Last June I had the opportunity to visit a true geological World Heritage Site : the Giant Causeway in Northern Ireland. A great place to visit and study ! As soon as I entered the site, I heard some people shouting "Look...these beautiful giant basalt crystals over there..." Sure enough, many people think so and in fact quite a few mentionings of "giant basalt crystals" can be found on the WWW as well...Time to correct an more or less widespread error. Here you can find more information and photos about the true nature of giant basalt columns, which sometimes strikingly resemble giant crystals.
Basalt was a great puzzle to early geognosts (what is the old term for geologists). Was it a volcanic rock formed by fire or a sedimentary rock laid down in some ancient ocean ? Which later dried out and with them the black rock called basalt, which formed polygonal cracks and joints during this process ?..In fact this sounded rather logical and so most geognosts of their time interpreted basalt as ocean sediment with typical dessication cracks !
It was the classic time of Johann Wolfgang Goethe and the pros and cons, the neptunists with their marine basalt formation genesis and the plutonists with their fire theory faught a severe scientific battle about this issue. The clash between the different schools centred around a small basalt hill in Western Bohemia called Kammerbühl (today : Komorní hurka) and it was Goethe himself, who visited this hill eleven times to bring light into the darkness. At last the great poet himself, which was a great nature scientist too, ordered an adit to be driven into the hill at his own cost and this was done. The adit finally found the volcanic root below the Kammerbühl hill and solved once and for all the scientific controversity : Basalt is a volcanic rock type !
But wait...what about the polygonal and often almost perfect hexagonal columns ? Well, if basalt is of volcanic origin, naturally these often magnificient columns have to be crystals, crystallized from the molten lava, isnt it ?
That is indeed, what many scientists thought for quite a while and you can find graphics in old geology books depicting "basalt crystals" resembling giant quartz crystals. However you always just find "prism faces" but never any "pyramid faces" on the basalt columns. And if you take a closer look with your lens or break any basalt column, you dont find any internal macrocrystalline pattern for real crystals, but just a mess of microscopic crystal grains of plagioclase, pyroxene, olivine and some minor other minerals such as magnetite and ilmenite. And if you take another look, you discover, that by no means all basalt columns display hexagonal shapes, but eventally also five sided, four sided or eight sided columns. So...no basalt crystals !
But what are these giant basalt columns than..if not crystals ? Here we come back to the old arguments of the neptunists. At least in one point they were quite right. Basalt columns form as sort of dessication cracks pretty much like drying mud cracks. The main difference is the temperature : basalt is cooling down quite rapidly and by doing so it starts shrinking. Of course this shrinking process starts at the surface, which is either air or more likely another, cool rock. And from that contact the shrinking process slowly develops into the depth, forming longer and longer columns. These can grow to many meter length, depending on the thickness of the original basalt lava flow. In some cases, such as the Devils Tower in Wyoming, the basalt columns may reach even 100 m length and more, but that surely is an exceptional length.
So basalt columns, magnificient as they are , are a result of cooling down and shrinking. Or is there possibly an alternative explanation ? Well, there is indeed, at least for the Giants Causeway : it is the legend of the giant Finn MacCool and his rival Fingal long, long time ago :
Finn McCool was an Irish Giant and lived on an Antrim headland and one day when going about his daily business a Scottish Giant named Fingal began to shout insults and hurl abuse from across the channel. In anger Finn lifted a large rock and threw it at the giant as a challenge, but the rock landed in the sea.
Fingal retaliated with a rock thrown back at Finn and shouted that Finn was lucky that he wasn’t a strong swimmer or he would have made sure he could never fight again.
Finn was enraged and began lifting huge boulders of rock from the shore, throwing them so as to make a pathway for the Scottish giant to come and face him. However by the time he finished making the crossing he had not slept for a week and so instead devised a cunning plan to fool the Scot.
Finn diguised himself as a baby in a cot and when his adversary came to face him Finn’s wife Oonagh told the Giant that Finn was away but showed him his son sleeping in the cradle. The Scottish giant became apprehensive, for if the baby was so huge, what size would the father be?
In his haste to escape Fingal sped back along the causeway Finn had built, tearing it up as he went. He is said to have fled to a cave on Staffa which to this day is named ‘Fingal’s Cave’.
The bridge was destroyed, but to this very day you can visit and see its remains on the opposite shores : The Giants Causeway in Northern Ireland and the equally impressive Fingals Cave on the Island of Staffa in Scotland.
Text (slightly altered)from Lisa McGarry on The Northern Ireland Guide
Whatever explanation you prefer, Giants Causeway certainly is a perfect place to visit when you are in Ireland and so are many other places with giant basalt columns, which you find around the globe.