, James Joyce and Oscar Wilde were influenced by the Celtic Revival, even if only as something to react against. Today, the remarkable nature writer and artist Tim Robinson lives and works in the ‘the West’, creating fabulous prose from his desire to know everything about tiny pieces of the land.

This is not a post about about Irish literature or history, much as I would love to write about the fascinating way they intersect and interweave. But these are deep waters, full of traps for the unwary Englishman and the amateur alike. But writing about the geology of the west of Ireland is something I’ve been trained to do.

The west of Ireland is dominated by the Atlantic. If you live there you learn to look towards the Ocean to see how soon the next shower is coming. Places like Achill Island feel like the prow of a battleship in stormy weather. Massive cliffs are moderated only by beaches covered in car-sized stones, shingled by the storms. On a typically brisk day, a car parked a mile inland becomes covered in sea-spray.

I have a big geological map of North America (you may have it too). Stretching over to include Iceland and Greenland, the eastern edge cuts only a tiny area of Europe – fragments of the west of Ireland seemingly adrift in the Atlantic ocean. It turns out that the Atlantic connection is more than poetic. For the best bits of its geological history, the north west of Ireland was part of the continent of Laurentia, now mostly found on the other side of the Ocean. The geological narrative I will be telling was hard-won – the big picture takes in Greenland, Scandinavia, Scotland, and the eastern USA and Canada. The west of Ireland is the keystone, joining different spans of knowledge together, holding up a great scientific structure.

Just as Irish writers created work with world-wide impact, so Irish geology has a wider role to play. How do we link the structures in ancient mountain belts to broad plate tectonic concepts? How long do orogenies last? How does magmatism affect metamorphism? How do we gain tectonic insights from sedimentary basins? The west of Ireland has much light to shed on all of these questions.

I will be writing a series of posts on the geology of the west of Ireland. I shall focus mostly on South Mayo and Connemara, but will take occasional trips elsewhere. This follows the pattern of the time I spent in Ireland, studying for my PhD. I’ll draw on this experience to give some insight into how science is really done. At times I’ll sound like I have all the answers, but I’ll also make it clear that really nobody does – science is always a work in progress. I’ll talk of the mistaken ideas of the past, plus the awkward facts that may ultimately overturn parts of today’s scientific consensus. Scientists are human too. I have a tale to tell of graduate-student-doubt, academic bitchiness and ultimate redemption at the hands of U-Pb geochronology.

The great comedy ‘Father Ted’ is another cultural product of the west of Ireland. So as Mrs Doyle would say “will you be having some more Irish geology blog posts? Ah go on. Go on now. Go on, go on, go on, go on, go on, go on, go on, go on…”.

How to make a rock from scratch

Posted on 20 January, 2013 by Metageologist

“If you wish to make an apple pie from scratch, you must first invent the universe.” Carl Sagan.

I have a handsome piece of rock in my hand. How did it come to be, how was it made? A perfectly acceptable geological answer is that it formed as molten rock cooled slowly underground. But that’s not the whole story, it doesn’t say what melted, and where that come from and…

So, taking my cue from Carl Sagan, here’s the full story.

Inventing the Universe

The Universe was created in a ‘Big Bang’ (if you want to know what happened before that, you are reading the wrong blog). At first, only three elements existed, Hydrogen, Helium and Lithium, basically just simple arrangements of sub-atomic particles. As the universe calmed down a bit, clumps of gas grew and grew, increasing the density in their centre. Eventually the pressure squashed atomic nuclei so much that they fused together, producing energy. The first stars were born.

These nuclear-powered furnaces produced light and heat, but also performed alchemy, turning simple nuclei into larger ones, thereby creating new elements. Getting from a nucleus with a few protons and neutrons to ones with over 100 (as seen in heavier elements) is not easy. Several generations of stars were required, gradually building larger and larger nuclei. The heaviest elements only form in the extraordinary conditions that occur in the least few seconds of a supernova, where large gobbets of protons and neutrons are forced together.

The modern universe has seen several generations of stars come and go, during its 13.75 billion years of life. As wells as stars, galaxies and the like, it contains chemically complex clouds of gas and dust, the mixed remnants of exploded stars. Hydrogen is still dominant but plenty of other elements exist. Over 150 organic molecules have been recognised, including vast quantities of ethyl alcohol, otherwise know as booze. Our own solar system formed from such a gin-soaked* cloud over 4 and a half billion years ago. The atoms that the rock in my hand, and you, and everything else you can see are made of was in there. Arranged rather differently, but there nonetheless.

