The central part of the Island of Pantelleria is occupied by a caldera depression of ellipsoidal form, the major diameter of which is about 7 kilometers. The collapse causing the caldera to form seems to have taken place after a complex eruption which led to the extrusion of a great endogenous dome, the formation of which was followed by a localized explosive activity. The succession of the acid volcanites of Pantelleria shows periodical evolutions of the magmas, undergoing various cycles of differentiation. After a period of quiescence a new cycle of differentiation started again. This may be explained by admitting that the differentiation was accompanied by an increase in the gas content. The violent degassing re-established conditions analogous to those which had existed at the beginning of the previous cycle.