Scrutable

Today the BBC has an article about how all Olympic curling stones are made from two kinds of granite mined only on Ailsa Craig, a small uninhabited island in the Firth of Clyde.

Unfortunately the photo captioned "The island has a unique granite which is only found on Ailsa Craig" shows columnar basalt. Mostly when we see columnar jointed rock in volcanic provinces, it is columnar basalt. Though it doesn't have to be. Other rocks can also exihibit columnar jointing, even sedimentary rocks on rare occasion. But I'm reasonably confident this is what you'd expect it to be, columnar basalt, as other articles point to columnar basalt on Ailsa Craig. And the Wiki article on Ailsa Craig notes that "microgranites" on Ailsa Craig were intruded as dykes. I have also found pictures of the granite mines and it looks like what you'd expect, not columnar jointed.

Any more Bad Geology you have spotted?

I did a quick google (because I know columnar jointing is possible in rhyolites) and I think you might be wrong.

The jointing is crucial - it looks like cooling-induced, columnar jointing, but is actually a result of the intersection of three
principal joint sets and results in four or five-sided columns with a mean diameter between 0.5 - 1.0 metre.

in no idea what this publication is.

Yeah, I think you've misunderstood the Wiki. Microgranite refers more to grain size than size of the intrusion (although smaller intrusions would be more likely to have smaller grains). And it is intruded by, not as, dykes of a different rock (although granites can and do intrude themselves as dykes). The BGS map for the island (it's the Girvan map) shows that the island is indeed mostly granite with a few non-granitic dykes. It would be nice to have a proper geological description of the island; I haven't properly searched for one.

Pishwish wrote: ↑

Mon Feb 07, 2022 6:59 pm

Yeah, I think you've misunderstood the Wiki. Microgranite refers more to grain size than size of the intrusion (although smaller intrusions would be more likely to have smaller grains). And it is intruded by, not as, dykes of a different rock (although granites can and do intrude themselves as dykes). The BGS map for the island (it's the Girvan map) shows that the island is indeed mostly granite with a few non-granitic dykes. It would be nice to have a proper geological description of the island; I haven't properly searched for one.

Is that smaller grain related to cooling faster because it's smaller, or is it more complex?

Pishwish wrote: ↑

Mon Feb 07, 2022 6:19 pm

I did a quick google (because I know columnar jointing is possible in rhyolites) and I think you might be wrong.

The jointing is crucial - it looks like cooling-induced, columnar jointing, but is actually a result of the intersection of three
principal joint sets and results in four or five-sided columns with a mean diameter between 0.5 - 1.0 metre.

in no idea what this publication is.

Well done, you are right, and the BBC were right. It is columnar jointed granite. Though pictures indicate that only portions of the stone on the island are columnar jointed.

It is interesting reading the article on how difficult it is to find an alternative stone to do the job required.

jimbob wrote: ↑

Mon Feb 07, 2022 10:08 pm

Pishwish wrote: ↑

Mon Feb 07, 2022 6:59 pm

Yeah, I think you've misunderstood the Wiki. Microgranite refers more to grain size than size of the intrusion (although smaller intrusions would be more likely to have smaller grains). And it is intruded by, not as, dykes of a different rock (although granites can and do intrude themselves as dykes). The BGS map for the island (it's the Girvan map) shows that the island is indeed mostly granite with a few non-granitic dykes. It would be nice to have a proper geological description of the island; I haven't properly searched for one.

Is that smaller grain related to cooling faster because it's smaller, or is it more complex?