The cloud contained elements in different forms, such as tiny grains of diamond, formed in supernova shockwaves. They are found today in meteorites, precious little gems older than our solar system. Other grains were present (notably ‘CAIs’ or Calcium-Aluminium inclusions) but many elements were in the form of gas or ice. These ‘volatile elements’ are distinguished from refractory elements found in grains. Unsurprisingly compounds we know of as gas or liquid were in the volatile component, but it also included elements we think of as solid, such as potassium and lead.

Inventing the solar system

Some eddy, some chance event, created a part of the cloud, denser than the rest, that ended up as our sun. As its nuclear furnace ignited, strong solar winds started pushing through the rest of the cloud, blowing out the gas and ice, leaving only dust and larger solid fragments. The gas, ice and volatile elements were pushed out beyond a ‘snow line’ about 4 earth orbits from the sun, where some ended up as part of Jupiter and Saturn, the ‘gas giants’.

The earth’s composition today is measurably different from chondrites, a class of meteorites that records the composition of the original cloud. Indeed the whole of the inner solar system is depleted in the volatile elements that were blown away in the gas and ice.

Over 100s of millions of years the inner solar system formed into the four planets we see today. This was a violent process. Chunks of rock called planetesimals formed and then smashed together. Mercury and the earth-moon system clearly show the marks of major collisions early in their history. This violence is convenient for us, as it provides evidence for what is going on deep within the earth.

Inventing the earth

Many meteorites are fragments of planetesimals that have been smashed into pieces. These fall into two main camps, stony meteorites and iron meteorites. To a first approximation, stony meteorites match rocks we find on the earth’s surface today. Iron meteorites are clearly exotic to us surface dwellers, but they would feel right at home in the centre of our earth.

As planets form they separate out into two chemically distinct portions- a silicate part and an iron rich part. Iron is the sixth most abundant element in the universe – stars make lots of it – and it is refactory. It’s the most common element in the earth. There is so much that some of it doesn’t bond with other elements but sinks down into the core. It takes some friends with it – siderophile (iron-loving) elements such as nickel, gold, platinum and iridium.

The remainder of the planet, the equivalent of the stony meteorites, is known to geochemists as the ‘bulk silicate earth’ and now makes up the earth’s mantle and crust. It is rich in lithophile or ‘rock-loving’ elements which like to bond with oxygen and hang out together. We can’t see the earth’s core directly, just infer its properties remotely, so iron meteorites give us a glimpse of a place we can never visit.

Geochemists are still settling the details, but the broad pattern is clear. Take the volatile elements away from the original cloud and you get the bulk composition of the earth. Extract out excess iron and friends and you are left with the bulk silicate earth. Here’s a rough graph of the composition of bulk versus silicate earth.

Taking out large amounts of iron into the core, leaves the other elements in increased proportion. Note how only six elements (oxygen, magnesium, silicon, iron, aluminium and calcium) make up nearly 99 percent of the bulk silicate earth. A silicate is a compound that contains SiO_4,, so looking at the numbers, its no surprise that these are common. What’s that? You’re wondering if iron and magnesium oxides are common? Oh yes indeed. The earth’s mantle (the vast majority of the silicate earth) is made of peridotite** which is made of olivine ((Mg,Fe)₂SiO₄) and orthopyroxene ((Fe,Mg)SiO₃) plus other minor minerals that contain calcium and aluminium.

Melting the mantle

Picture your favourite rock. Unless you are odd, its not peridotite. So where do the pretty rocks come from? They are found on the earth’s continental crust, which is volumetrically unimportant, but much more varied than the mantle. Here a whole range of chemical processes are active: weathering, biological activity, metamorphism. I’ll stick with just one as it made the rock in my hand*** and it is how crust forms from the mantle: melting.

The mantle melts for a variety of reasons (great overview here) and it is yet another process of chemical change. The proportions of the major elements are different between peridotite and the molten rock, plus the melt is richer in the minor elements, which aren’t particularly at home within peridotite. Melting and re-melting has allowed the continental crust to be enriched in the interesting 1% of elements and produce rocks very different from peridotite.

Note how oxides such as sodium and potassium which are negligible in the bulk earth are much more common in the continental crust. The same applies to most other rock-forming elements.

My rock, the red bars above, is a gabbro from Ireland which was melted directly from the mantle. It contains pyroxene, which handles the iron and magnesium, plus calcic plagioclase, which mixes the aluminium and calcium with silicate and a sniff of sodium. The gabbro now forms part of the continental crust and as it is eroded away it will end up enriching sediments and going through yet another cycle of chemical change.

Water, water everywhere

I’ll try your patience with a final wet coda. The magma from which my rock crystallised was pretty dry, but it intruded into wet rocks (metamorphosing sediments). After the magma crystallised, it cooled and water from the surrounding rocks crept in and created new wet minerals. Where did this water come from?

