The photo above shows basalt columns stacked on the pyramid shaped rock islet known as “The Herdsman,” located just off the Isle of Staffa in the Hebrides Islands of Scotland. These curious columns are mostly hexagonal, averaging between two and three ft (0.7-1.0 m) across. Image credit: Steve Irvine
They formed in the cooling core of a volcano that erupted here about 55 million years ago. The light coloring of the columns at the base of the islet results from small shells of marine animals, whereas the orange color at the top is due to lichen growth. The dark color of the rock in the mid-region is the normal color of the basalt. This area of the islet is too high for marine organisms to colonize, but also gets too much salty sea-spray for lichen to flourish. Photo taken on June 20, 2012.
Author: Steve Irvine