C. Romagnoli

Volcanic, tectonic and mass-wasting processes offshore Terceira Island (Azores) revealed by high-resolution seafloor mapping

Terceira Island, in the Azores Archipelago, lies at the intersection of four submarine volcanic ridges. New high-resolution bathymetric and seismic reflection data have been used to analyze the main volcanic, tectonic and mass-wasting features of the island offshore. Volcanic features such as linear volcanic centers, and pointy and flat-topped cones are mainly concentrated on the narrow western and north-western ridges, characterized by an overall rugged morphology. Fault scarps dominate mainly the broad eastern and south-eastern ridges, which are characterized by an overall smooth and terrace-like morphology. On the eastern ridge, faults form a series of horsts and grabens related to the onshore Lajes Graben. The strikes of the fault scarps, linear volcanic centers and alignment of volcanic cones on the ridges reveal two main structural trends, WNW–ESE and NNW–SSE, consistent with the main tectonic structures observed on the Azores Plateau. In contrast, a large variability of strike was observed in inter-ridge areas, reflecting the relative importance of regional and local stresses in producing these structures. Mass-wasting features are subordinate and mostly represented by hundred meter-wide scars that indent the edge of the insular shelf surrounding the island, apart from two large, deeper scars identified on the southern steep flank of the western ridge. Finally, the remarkable morpho-structural differences between the western and eastern ridges are discussed in the framework of the evolution of the Terceira volcanic edifice and hypothesized to reflect successive stages of ridge evolution.

Late-Quaternary ancient shorelines at Lipari (Aeolian Islands): stratigraphical constraints to reconstruct geological evolution and vertical movements

Indicators of three fossil shorelines, located at elevations of 43–45 m asl (I order), 23–27 m (II order) and about 12 m (III order), are recognized on the island of Lipari. Detailed evaluation of the stratigraphic relationships to the volcanic substratum allows their correlation with Late-Quaternary eustatic highstands of Tyrrhenian age, corresponding to marine oxygen isotope substages 5e (124 ka), 5c (100 ka) and 5a (81 ka). Marine deposits related to the ancient shorelines represent useful stratigraphic markers, as related unconformities constrain the geological evolution of Lipari. This is interpreted as a result of two main stages of volcanic activity (pre- and post-Tyrrhenian) spaced out by one stage characterized by prevalent marine erosional episodes (Tyrrhenian). Chronological and altimetrical data concerning the ancient shorelines have been used to estimate the vertical mobility of the volcanic edifice of Lipari, which is characterized by a continuous uplift at an average rate of 0.34 mm/a during the last 125 ka: this estimation is fully concordant with the values obtained, during the same time period, from the nearby islands of Filicudi (0.31 mm/a) and Salina (0.36 mm/a), suggesting a similar uplift trend induced by the prevalence of regional tectonic processes.

Coastal hazard due to submarine canyons in active insular volcanoes: examples from Lipari Island (southern Tyrrhenian Sea)

The recent high-resolution multibeam bathymetry surveys around Lipari Island allowed to evidence several submarine canyons, whose head often cut back up to very shallow water and at a few tens of meters far from the coast. These canyons are mainly located in the eastern and southern side of the island and are characterized by an ongoing retrogressive (landward) erosion, that also controlled the shape and the evolution of the coastline. The canyon heads are formed by minor slide scars. By coupling slide scar morphometry and simple numerical model we have been able to roughly estimate the potential tsunami wave amplitudes generated by related slope failures. Moreover, the retrogressive erosion of canyon heads can be claimed as a cause of the enhanced subsidence reported in the last few thousand years in the eastern part of Lipari, where the main villages are located. Based on these evidence, we propose a first assessment of the coastal hazard due to marine retrogressive activity in the largest and most densely populated island of the Aeolian Archipelago.

Small-Scale Bedforms Generated by Gravity Flows in the Aeolian Islands

Small-scale crescent-shaped bedforms were identified within flat-bottomed channelized features carving the submarine volcanic flanks of the Stromboli and Salina edifices. The bedforms are downslope-asymmetric, with a sub-horizontal stoss side and a steep lee side. They have a wavelength of tens of metres and a wave height of metres, and their crest-lines are roughly perpendicular to the maximum slope. Based on their size, texture and similarities to other bedforms, they can be interpreted as cyclic steps, i.e. a class of upslope-migrating turbidite sediment waves. Although their genesis must be similar, some differences are observed between the two cases. At Stromboli, repeated multibeam surveys at an 11-year scale showed a significant upslope migration of the bedforms because they lie on the bottom of a channel connected to active coastal dynamics, whose headwall is located at a few metres depth. By contrast, the surveys showed no significant morphological variations at Salina, where the bedforms are larger than at Stromboli and the channel headwall is located at about −100 m, suggesting that these feature are mostly inactive at present.

Relative Sea Level Rise, Palaeotopography and Transgression Velocity on the Continental Shelf

After the Last Glacial Maximum, some 21,000 years BP, the sea level rose from −130 m to its present-day position. This process of marine transgression inundated or eroded palaeolandscapes to varying degrees, resulting in the landward movement of the shoreline. The transgression velocity (TV), i.e., the velocity at which the shoreline migrated landwards, depends on evaluating the balance between the rate of relative sea level rise and the slope of the transgressed palaeotopography. It has a key role in determining the possibilities for reconstructing palaeoenvironments, the potential preservation of archaeological sites and the socio-economic and psychological impact of sea-level rise on past human populations. In this chapter we present a simple conceptual and computational approach to reconstructing the transgression velocity on shelf areas, making use of Digital Terrain Models (DTMs) of seafloor topography coupled with relative sea level curves, and discuss the different outcomes and limitations at different spatial scales, ranging from the continental (European seas) to the ultra-local scale.

Volcanic Landforms and Landscapes of the Aeolian Islands (Southern Tyrrhenian Sea, Sicily): Implications for Hazard Evaluation

The Aeolian Islands are a Quaternary active volcanic structure in Southern Italy. These volcanic islands are characterized by an outstanding display of volcanic landforms (stratocones, lava flows, domes, fissures, dykes, calderas, lateral collapses) derived from repeated episodes of volcanic activity and volcano-tectonic collapse under the control of regional tectonic trends. Stromboli and Vulcano are particularly characterized by ongoing eruptive and gravity-driven instability processes. Geomorphic evolution there plays a fundamental role on the localization of eruptive vents and conduits and the distribution of volcanogenic flows, with important insights on volcanic hazard and risk assessment.

Large-Scale Bedforms on Volcaniclastic Aprons Around the Aeolian Islands (Italy)

Large-scale bedforms, with wavelengths ranging from several tens of metres to over 1 km and wave heights ranging from a few metres to tens/hundreds of metres were observed on multibeam bathymetry in the median and lower part of submarine volcaniclastic aprons surrounding the Aeolian Islands, where a marked decrease in slope gradients below 4°–8° is present. The bedforms are characterized by crest-lines with an arcuate or sinuous shape, trending roughly perpendicular to the maximum slope direction. Based on their size, geometry and texture, they can be interpreted as coarse-grained sediment waves, which are also found in the proximal part of other modern and ancient turbidite systems. In the study areas, they are found within channelized features or on fan-shaped features lying at or close to major sedimentary sources, such as subaerial/submarine depressions left by sector collapses.