Water**** was driven off from the inner solar system by the early solar wind. It’s extremely volatile, so why is it on the modern earth? Over the whole earth there isn’t much (only about 500 parts per million) but its concentrated near the surface. One popular theory is that it came via meteorites – wet ones from beyond the snow line.

This is an extraordinary idea, especially when you consider how important water is. Despite being such as small proportion of the overall earth, water drives processes that influence the entire planet. Subduction, the process where oceanic crust (sometimes) sinks down to the base of mantle, is facilitated by water. Water is involved in the formation of eclogite which makes oceanic plates more dense and allows them to sink. It also drives mantle melting that forms continental crust that allows us to keep our feet dry. Venus lacks plate tectonics and is drier than the earth – is this explained by how many wet meteorites fell on one planet and not the other?

If a defining feature of earth is only here by chance, it certainly puts the search for ‘earth-like’ planets into context. When we find planets of earth size within the ‘goldilocks’ zone (of orbits that allow liquid water) slight differences in their history may mean they are still far from ‘earth-like’ in their ability to support life.

Whatever the events necessary to create life on earth, one of the things it does is make apple pies. My work here is done.

——————————————————————————————————————-

*being pure alcohol it’s nearer vodka than gin (juniper didn’t exist), which is a shame. No tonic in space either.

**olive and pyroxene are stable only near the top of the mantle. At greater depth and pressure other more exotic minerals (with similar chemistry) are stable. But that is another story

*** it’s made typing quite difficult, I should probably put it down now

**** I say water when I should perhaps use water/hydroxyl/hydrogen, but you’ll forgive the simplification, I’m sure

Picture of peridotite from the incomparable hypocentre on Flickr under creative commons.

This post draws a lot on the book Destiny or Chance revisited by Stuart Ross Taylor.

A bit of Scotland in an English playground

Posted on 29 December, 2012 by Metageologist

There is a park near my home that my children like. As is the way of things, this means I stand around it a lot, ready to rub bruised knees or produce biscuits or push ‘faster!’, but otherwise redundant. My attention often wanders to the big blocks of stone in the park – they are worth looking at.

To start with, here’s some ‘granite’.

The white material is a medium to coarse grained igneous rock of ‘felsic’ composition – granite (loosely speaking). The dark area is a portion of material within the granite. It may be a xenolith, a piece of rock that fell into the magma, but it looks to me like diorite, possibly the result of magma mingling.

There are few blocks of mafic igneous rock:

This ‘gabbro’ above shows both fresh and weathered surfaces. Plagioclase feldspar is colourless and weathers white while the dark minerals (pyroxenes?) sit within it. Note the rusty iron patch at the top.

Another block of gabbro has a slight sniff of layering to it.

Mafic magma is molten from about 1200°C, whereas more normal continental rocks (sediments say) can melt from 700°C. Put the two together, therefore and you expect some melting, producing migmatites.

This block is of high-grade metamorphic rock, with a gneissic foliation. A thin granite vein cuts through and has itself been folded.

Here’s some more metamorphic rock, with a folded foliation and a mica sheen.

Our final type of rock is sedimentary, a conglomerate.

Notice the variety of clast types. We’ve some red sediments, some ‘granite’, vein quartz, quartzite and more. Here’s a closer look.

I’ve no idea where these blocks came from, but I know it’s not nearby. They are part of the park landscaping and so were brought from somewhere else in Britain, on a lorry. The blocks are rounded and weathered, so they are not blasted from a quarry. I think they are glacial blocks. Assuming they came from the same glacial deposit I suggest they are from North East Scotland. There is a series of syn-orogenic mafic intrusions in this corner of the world that sit within the high-grade parts of the Buchan and Barrow metamorphic areas. Granites are two-a-penny in Scotland and the conglomerate looks like the post-orogenic ‘Old Red Sandstone’.

These rocks are very similar to those in my PhD field area, so to have them turn up close to home is rather splendid.

All photos by me. I was hoping for a sunny day to take them, but I’ve given up waiting. The photos give you an authentically gloomy and dark view of rocks from Scotland, at least.

Cratons – old and strong

Posted on 19 December, 2012 by Metageologist

Cratons are pieces of continents that have been stable for a over a billion years. As earth’s plates drift along, mountains periodically rise and fall, plate boundaries appear and disappear. But cratons are like great-grandmothers at family gatherings, while younger crust moves excitedly around them, they sit quietly, occasionally remarking on how different things were when they were young.

