How was the Giants Causeway formed

asks Mayastar:
A. The Giants causeway was made at the same time as the Isle of Mull, Staffa and Skye - about 65 million years ago.

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Q. Were they once joined
A. No, the eruption that caused them placed them as far apart as they are today.

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Q. How were they formed
A. It all started about 150 million years ago. The Atlantic Ocean didn't exist and the British Isles were joined to what is now Canada and Greenland on one side, and Europe on the other. However, it's generally accepted that the European and American land masses were already try to break away from each other at this point by means of convection currents.

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Q. What are they
A. Convection (heat) currents tear at the land a little at a time along existing rifts. But they alone were not strong enough to break the continents apart. What was needed was a 'mantle plume'.

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Q. What's a mantle plume, then
A. A mantle plume is a rush of heat from the Earth's core which passes right through the mantle - the part of the Earth between the core and the crust - until it reaches the base of the crust. Here, it spreads out, and provides enough power to split apart the land masses that the rifts and convention currents have already weakened.

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So, about 65 million years ago, a plume forced the European side (including the British Isles) eastwards, and the American side westwards. That plume still exists - it created Iceland, and continues to push Europe and America apart.

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Q. So did the plume create the Giants Causeway
A. Yes. It melted material from just below the crust and this reached the surface as basalt lava. Basalt lava is what formed the crust beneath all the world's oceans as the continents moved apart.

However, in northern Ireland and western Scotland, some basalt lava was forced up through cracks in very old weaknesses cutting through northern Britain, such as the Great Glen Fault (creating the lavas of Mull and Fingal's Cave) and the Highland Boundary Fault, which can be traced across Ireland (creating the lavas of Antrim - including the Giants Causeway).
The peculiar shapes in the Giants causeway were created by the way this type of lava cools.

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Q. What is that
A. As basalt lava cools it shrinks. Certain places will start to go hard first and the lava will solidify in increasing circles around them, shrinking as it does. Lava in-between two cooled centres will be pulled in both directions, and a crack will form between them. The cool spots are random, although the cracks tend to be hexagonal, but they vary a lot.

The lava also starts to cool from the top (because of air) and from the bottom (because of the rocks it has travelled over), so the crack pattern forms first at the top and bottom of the lava, then travels downwards and upwards as the lava cools in the middle, forming the basalt columns seen on the Giants Causeway and in Fingal's Cave on Staffa.

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By Sheena Miller