Despite my relatively short career as a geologist, it was a hard choice. There was a spectacular fault outcrop in Arkitsa, Greece; some impossible-looking resistant beds sticking straight out of the forest near Benés in the Catalan Pyrenees; and the classic 'ripples on a vertical surface' to illustrate tectonic forces, also in the Catalan Pyrenees. However, I decided on this photo in the end, mainly because now I am a proper volcanologist I felt I should choose a volcano-related picture.
The photo is from Santorini, and shows a volcanic bomb. The layer it has impacted into is about 2m thick. While this is far from the largest bomb on the island, the way it is exposed here really does emphasise the power released when a volcano decides to blow.