Volcanism in the Azores: a marine geophysical perspective

In contrast to oceanic islands produced by mantle melting anomalies or “hotspots” lying in mid-plate settings, the central and eastern Azores islands are distributed on the Nubia-Eurasia tectonic plate boundary and experience frequent tectonic earthquakes. As they lie in an extensional to trans-tensional tectonic environment, volcanism is organised into submarine and subaerial ridges oriented perpendicular or oblique to the direction of plate separation. Much of the marine geophysical work undertaken to study these features and the submarine flanks of the islands has involved seabed mapping with sonars of various kinds, beginning with the GLORIA long-range sidescan sonar deployed in the late 1970s and continuing more recently with the deeply towed sidescan sonar TOBI and multibeam bathymetric sonars in dedicated expeditions and ship transits. These datasets give us a view of the topographic structure of the ridges and of the morphologies of the volcanic and tectonic features comprising them. Sonars also have been used to investigate the incidence of large- and small-scale landsliding around the islands, as well as the morphologies of lava flows originating from land and entering the sea. The overall picture emerging is one of landslides, cones, terraces and ridges analogous to those of Hawaii, Canaries and other oceanic island groups but with notable differences. For example, large-scale landslides appear to be rarer in the Azores and submarine cones tend to be steep-sided and pointed, not flat-topped or cratered as they are in some parts of the Hawaiian Islands. Faults are also more common in the Azores tectonic environment.