It's a while since I did any igneous petrology, but while it is a bit more complex you are broadly correct. Coarse-grained rocks such as granite typically form in large plutons where they have plenty of time to cool. Medium-grained rocks (such as the microgranite mentioned) typically form in minor intrusions such as dykes and sills where cooling while be more rapid. You can often identify a "chilled margin" to such intrusions where more rapid cooling due to the surrounding country rocks leads to a finer grain size than the central part of the intrusion. Fine-grained rocks such as rhyolite or basalt are typically volcanic rocks that have cooled rapidly on extrusion. Ultimately if cooling is rapid enough you end up with volcanic glasses like obsidian.

wilsontown wrote: ↑

Tue Feb 08, 2022 11:01 am

jimbob wrote: ↑

Mon Feb 07, 2022 10:08 pm

Pishwish wrote: ↑

Mon Feb 07, 2022 6:59 pm

Yeah, I think you've misunderstood the Wiki. Microgranite refers more to grain size than size of the intrusion (although smaller intrusions would be more likely to have smaller grains). And it is intruded by, not as, dykes of a different rock (although granites can and do intrude themselves as dykes). The BGS map for the island (it's the Girvan map) shows that the island is indeed mostly granite with a few non-granitic dykes. It would be nice to have a proper geological description of the island; I haven't properly searched for one.

Is that smaller grain related to cooling faster because it's smaller, or is it more complex?

It's a while since I did any igneous petrology, but while it is a bit more complex you are broadly correct. Coarse-grained rocks such as granite typically form in large plutons where they have plenty of time to cool. Medium-grained rocks (such as the microgranite mentioned) typically form in minor intrusions such as dykes and sills where cooling while be more rapid. You can often identify a "chilled margin" to such intrusions where more rapid cooling due to the surrounding country rocks leads to a finer grain size than the central part of the intrusion. Fine-grained rocks such as rhyolite or basalt are typically volcanic rocks that have cooled rapidly on extrusion. Ultimately if cooling is rapid enough you end up with volcanic glasses like obsidian.

Cheers.

As an aside and at a far smaller scale, when we look at the grain structure of deposited polysilicon in trench MOSFETs (power devices so sometimes say 5um deep and 0.5um wide, you can see it forms from the edges and has a seam down the middle (unsurprisingly). It looks remarkably like the seam you get if you bite into an iced lolly on a stick.

This is the best image I could find online - I have seen clearer ones at work

https://www.researchgate.net/figure/Cro ... _281942940

I was interested in why they now make compound stones using two types of the island's granite. It seems the blue hone is more resilient as it has very low water absorption and consequently isn't abraded by freeze/thaw so, as shown in the photos, the bulk of the stone is common green granite and a disc of the rarer blue type is set into the base and used to form the running surface.

A geologist (not me) blogs

https://blog.nms.ac.uk/2022/02/08/the-r ... ne-island/

Does this count

https://news.artnet.com/art-world/stone ... al-1998477

Scientists Have Conducted Tests That Reveal Stonehenge Is Made From a Nearly Indestructible Ancient Material

jimbob wrote: ↑

Wed Feb 09, 2022 11:22 am

Does this count

https://news.artnet.com/art-world/stone ... al-1998477

Scientists Have Conducted Tests That Reveal Stonehenge Is Made From a Nearly Indestructible Ancient Material

So they are sedimentary rocks with a sort of concrete composition?

Taking a look at the photomicrograph they have in the article helps a little bit - you can see that the rock is made up of quartz grains, and you can also identify that are quartz overgrowths that surround the original grains - that's what the arrows in the image are pointing out. So you would have had an initially porous sedimentary quartzite that has been cemented by quartz precipitation from groundwater circulation. Quartz precipitation is a temperature-controlled process and is enhanced at greater than 90-100 C. Quartz is hard (7 on Moh's scale) so it would indeed be a very low-permeability, durable material.

I'm not entirely sure what is Bad about that article, apart from the spelling.*

OK, "nearly indestructible ancient material" is a rather silly way of describing a particularly tough and hard rock. But they make clear what they mean in the article itself. It isn't actually peddling erroneous geology.

There is a risk, perhaps, of deducing some particular prescience by the ancients, though I don't think the article tries to do that. What happened is largely what you might call selection - what survives is often what was, by chance, best able to survive. Stonehenge lasted better than such constructions because it was made of particularly tough hard rocks. Sarsen rocks lie around persistently, and hence to be found for stone circle building, because they are particularly tough hard rocks. Stonehenge is located in an area where such sarsen stones are common.

