Characterising the irregular coastlines of volcanic ocean islands

Abstract

Volcanic ocean islands grow into irregular shapes because of eruption along radiating volcanic rift zones, caldera collapse, slope failure, coalescence of adjacent volcanoes and marine and subaerial erosion. With the aim of searching for variations with environmental parameters which might reflect variations in these processes, this study calculates and analyses characteristics of island irregularity from the World Vector Shoreline database. Islands are idealised as flat plates bounded by the digitised shorelines from which perimeter distance, elongation and moment of inertia are calculated to represent irregularity. These characteristics are highly variable and do not correlate strongly with any of the parameters studied here, reflecting the complexity of processes shaping island coastlines. On closer scrutiny, some weak variations with seafloor age and spreading rate are due to some highly irregular islands on old seafloor mostly formed at slow-spreading ridges (Canaries, Cape Verde, Comores, Molokai and Madeira). Since most of these islands lie on the African plate which is slowly moving relative to the earth's mantle, this may reflect a fortuitous superposition of adjacent volcanoes by stationary hot-spots, although their embayed outlines suggest that landslides are also important. The analysis also resolves the orientations of islands with respect to the tectonic fabric of their underlying oceanic basement, revealing that islands on seafloor originally created at slow-spreading ridges are preferentially oriented perpendicular, parallel and at 30° to the fabric and that islands on young seafloor show a slight tendency to be aligned fabric-parallel.