Characterising the Irregular Coastlines of Volcanic Ocean Islands


Neil C. Mitchell

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 length, 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, Comors, Molokai and Madeira). Since most of these islands lie on the African plate which is slow moving relative to the mantle, this may reflect a fortuitous superposition of adjacent volcanoes by stationary hotspots, 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 underlying oceanic basement, revealing that islands on seafloor created at slow-spreading ridges are preferentially oriented perpendicular, parallel and at 30û to the fabric and that islands on young seafloor show a slight tendency to be aligned fabric-parallel.

Mitchell, N. C., Characterising the irregular coastlines of volcanic ocean islands, Geomorphology, 23, 1-14, 1998.


Return to Mitchell home page