Of course my own proposal for Bad Geology turned out to be totally wrong, so who am I, etc.

*Sarsen, not sarcen.

Indeed, it's just a silly clickbaity title for an article about an analysis of the sandstones and their provenance*. Quartz-cemented sandstones (quartzites) are not uncommon and are known for their durability (a hammer blow will just bounce back).
It would be like "Archeologists carry out tests on wood from an Egyptian tomb and discover it's AMAZING origin." Yeah, we examined this funerary object and it's made from Lebanese Cedar, consistent with the known trade routes of the time.

*I mean, the results are interesting, in this and in the more sober reuters article.

Oh it was just the headline.

"Surviving ancient artefact built from particularly erosion-resistant stone" doesn't have quite the same ring to it.

wilsontown wrote: ↑

Tue Feb 08, 2022 11:01 am

jimbob wrote: ↑

Mon Feb 07, 2022 10:08 pm

Pishwish wrote: ↑

Mon Feb 07, 2022 6:59 pm

Yeah, I think you've misunderstood the Wiki. Microgranite refers more to grain size than size of the intrusion (although smaller intrusions would be more likely to have smaller grains). And it is intruded by, not as, dykes of a different rock (although granites can and do intrude themselves as dykes). The BGS map for the island (it's the Girvan map) shows that the island is indeed mostly granite with a few non-granitic dykes. It would be nice to have a proper geological description of the island; I haven't properly searched for one.

Is that smaller grain related to cooling faster because it's smaller, or is it more complex?

It's a while since I did any igneous petrology, but while it is a bit more complex you are broadly correct. Coarse-grained rocks such as granite typically form in large plutons where they have plenty of time to cool. Medium-grained rocks (such as the microgranite mentioned) typically form in minor intrusions such as dykes and sills where cooling while be more rapid. You can often identify a "chilled margin" to such intrusions where more rapid cooling due to the surrounding country rocks leads to a finer grain size than the central part of the intrusion. Fine-grained rocks such as rhyolite or basalt are typically volcanic rocks that have cooled rapidly on extrusion. Ultimately if cooling is rapid enough you end up with volcanic glasses like obsidian.

There's also an issue about composition. An 'acidic' magma is high in quartz, and if cooled quickly forms a rhyolite or slowly a granite. One low in quartz ('alkali') forms a basalt with quick cooling, or gabbro if slow.

You can tell which composition a lava has by watching it - the fast moving rivers of lava from Hawaii are alkali; the slow moving sticky ones are acidic

We do talk about "acid" and "basic" igneous rocks but that shouldn't be understood as making any sense from a chemistry perspective.

wilsontown wrote: ↑

Sun Feb 20, 2022 8:57 pm

We do talk about "acid" and "basic" igneous rocks but that shouldn't be understood as making any sense from a chemistry perspective.

Which ones are you allowed to lick on an alkali diet?

Yeah, you wouldn't use the term alkali to refer to what will become basalt. Alkali usually refers to alkali metals, as in Calc-alkali feldspars.

Mafic, intermediate and felsic are the usual terms for the more common melt/rock compositions. Wikipedia has a nice diagram showing how mineralogy varies from felsic to ultramafic compositions. Of course, the differences in chemistry are more than just SiO2 and feldspar composition. (If I remember right, the felsic end tends to have more OH and less Fe and Mg).



And slow moving lavas to tend to be less viscous, but that might not always be due to composition (particularly on Hawaii, where you get 2 very different types of lava flow behaviour--pahoehoe and aa--that as far as I recall is not due to composition).

Pishwish wrote: ↑

Sun Feb 20, 2022 10:33 pm

Yeah, you wouldn't use the term alkali to refer to what will become basalt. Alkali usually refers to alkali metals, as in Calc-alkali feldspars.

