Amazing Natural Phenomena – Columnar Joints

Columnar joints is the name geologists give to these column like rock formations found in many parts of the world. Apparently, the first one that got talked about a lot was the Giant’s causeway (the first image) somewhere of the coast of northern Ireland. There are apparently a whole bunch of them around the world. The only one I have seen (just yesterday) is the Devil’s postpile (the second image). I as always asked Wikipedia to see what was known about them. As will all natural structures that appear too ordered or too functional to be “natural”, there is a story that people invented to explain their origin. For example, Wikipedia has this story for the Giant’s Causeway

“Legend has it that the Irish giant Fionn mac Cumhaill (Finn McCool) built the causeway to walk to Scotland to fight his Scottish counterpart Benandonner. One version of the legend tells that Finn McCool fell asleep before he got to Scotland. When he did not arrive, the much larger Benandonner crossed the bridge looking for him. To protect Fionn, his wife Oonagh laid a blanket over Fionn and pretended he was actually Fionn's baby son (in a variation, Fionn fled after seeing Benandonner's great bulk, and asked his wife to disguise him as the baby.) In both versions, when Benandonner saw the size of the 'infant', he assumed the alleged father, Fionn, must be gigantic indeed. Therefore, Benandonner fled home in terror, ripping up the Causeway in case he was followed by Fionn. Another variation is that Oonagh painted a rock shaped like a steak and gave it to Benandonner, whilst giving the baby (Fionn) a normal steak. When Benandonner saw that the baby was able to eat it so easily, he ran away, tearing up the causeway...”

Cool huh?! Next question to ask is do we have a physical explanation for how these apparently ubiquitous structures form. Well, volcanologists have been thinking about these for a number of years and a lot is known. The physics as such is too complicated and I don’t understand it well enough to tell you clearly what is going on. But, loosely speaking, the structures are a result of two things. One is the fact that hot lava cools from outside. So, there is a temperature gradient in the material when going from out to inside. This produces a stress instability in the system. The second thing is that ground water is being boiled off by the lava continuously and there is come kind of a capillary effect associated with this. These two things together give rise to the structures we see. If you want to know more, you should go to the webpage of this guy, who is reproducing these structures in his lab using cornstarch and trying to sort out the exact details of the hydrodynamic and elastic instabilities that drive this particular pattern. And even if you are not up to further reading on this, check out this video from Discovery Channel, where the investigator talks about his experiments.
So, even though I still don’t understand the details of what is going on, I went to bed peacefully convinced that there were no giants or giant’s cunning wives required to explain the phenomena :)).