An INGV research team has identified and morphologically characterized two underwater volcanic fields located a few tens of kilometers from the coast of Sciacca, in the Strait of Sicily, with the aim of improving the estimation of volcanic hazard
A study recently published in the journal “Frontiers in Earth Science” made it possible to improve the knowledge of the Graham Volcanic Field, a hitherto little known active volcanic area located in the Strait of Sicily, about 40-50 kilometers off Sciacca (AG). The research, curated by Danilo Cavallaro and Mauro Coltelli, researchers of the Etna Observatory of the National Institute of Geophysics and Volcanology (OE-INGV), made it possible to investigate a volcanic field located relatively close to the coast and in a very busy stretch of sea from maritime traffic, with the aim of helping to refine the estimate of the volcanic hazard that impacts on the coast and on the safety of navigation.
“Our research”, explains researcher Danilo Cavallaro, “is centered on a detailed morpho-bathymetric study of the Graham Volcanic Field. It is a volcanic field formed by a dozen small volcanic buildings, which also includes the small cone that represents what remains of the ephemeral Ferdinandea Island, formed during the well-documented Surtsey-type eruption of 1831”.
The study is based on high resolution multibeam bathymetric data and ROV (Remotely Operated Vehicle) video thanks to which it was possible to carry out a morphological analysis of the volcanic, erosive and depositional elements that characterize the volcanic field. The cones lie on a seabed whose depth varies between 150 and 250 meters and show heights varying between 100 and 150 meters, reaching up to -9 meters below sea level in the cone of the former Isola Ferdinandea.
They consist of poorly consolidated pyroclastic material, with the exception of pointed spiers present on the top of some cones, made up of massive basalts which represent what remains of the supply ducts. The analysis of the morphometric parameters of the cones, together with that of the erosive and depositional elements and related to the variations in the sea level, has allowed us to confine the age of the volcanism which originated the Graham volcanic field to around 20.000 years ago .
"The spatial distribution and the shape of the submarine volcanic edifices that make up the Graham Volcanic Field", adds the researcher Mauro Coltelli, "allowed us to validate the interaction between tectonics and volcanic activity in the formation of this volcanic field, since the cones are located along alignments oriented from North-West to South-East and from North to South, corresponding to the main tectonic guidelines of the Strait of Sicily".
The research has also made it possible to identify and morphologically characterize another volcanic field, called Terrible Volcanic Field, consisting of about thirty small cones probably older than those of Graham.
The correlation between submarine volcanic processes and the shape of the cones confirmed the nature of the volcanism that originated the volcanic fields. This volcanism represents a peculiarity because it is set in correspondence with a transcurrent belt that affects a continental crust, and therefore in a geodynamic environment different from those typical of other volcanic fields, such as for example subduction zones or oceanic ridges.
Furthermore, the morpho-bathymetric study has identified numerous submarine landslide deposits and depressions caused by the violent emission of gas (pockmarks) near the two volcanic fields studied, suggesting the presence of submarine landslide phenomena and widespread releases of gas accumulated in the subsoil.
Rome, 10 December 2019
#ingv #vulcani #canaledisicilia #graham #volcanic danger
Abstract
Cavallaro, D., Knives, M., 2019. The Graham Volcanic Field Offshore Southwestern Sicily (Italy) Revealed by High-Resolution Seafloor Mapping and ROV Images.
The understanding of submarine monogenetic volcanic fields, especially if located near to coastal areas, is fundamental for volcanic risk assessment. Using high-resolution bathymetric data and ROV images, the submarine Graham volcanic field, located 40–50 km offshore southwestern Sicily (Italy), has been described in detail. The field comprises a ten of monogenetic volcanic seamounts aligned along a NS trending belt at 150–250 m water depths and includes the relict of the short-lived “Ferdinandea Island” produced during the well-documented 1831 “Surtseyan-type” eruption. The present-day morphology of the cones is the result of the interplay between volcanic activity, wave and current erosion, mass-wasting and depositional processes, in relationship with sea-level change, acting in both subaerial and submarine environments. The analysis of the morphometric parameters allowed a detailed morphological classification of the cones. The seamounts are composed of poorly consolidated tephra and show steep slopes and pointy or flat tops, often characterized by sub-vertical knolls. Taking into account analogies with other volcanic seamounts worldwide, the analysis of some morphological characteristics, such as presence and depth of terraces on top and along the slope of the cones in relationship with sea-level fluctuations, allowed us to hypothesize a Late Pleistocene-Holocene age for the volcanism forming the field. The probably older Terribile volcanic field was also identified on the adjacent Terribile Bank and analyzed. Numerous mass-transport deposits and pockmarks were identified in the surroundings of the volcanic fields, suggesting the occurrence of diffuse slope failures and fluid releases, respectively. The distribution and shape of the cones within the volcanic fields provided important insights into the interaction between volcanism and tectonics. The alignment of the cones and the main axis of the clusters in which they are grouped revealed two preferred directions, NS and NW-SE, respectively, which are consistent with those of the main tectonic structures of the Sicily Channel. The detailed bathy-morphological analysis of the cones proved the monogenetic nature of this volcanism, which represents a peculiarity since it took place outside the typical geodynamic settings of other volcanic fields worldwide such as subduction or oceanic rift zones, and far from long-lived volcanic systems.
Image - Clockwise: (A) The study area with location of the named submarine volcanic fields Graham Volcanic Field (blue box) e Terrible Volcanic Field (white box); (B) The alignment of the volcanic cones that form the Graham Volcanic Field with the clusters in which they have been grouped highlighted; (C) A 3D image of Banco Graham with in the upper right corner the cone representing what remains of the ephemeral Ferdinandea Island and next to it S5, the largest cone of the entire volcanic field; (D) Geodynamic overview of the study area.