Mafic, intermediate and felsic are the usual terms for the more common melt/rock compositions. Wikipedia has a nice diagram showing how mineralogy varies from felsic to ultramafic compositions. Of course, the differences in chemistry are more than just SiO2 and feldspar composition. (If I remember right, the felsic end tends to have more OH and less Fe and Mg).



And slow moving lavas to tend to be less viscous, but that might not always be due to composition (particularly on Hawaii, where you get 2 very different types of lava flow behaviour--pahoehoe and aa--that as far as I recall is not due to composition).

Thank you. Looks like things have developed a bit since I studied geology in the 1970s....

Trinucleus wrote: ↑

Sun Feb 20, 2022 7:43 pm

There's also an issue about composition. An 'acidic' magma is high in quartz, and if cooled quickly forms a rhyolite or slowly a granite. One low in quartz ('alkali') forms a basalt with quick cooling, or gabbro if slow.

You can tell which composition a lava has by watching it - the fast moving rivers of lava from Hawaii are alkali; the slow moving sticky ones are acidic

To get a fine-grained glassy rock, I understand you need fairly quick cooling, but not so fast it becomes extensive glass like obsidian. And I think columnar jointed rocks tend to be where there is a submarine or subglacial eruption, which would deliver some fairly fast cooling. Since that fast cooling is, as you suggest, in general inconsistent with granite, which is mostly coarse-grained, it must be some fairly unusual conditions that produce either fine-grained granites or columnar jointed granites.

What other fine-grained glassy rocks are there?

Ah, I wouldn't count on my knowledge (I'm already cringing at calc-alkali feldspar). I went to college 30 years ago, but it was big on granites, so I remember a lot more of that stuff. I'm sure a lot has changed as new data comes along and theories fall in or out of fashion. (Geology allows more speculaton than lots of other sciences, since you can't really run a crystallisation experiment for a couple of million years. Lots of "rules" are based on well informed conjecture, but a few have been turned on their head, or at least recognised as not being proven. I never really understood how large crystals indicated long cooling times in magma, yet pegmatites have massive crystals and yet could not have cooled that slowly.) Wiki is handy for reminding yourself of what you learned, occasionally you get a bit of a shock (oomycetes are algae now? Anapsids aren't considered primitive anymore?) Still, it could be worse, I can't imagine learning geology before plate tectonics was accepted and everything was all about geosynclines and such.

Pishwish wrote: ↑

Mon Feb 21, 2022 7:38 pm

Still, it could be worse, I can't imagine learning geology before plate tectonics was accepted and everything was all about geosynclines and such.

O level geology (1971) was "a geosyncline formed" with absolutely no explanation of how or why. At uni a few years later we had a lecture from Dan Mackenzie who was an actual famous geologist who had managed to explain continental drift as plate tectonics which beautifully explained so many phenomena, not just South America fitting into Africa.

Nice to know I've been at the cutting edge

I remember seeing the BBC documentary The Restless Earth, when continental drift was becoming accepted. Gosh that was almost fifty years ago.

I was quite impressed by how, in "A short history of nearly everything," Bill Bryson explained how geologists came to accept plate tectonics.
I think the real clincher was the magnetic signature of ocean floor basalts (to non geologists: the earth's magnetic field flips from North to South over time, for reasons not fully understood, and igneous rocks record that orientation at the time of cooling. This produces a barcode like pattern of normal and reversed magnetic stripes as you move away from either side of the mid-oceanic ridges, where plates are pushed apart and new ocean floor is formed.) We were told the reason why we have that data is because the US navy needed to know background magnetism to better hide their submarines. I liked one lecturer's analogy of the ridges being like giant, incredibly slow tape recorders producing 2 copies of the magnetic field going as far back as the Jurassic.

The pre-pangea plate tectonics were harder to figure out, on account of those ocean floors not being there anymore. Older igneous rocks on the continent provide some data but someone (Richard Fortey?, I think he was big into trilobites) figured out that you could reconstruct continental configurations based on when various creatures were able to cross a shrinking ocean to occupy equivalent ecological niches. (I am probably getting the terminology wrong here).