Geology

The giant basalt columns which gave rise to the island’s Norse name Staffa – literally staff-ay or staff (pillar) island – are truly remarkable. Columnar basalt formations are not rare, but those on Staffa are spectacularly large and regular. The comparison with the Giant’s Causeway in County Antrim was noticed many centuries, if not millennia, ago. However, although both were formed around 61 million years ago, they were formed by different volcanic eruptions, at different times, and were never directly connected.

Staffa and the outlying Treshnish Isles, as well as much of Mull, are the remains of a great pile of basaltic lava flows, much of which has since been eroded away. The columns formed as a result of thick lava flows being erupted across damp or wet ground. The vertical columns were formed after the lava had mostly solidified, by the shrinkage cracking of hot but solid rock. The flatter the ground underneath the lava flow, the more slowly and evenly the flow cooled and the larger, more vertical and regular the columns. Smaller, curved or angled columns formed under more irregular cooling conditions, where the lava flow was thinner or was cooling against sloping ground below or at the sides of the flow.