Basalt lava flow, Staffa

Huge lava flows like the one in the image below, which also forms the Giant’s Causeway in Northern Ireland, show that about 60 million years ago north-west Britain was volcanically very active, like Iceland is today. The lava flows were erupted on land (there was no sea here then) and flowed across a wide area of what is now western Scotland, Northern Ireland, and East Greeland!

As the newly-formed basalt lava cooled, it contracted and broke into sections which, in the lower part of the flow, produced regular 6-sided columns. The rubbly upper part of the flow contains many gas bubbles trapped as the lava solidified, and so did not break as evenly.