English: Accretionary Lapilli These little concentric rings are cross-sections of accretionary lapilli. They are formed when ash particles erupted from a volcano in moist clouds, and they accrete like hailstones into spheres. There is a thicker layer of these here than at Stoer, suggesting that the source which produced them was closer to here than to Stoer. It has recently been suggested that they may have been produced in a meteor strike rather than a volcanic eruption, but there is no consensus about this idea.