An example of river pattern evolution produced during the lateral growth of a central polygenic volcano: the case of the Alcantara river system, Mt Etna (Italy)

Analysis of numerous geognostic soundings and geophysical investigation data, indicates the presence of a wide depression, identifiable as the ancient valley of the Alcantara River, below the northern flank of Mt Etna. Surface and subsurface data were used to reconstruct the complex stratigraphic succession filling the paleo-Alcantara valley, consisting of superimposed lava flows, interlayered with thick lentiform clastic bodies. The evolution of the Alcantara river system was defined with reference to the different phases of growth of Mt Etna. The first correlation between volcanic activity and the drainage system refers to the initial phases of alkaline volcanism (170–100 ka). During this period, the southern watershed of the paleo-Alcantara valley acted as a morphological barrier to the products of the various volcanoes which gradually grew in the sector now occupied by the Valle del Bove. The paleo-Alcantara riverbed was sporadically invaded by lava flows generated by peripheral activity of the central edifice. A radical modification of the hydrographic pattern of the region began about 45–40 ka as a consequence of the growth of the Ellittico volcano, the main eruptive centre of the Etnean edifice. The gradual expansion of this volcano led, over a period between about 30 and 25 ka, to the northward deviation of the paleo-Alcantara riverbed. The consequent erosion of a section of the watershed with the adjacent hydrographic basin of the paleo-S. Paolo river resulted in the capture of the latter. From then on, the hydrographic pattern of the area gradually took on the characteristics it has today. A water circulation system developed within the thick volcanic pile filling the paleo-Alcantara valley, with the formation of an aquifer, which is today one of the most important hydro-structures of the whole volcanic edifice and is, in part, exploited in the eastern side of Sicily.