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.