Erosion processes on the rocky coasts of Ponza Island (Italy)

The Island of Ponza is part of the Pontine Archipelago located at an average distance of about 50 km from the coast of southern Lazio in Italy. From the geographical point of view, this archipelago is siyuated between the oute edge of the continental shelf and slope, which connects with the abyssal plain of the Tyrrhenian Sea. Its origin is linked to volcanism that has charcterized this margin of the Apennines during the Plio-Pleistocene extensional phases. At present tectonic and volcanic events (the latest emission date back about 200.000 years) can be considered concluded even if the area is still affected by a weak earthquake activity. The coast of the island is characterized by cliffs cut off in volcanic rocks, mostly sub-vertical, with heights ranging from about 10 m to over than 130 m. Not lacking at the base of them, as well as inside inlets and bays, often coinciding with secondary craters, sandy-pebbly beaches. The concentration of energy on headlands has also formed arches and sea stacks. Evidences of marine abrasion, active since the Early Pleistocene, are present in some relics od surfaces at altitudes of 200-270 m, 100-120 m and 45-50 m. To these raised surfaces are respectively associated gravelly beach deposits, a strongly altered paleosoil and weakly cemented eolian deposits. Marine erosion takes places at the foot of the cliffs, whereas on the face of them works the abrasion of the wind, the run off of the water and its inflitration in the lines of weakness, the weathering of the rocks by meteroic agents and, even, the seismic action. The lithological nature of the volcanic formations, characterized by high fracturing as well as fault plane and fractures, which also isolate blocks of considerable size, determines conditions of great danger with frequent falls involving rock masses from small to extremely large sizes. The danger is usually heightened, when the top of the cliff is mantled by epiclastic deposits from loose to weakly cemented. These deposits, due to the flow of water and the physical action of plant roots, can became unstable, resulting in slidings and rapid flows. In these mass movements are also involved other volcanic material detached from the cliffs. The continuous retreat of the cliff is proved by: the constant presence of significant accumulation of debris at their foot, despite the wave action; absence or lack of evidence, on the exposed face of the cliff, of notches ar sea terraces immediately above to the present sea level, related to eustatic sea level changes occurred in the latest Pleistocene (this phenomenon shows a low coefficient of morphological preservation and thus a "high speed of erosion"; the presence of truncated valleys; the uncovered of the underground ducts of the Roman age.