Every continent has cratonic areas, notably the core of North America, Scandinavia, Siberia, India and most of Australia. They may be covered by a thin shawl of sediment, but often they expose ‘basement’ rocks such as gneiss. Economically they are very important – most of the world’s diamonds come from cratonic areas as do many other valuable deposits.

Cratons are stable because they are strong. The geology of the Himalayas illustrates this – the modern day plate boundary between Indian and Asia is at the southern edge of the Himalayas. The cratonic Indian plate is barely deformed, in great contrast to the vast pile of deformed soft young crust in the Tibetan Plateau to the north.

Cratonic crust is strong, being unusually cold and dry, but that is only part of the picture. Continental crust is the upper portion of continental lithosphere. It’s lithosphere that puts the plates into plate tectonics, it’s a rigid layer on the earth’s surface, as opposed to the hot flowing mantle that lies beneath (the asthenosphere). Lithosphere consists of the crust and an underlying layer of mantle that has become ‘stuck on’. This layer of lithospheric mantle can be 2 to 3 times thicker than the crust above and so contributes a great deal to the strength and stability of continental lithosphere.

It turns out that the lithospheric mantle beneath cratons is unusual. In oceanic crust, as it ages it cools and eventually it sinks down into the mantle again. It is thought that continental lithospheric mantle can fall off as well, but in cratons this doesn’t happen. Finding out why is an interesting challenge but before we start that journey, a brief interlude….

Geochemical interlude

My feelings about geochemistry have changed over time. As a doctoral student I was exiled out into an annex 10 minutes from the main department because a new piece of geochemical apparatus required a lab to be expanded, swallowing our little rock-filled office. I could relate to metamorphic petrology and analysis of mineral chemistry, but the isotope stuff, seemingly requiring months of juggling Hydrofluoric acid to get a couple of data points, left me cold. Years later, with a broader, wiser perspective I find myself marvelling at the way isotope geochemistry leads to so much awesome science.

There are two flavours of isotope geochemistry, stable and radiogenic. Many chemical isotopes are unstable and over time spontaneously turn into different elements, the bits left over being thrown out as matter and energy – radiation. For geologists, this real-life alchemy, known as radioactive decay, is mostly used to tell how old something is. The rate of radioactive decay is constant, so measuring the current abundance of isotopes and working backwards tells you the time elapsed since an event happened. Getting from measurements to age involves assumptions about the sample which may be incorrect – my own research successfully predicted a published age was incorrect. Nevertheless modern geochemists have a wide range of extremely robust and accurate techniques at their disposal. Datable events include a crystal forming from a magma or growing in a metamorphic rock, a surface being exposed to the atmosphere, a critter growing, a mineral cooling below a particular temperature, when magma was extracted from melting rock, even the age of ground water.

Stable isotope geochemistry relies on the fact that two isotopes of the same element, for example Hydrogen and Deuterium, are not precisely the same. Most physical and chemical processes treat them identically, but some do not, the slight different in mass means slight differences between isotopes. For example during evaporation of water the lighter H₂¹⁶O will vaporise first, creating a difference in isotopic composition between the water vapour and the remaining liquid. This phenomena of isotopic fractionation affects water in other ways, such that there is a relationship between fossil oxygen isotopes (in ice or fossil sea shells) and the temperature of the sea they came from. This tremendously useful technique is just one example of the seemingly magical way tiny differences in the physics of atoms can shed light on diverse geological problems. There are many other applications, but the technique shines most when we have very little other evidence to go on. Research into the formation of the earth and moon relies heavily on studies of arcane isotopes. We have only limited information about lithospheric mantle from beneath cratons (remember them?) and isotope geochemistry tells us many interesting things…

Back to the cratons

A recent review by Cin-Ty A. Lee, Peter Lufﬁ, and Emily J. Chin of Rice University focuses on the creation and destruction of continental mantle (I’ll mostly discuss creation).

If lithospheric mantle beneath cratons was the same composition as the rest of the mantle then over time it would cool and become denser and unstable. A consensus has emerged that it does not because at some point in the past it was involved in melting and up to half of its volume was removed. The rock left behind after the melt flowed away is known as depleted peridotite and it is stronger and more buoyant than normal mantle, so it remains stable for billions of years.

Fragments of mantle peridotite in lava.

Pieces of continental lithosphere do reach the surface, sometimes as huge slabs such as at Ronda in Spain. Some weird volcanic deposits start at mantle depths and bring up pieces of the surrounding rocks. Sometimes they contain diamonds, but also pieces of the lithospheric mantle. Armed with these samples, geochemists have applied a massive array of techniques.

