Showing Off About Science

On the 3rd April 2012 I did a set at Science Showoff held every month at the Wilmington Arms in London. There are 10 acts who each have 9 minutes to talk about ANY aspect of science using any technique they want – it can be music, practical demonstrations or simple Powerpoints (there have been rock guitarists singing about physics, opera singers chatting about STD’s…the lot!). I was kindly asked by the organiser to do a set on geology, as it was a science they had not yet had on stage. It ended up going down pretty well on the night, so I thought I would put the presentation and my spiel up on my blog. The pictures were all originally Powerpoints that I designed so some might not work quite so well in a blog but nevertheless – enjoy!

I am a geologist. I know what you are thinking.

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No, seriously. I love geology, I have done it since I was a wee girl and I have to say I have had some of my fondest near death experiences, along with the odd moment when health and safety would have a heart attack.

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(It was a disused asbestos quarry for your information). And of course those odd moments where I got so depressed at being on my own, on a hill, trying to dry my socks in what can only be called rubbish weather and looking at rocks. So, I might have defaced some geological heritage (!).

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Apart from all that, we did do some proper geology though. Here’s a little story of one rock that caught my attention during my dissertation…

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Initially I can tell that there are two different rocks here, one on top and one on the bottom. I can also say that one is likely to be more silica rich than the other due to the colour – the lighter one will likely contain minerals such as quartz (SiO2) and feldspar (Na, Ca), and the darker one will be more likely to contain olivine ((Fe,Mg)2SiO4) and pyroxene XY(Si,Al)2O6. This tells me about the way in which they crystallised and from where. The dark rock is likely to be primitive, from melting of mantle and very quickly cooled to rock. The lighter one probably spent a little more time in a magma chamber, meaning there are more evolved chemical species comprising the rock.

The size of the crystals making up the rock also tells me something about it: the groundmass is very fine in both of them, but especially so next to the boundary of the two. Both rocks also contain much larger crystals (up to 1cm large). This means that both rocks spent very little time in a magma chamber, but both did have enough time to grow some large crystals. This ties in nicely with the chemical observation, because the lighter rock has comparatively more large crystals than the dark rock, indicating more time for the crystals to grow large in a chamber. These facts also indicate that these rocks were likely to be extruded as a lava flow rather than intruded as a dyke.

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The dark rock contains olivine and augite as very small crystals and plagioclase (Na, Ca) feldspar as much larger ones. From these simple mineral associations I can confirm this is a basalt. The lighter rock contains quartz, a mix of Na and K feldspar, and magnetite telling me it is a rhyolite. This basically means that the rocks contained various silicates: magnesium, iron, aluminium, calcium, sodium and potassium. Both of the classifications fit in really well with my preliminary thoughts.

What is confusing is that rhyolite is formed from evolution of basalt magma – through the relative enrichment of the magma in silica with continued crystallisation giving them different mineral associations, basalt with Al, Mg, Fe and Ca and rhyolite with Si, Na and K – so how can they be cooled as melt together at the same time?

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The answer could be in the quartz crystals. These crystals have an unusual appearance indicating that they could have been resorbed into the melt at some point during their evolution. This could happen if the crystals were unstable in the new melt chemistry – for example if a basalt was intruded into an evolved silicic rhyolite melt, the laws of thermodynamics would tell you that the silica would be assimilated into the melt fraction that was undersaturated in silica.

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So somehow we have a magma chamber, likely formed from shallow large scale melting of the crust to produce a basaltic magma that began to evolve. Then, at some point in its evolution, there was possible ingress of another wave of basaltic magma via some sort of intrusion. This is common at ocean ridge systems – but how can we decide whether or not this is magma originating from an ocean ridge tectonic setting?

This is where the Rare Earth Elements come in.

Geologists wanted to know what distinguished these rocks from each other, not purely based on their simple chemical compositions which can be similar from more than one setting but also on their trace element compositions. Could these give a more definitive answer as to the tectonic provenance of the rock?

It turned out the answer was yes. And it was beautifully logical. At subduction zones, it was possible to see trace amounts of LILE elements in the basalts. This is important because most LILE elements are soluble, so that it proves there is incorporation of seawater into the mantle during subduction. Similar principles apply for all kinds of rocks – whether they are from island arcs, mid ocean ridges, continental mountain chains or those other random islands in the middle of the sea no one really knows what they are!

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So what is our rock then? Firstly, I did not actually do any X-Ray Fluorescence on it to find out, but the likely story is that they were from a magma chamber formed through stretching of the crust – similar to ocean ridge magma formation. The timing of the eruption of these lavas was concurrent with the opening of the North Atlantic, that caused widespread stretching of the crust, along with the emergence of the Iceland plume which may have caused the extent of volcanism seen across the Hebrides.