Development of volcanic insular shelves: Insights from observations and modelling of Faial Island in the Azores Archipelago

A variety of geological processes contribute to the development of volcanic island shelves, although their relative importance remains unclear. We have mapped the morphology of Faial Island's shelf in the Azores from geophysical (multibeam sonar, chirp and boomer seismic reflection) and geological data (sediment sampling). A wave erosional model and a eustatic record of glacially induced oscillations of sea level, which modulated the elevation at which the waves operated through time, was used to determine the contribution of mechanical wave erosion to shelf development. Wave regimes varied according to the orientation of the coast. The model was run for 10 to 800 ka, depending on the age of the various coastal sectors, based on published radiometric data. The modelling replicated the shape of the island's shelf reasonably well, supporting the contention that it developed primarily through mechanical wave erosion operating in an intertidal zone that migrated landwards and seawards with changes in the level of the sea. Disparities between modelled and actual shelf morphologies may reflect varied geological resistance to erosion, errors in the radiometric dates, more recent, unrecognised volcanic deposits, sediment deposition and tectonic vertical movements. There were some variation in rates of shelf development over different time scales, but over the long-term, the relationship between shelf age and width was essentially linear. The depth of the shelf break in some areas suggests that there have been differential vertical movements of the land around the island, probably related to local faults. The poor development or absence of terraces on the shelf may be attributed either to recent volcanic progradation or to sediment infilling of the space created by earlier erosion. However, poorly developed terraces on the southern shelf of Faial are similar to those predicted by the model, given appropriate adjustments for vertical movements of the land.