The geochemistry of melting rock is well understood and it allows us to infer many things about the conditions under which it happens. For example comparing ancient with modern samples, we can tell that older samples melted at higher temperatures (1,500 to 1,700◦C 1,300 to 1,500◦C) and saw more melt extraction (50 to 30% versus <30%). This is consistent with the fact that the early earth was hotter.

Melting is affected by pressure as well as temperature – deeper mantle rocks contain different minerals (at these pressures, garnet at depth, spinel above) which melt in subtly different ways. Cratonic peridotites melted at shallow depths (90km) but were later moved deeper (180km).

Not all samples are of peridotite, next common are pyroxenites, sometimes referred to as eclogites. Chemically these are similar to basalts formed at modern mid-ocean ridges. Their oxygen isotopes show wide variation, such as is seen in oceanic crust following hydrothermal alteration. Other physico-chemical processes could in theory cause similar patterns, but it seems only low-temperature processes explain the patterns seen. The fact that they were once part of oceanic crust makes these pyroxenite rocks very different from the peridotites (which have only ever been part of the mantle).

Further evidence that pyroxenites were once near the surface comes from sulphur isotopes from eclogitic inclusions within diamonds. Fractionation of these isotopes occurs in ways only explained by chemical reactions high in the atmosphere. These diamonds with eclogitic inclusions themselves have carbon isotopes that are extremely ‘light’. Light carbon is a sign of life – organisms preferentially contain light carbon. In these ancient rocks, life probably means bacteria. So we can lengthen the list of “lovely things bacteria help to make” – cheese, soy sauce, wine, yogurt, healthy digestion and now certain types of diamonds.

NB for visitors from the German wikipedia page on Diamonds, I’m delighted you’re here. Here is a better reference for the science on this.

This is a wonderful thing. A bacterial mat sitting on the sea-floor billions of years ago ends up being stuffed down into a subduction zone. It enters a world of crushing pressure and extreme temperature where it is transformed utterly, its carbon forming a diamond. Later a weird eruption happens and the diamond is squirted back up to the surface for us to marvel at. Only isotope geochemistry (and some generous assumptions on my part) allows us to tell this story.

How does continental mantle form?

Our authors discuss how continental mantle forms (spoiler: we don’t really know). One model is that a large plume of hot mantle material rises up underneath a continent. This would indeed form lots of depleted peridotite (underneath a big pile of lava). However this model is inconsistent with the evidence described above – melting would be expected at greater depths (200km) than is seen.

A plume model also fails to explain the eclogitic parts of the mantle. One mechanism to explain how portions of oceanic lithosphere end up in the sub-continental mantle is that when subducted instead dropping steeply down they remain buoyant and stack up beneath the continents. In this model, the sub-continental lithosphere grows by progressive capturing and stacking of oceanic lithosphere. Its been argued that parts of the Farallon plate were captured beneath western north America in recent times. Hotter oceanic lithosphere (common in the Archean) subducts at a shallower angle and so is more likely to end up getting stuck to the base on the contintent.

This model explains the evidence for shallow melting and subsequent burial, plus the presence of former oceanic lithosphere. Indeed it predicts a larger proportion of eclogitic pyroxenite than is observed. Our authors propose a process of “viscous drainage” whereby the crustal portion of the stacked ex-oceanic lithosphere (now dense inclined sheets of garnet pyroxenite) slowly ‘drains’ down and out, leaving the peridotite ‘framework’ behind.

Models of the creation of continental crust emphasise the importance of island arcs. An oceanic island arc (where oceanic crust, subducts under oceanic) may collide with a small continent, turn into a continental island arc and ultimately become a new piece of continental crust. This mechanism will not give thick lithosphere on its own however – so some process of ‘orogenic thickening’ (squeezing it so it’s thicker) is required.

You will perhaps have sensed that the paper drifts into speculation a little. This is appropriate in a review paper and inevitable as we are on the ragged edge of what we can know about these rocks, so distant in space and time. The paper ends with a ‘future directions’ section full of as yet unanswered questions.

“How do continents form?” is a simple question to ask but we still can’t give a complete answer to it. We still don’t know the secret to great-grandma’s longevity.

Peridotite picture from dun_deagh on Flickr under Creative Commons.

Craton map from Pearson and Wittig under Geological Society of London fair use policy.

PEARSON, D., & WITTIG, N. (2008). Formation of Archaean continental lithosphere and its diamonds: the root of the problem Journal of the Geological Society, 165 (5), 895-914 DOI: 10.1144/0016-76492008-003
Lee, C., Luffi, P., & Chin, E. (2011). Building and Destroying Continental Mantle Annual Review of Earth and Planetary Sciences, 39 (1), 59-90 DOI: 10.1146/annurev-earth-040610-133505