Slieve Gullion itself is in fact a more recent addition than the ring dykes which surround it and is made up of layers of igneous rock. As with the ring itself, there has been some debate as to their origins. One suggestion is that a huge explosive eruption of the volcano created a vast crater, or caldera, into which lavas were excluded in layers. Another more plausible explanation is that the lavas were extruded in layers. There is evidence of many highly unusual features developed by the interaction of basic and acidic magmas.