On the north-east coast of County Antrim in Northern Ireland lies an unusual rock formation capable of drawing in millions of visitors from around the world every year. They flock to see a vast plateau of polygonal basalt columns, known as the Giant’s Causeway, which looks like a carpet of enormous steppingstones extending out into the Irish Sea. The basalt pillars that make up this amazing rock formation range in size from a matter of centimetres to tens of metres.
Although the Giant’s Causeway is so-named due to an ancient legend, its formation actually began some 60 million years ago when volcanic activity forced tectonic plates to stretch and break, causing magma to spew up from inside the Earth and spill out across the surface as lava.
The temperature of erupting lava can range from between 700 and 1,200 degrees Celsius (1,292 and 2,192 degrees Fahrenheit). However, upon contact with the surface it will immediately begin to cool. At first this cooling is extremely rapid and causes a hardened crust to form on top of the lava, which insulates the still liquid lava below. Because the lava is insulated this cooling becomes increasingly slow over time. While you could probably walk on the lava’s crust after about half an hour, thick lava flows can take many years to cool completely and become totally solid.
While the temperature falls the lava dries out, and it’s this drying that causes the solidifying lava to crack and form regular pillars of basalt rock. The size and shape of each basalt column is determined by the rate at which the lava cools and dries, and therefore the speed at which what’s called the ‘drying front’ moves. Scientists from the University of Toronto discovered that the slower the cooling the larger the basalt columns.