A few days ago I was having a stroll north of Changgyeonggung palace, when I found a mysterious rock on display in the park.
The rock looked like a Roman mosaic, a tessellated pavement of stone. My ‘fifth-and-a-half sense’ of structural geologist immediately started to point to a very specific solution to this geologic puzzle, and I could not resist to this very juicy opportunity to make a quiz about it on Facebook and Instagram. So I asked.
I have to say that I was very surprised by many comments. There were many very smart and educated guesses, which made me feel proud of my followers. Many proposed a very simple but effective explanation for the presence of these regular sets of surfaces.
This was my thought exactly. Indeed, if we move around and look at the rock from different angles (you can actually do it on Google street view), there are many observations we can take that support this hypothesis.
First of all, the mistery boulder has two rocks in it! If you look at the photo above, the mistery boulder is made of a granular granitoid, consisting of large crystals of feldspar and quartz (bottom half of the image), in sharp contact with a pinkish and more homogeneous rock that contains those well-developed surfaces. The latter is well-exposed on the polished surface of the boulder, resulting in a spectacular mosaic-like pattern. However, it is possible to see that the pink rock was originally an element cross-cutting the granitoid [slider].
A small chunk of granite is visible at the base of the boulder. There, and only there, we can see that the pink rock is actually a narrow dike or vein that intruded the granitoid. If we keep walking around, we can admire the tessellated structure of the dike, which, hence, results from the presence of three regular sets of joints that intersect at nearly 90° [play with the slider below]
Many rocks contain three sets of orthogonal joints, which typically form due to decompression, as erosion removes the overburden and rocks are exposed at the surface, or due to tectonic forces that produce regular sets of fractures. In addition, granitic rocks originated from the crystallization of magma, like these ones, may develop regular fractures as they contract during cooling. Overall, this sounds like a reasonable explanation to me, but there is a problem. The problem is that I received other smart and educated guesses that made me doubt of my first guess.
Minerals fracture along specific crystallographic directions – cleavage planes – along whom they are weaker. I avoided naming the composition of the ‘pinkish dike’ but that pink color betrays the presence of abundant alkali feldspar, which, by happy coincidence, fractures along three sets of orthogonal cleavage planes. This is why many also proposed that this was some sort of subparallel association of feldspars or even a gigantic feldspar. Now, I do not think this is the case, because there must have been quite a peculiar combination of geological processes to produce something like this in a very narrow dike, which rather seems fine-grained. Nevertheless, those comments highlighted many problems that needed to be investigated and solved, like the fact that the joints seems to be concentrated only in the dike and not in the host granite.
At this point, I became paranoid, I started to search for this rock in Google, in English and Korean, I found a photo of the sign next to the rock, which -sadly- contained only an explanation of the sundial next to it (why are people never interested in rocks? 🙁 ). Then, after a long and unsuccessful research, I brought my pain to Twitter. I was not looking for anything special, just an outcrop-equivalent of this rock or maybe a small Korean paper that studied the problem. Finally, @Golos_Tikho saved me and found this.
So my guess was correct: in the area there are thin veins (or dikes) in the granite that selectively develop a lot of joints (at least compared to the granite). Indeed, if we look more carefully at the boulder in Changgyeonggung, we can see that the joints actually continue also in the granite surrounding the dike.
This indicates that their development was the result of forces that were experienced by the granite and the dike alike. However, it is clear from the last slider that the dike and the granite respond differently to fracturing. The coarse-grained granitoid seems not to be the ideal rock where to develop planar joints. On the other hand, the more homogeneous and fine-grained dike is able to fracture pervasively. I think this is the likely explanation for the formation of this rock, but if you don’t like it, bananamcbean has a better one:
Introducing the ‘mistery rock’ series
This was super fun for me (and I bet also for you), so I promise I will do it again, whenever I encounter something odd! Stay tuned!
← look for this marker on the map!
I would like to thank Michele Lustrino, Martha Carr, Sandra McLaren, Marta Codeço, Marco Palumbo, Jean Marius Palincas, Jules Blm, Helfe, Deemery, Kate Luxford, Mark, Finn, and Maria for supporting my blog! Thank you very much! If you like my geological posts and you wish to support me, you can do it by offering me a coffee at ko-fi!