“Legend has it that the Irish giant Fionn mac Cumhaill (Finn McCool) built the causeway to walk to
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 :)).