A gneiss lecture for Yvonne

Hey Gail! What is this stone doing on our kitchen floor?

Well Bertie it's a present from our friend Yvonne. She picked it up on a beach on her recent trip to the Isle of Lewis. It's nice isn't it?

Oh ha ha very droll. Of course I know it's gneiss. Lewisian gneiss in fact. But really, isn't there something a bit coals-to-Newcastle about giving a lump of rock to a geoscientist?

Bertie now, we must learn to be more appreciative. Yvonne told me she wants in return a lesson on the whole topic of the Lewisian gneiss - she is keen to be taught some geology and this should be encouraged. Maybe you could put on your Bertie Boffin hat and help me prepare something for her?

OK then, that's a great idea. Well let me see.

First off, the Lewisian is very old. Even older than Yvonne. Archaean in fact! It is the oldest rock formation in the UK, although if you go to Australia or Greenland or Canada, you can easily find rocks which are even older.

Secondly, it is a metamorphic rock, which means it has been changed, i.e. metamorphosed, from its initial state. Originally this boulder was probably a granite. Just like the stone from which our house, and Yvonne's, are built.  Perhaps Yvonne could imagine our houses were somehow buried tens of kilometres under the earth, where the heat and pressure are so intense that the granite minerals very slowly recrystallise into the type of rock we call a gneiss.

In general, we explain how rocks get deeply buried in the earth, and then rise up again, by applying plate tectonics theory, as in the image above. But way, way back in the Archaean era (when of course there were no houses as there were no humans nor other life forms excepting maybe a few bacteria) the plate tectonics thing was only just getting going and the earth was a whole lot hotter than it is now, so the favourite geologist's dictum, 'the present is the key to the past', is a bit hard to apply to this phase of Earth's history.

So around 1.7 to 3.0 billion years ago, when the Lewisian was forming, geologists now believe that rather than the current set-up of tectonic plates, there were all these 'terranes' crashing into each other, and, truth to tell, it was a long time ago (did I say that already?) and it all gets very, very complicated.

Gosh you know what? I've just remembered. The son of one of our Torridon neighbours, a fine young chap called Dr John MacDonald of Glasgow University, is a proper expert on the Lewisian formation. He has written papers with titles like:

Temperature–time evolution of the Assynt Terrane of the Lewisian Gneiss Complex of Northwest Scotland from zircon U-Pb dating and Ti 

I am thinking we should invite Yvonne over to Torridon (where, after all, we have a bunch of Lewisian rocks just around the corner) and if Yvonne asks nicely, then John, a real bona fide lecturer, can tell her all she would ever want to know about Scotland's most ancient rock formation. And possibly more...

Lewisian rocks (pink): distribution in NW Scotland

Fine Bertie, but in the meantime I expect Yvonne will want to learn about the pretty pink bits in our rock. 

Oh yes of course. The pink splodges are in fact a mineral called potassium feldspar. It's the same mineral that gives the granite buildings in the Deeside town of Banchory their pinkish tinge. Although in Aberdeen the granite is grey because it crystallised from a magma of slightly different chemical composition.

Er Gail, I am feeling a bit tired. This teaching business is hard work isn't it? 

Also, could you please tell Yvonne that a better present would be one of those delicious chewy things from the 'Pet Comforts' shop, rather than some random old rock? 

Or should I, next time she pops over for a cuppa, tap in to Yvonne's own area of expertise and demand a Jungian analysis of that dream I had last night about chasing sheep?