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Shelves from volcanic ocean islands result from the competition between two main processes, wave erosion that forms and enlarges them and volcanic progradation that reduces their dimension. In places where erosion dominates over volcanism, shelf width can be used as a proxy for the relative age of the subaerial volcanic edifices and reconstruction of their extents prior to erosion can be achieved. In this study, new multibeam bathymetry and high-resolution seismic reflection profiles are exploited to characterize the morphology of the insular shelves adjacent to each volcanic edifice of Terceira Island in order to improve the understanding of its evolution. Subaerial morphological and geological/stratigraphic data were also used to establish the connection between the onshore and offshore evolution. Shelf width contiguous to each main volcanic edifice is consistent with the known subaerial geological history of the island; most of the older edifices have wider shelves than younger ones. The shelf edge proved to be a very useful indicator in revealing the original extent of each volcanic edifice in plan view. Its depth was also used to reconstruct vertical movements, showing that older edifices like Serra do Cume-Ribeirinha, Guilherme Moniz, and Pico Alto have subsided while more recent ones have not. The morphology of the shelf (namely the absence/ presence of fresh lava flow morphologies and several types of erosional, depositional, and tectonic features) integrated with the analysis of the coastline morphology allowed us to better constrain previous geological interpretations of the island evolution.

On 30 December 2002, a major instability event, deeply involving the submarine slope, occurred on the Sciara del Fuoco, on the western flank of Stromboli volcano, in the Aeolian Islands. Tsunami waves with a maximum runup of over 10 m in Stromboli were generated, having a measurable impact as far as the Sicily coast. Just 10 months before the event, a multibeam bathymetry had been collected in the area down to 1000 m of depth. A repetition of the survey after the slide allowed the unique opportunity to verify the occurrence of a large submarine slide and to define volumes involved and morphology generated by the event, through the comparison of the preevent and postevent bathymetric grids. A morphological characterization of the slope before and after the submarine landslide is presented, showing how the preexisting features interacted with the slide event in controlling the instability. Mechanisms of the submarine failure are discussed on the basis of the geometrical characters of the landslide event, structural and stratigraphic setting of the submerged slope, and geotechnical considerations on the behavior of slope material

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