From Imaggeo.egu.eu: http://imaggeo.egu.eu/view/1166/. Columnar jointing forms from volcanic rocks that cool in such a way that cracks form at the edges of the hot rock/lava. As the rock cools the cracks deepen into the lava, and tend to form columns that have between 3 and 12 sides. The cracks form like this due to stress differentiation within the lava when cooling.
-2.1842 W, 36.7206 N