Contribution on geological and volcanological aspects of Monte di Vezzi (Ischia Island, Italy) to study the landslide events of April 2006

Abstract

Volcanic activity at Ischia Island encompasses five phases, aged between 150 ky B.P. and 1302 A.D. Different stages of volcanic activity implied different volcano-tectonic events, summarized as follows: a) Epomeo horst resurgence, b) built of the structural highs of Mt. Vico, Mt. Panza (at west), Mt. di Vezzi, Mt. Torone, Mt. Campagnano (at east), c) Ischia graben development in the eastern sector of the island. According to several authors Mt. di Vezzi, constructed in the second phase of activity (150 ky-75ky), is a lava dome. In this paper we define it as a complex volcanic morphology, firstly built by strombolian activity passing to a lava fountaining phase. This produced a spatter lava bank and culminated with the emplacement of a massive “dome flow”, which shows fluidal bending structure. After the deposition and during cooling the lava body experienced thermally induced stress joints, notwithstanding that the results of mesostructural analysis indicate that tectonic movements related to the regional stress field contributed to joints development. The geological model of the Mt. di Vezzi area made it possible to identify the following main geological factors that concurred to cause the instability of the slope: - the different mechanical behavior of terrains along the boundary lava-welded scoria deposits; - the diffuse presence of joints in the upper lava deposits; - the presence of a 6 m wide morphological step at the previously quoted boundary, due to the withdraw of the upper part of the slope where the fractured lavas crop-out; - the piling up along this step of thick loose recent pyroclastic products (up to 5 m of thickness) both primary and reworked; - the presence in the intermediate part of the slope of scoriaceous layers 25-30° northward dipping. The landslides occurred on the northern Mt. di Vezzi slope, and shows geomorphological characteristics and mechanical behavior other then those of the landslides typical of the island. Because of this, further studies are needed on the slope evolution at Ischia island, mainly at Mt. di Vezzi, where a residual risk still exists both in the sites of May 2006 landslides as well as in the nearby areas showing similar geological features.