Comment on “Construction and destruction of a volcanic island developed inside an oceanic rift: Graciosa Island, Terceira Rift, Azores” by Sibrant et al. (2014) and proposal of a new model for Graciosa's geological evolution

Volcanoes rising above sea level within extensional oceanic plate boundaries provide accessible locations with which to study the effects of plate tectonic and volcanic processes of such areas. However, relying solely on subaerial observations can lead to biased interpretations. Reconciling the information provided by multibeam echo sounders on the submarine parts of volcanic islands with geology and geomorphology observable above sea level can potentially provide more robust interpretations. In this comment of the study of Sibrant et al. (2014), which is based almost solely on subaerial observations, we show how the published multibeam sonar data around Graciosa reveals that their proposed successive phases of destruction of the volcanic edifices composing the island by massive landslides is incompatible with the high-resolution bathymetry. The data reveal no large-scale debris avalanche deposits or characteristic flank collapse scars where Sibrant et al. (2014) propose these landslides to have occurred. Instead, the data show volcanic constructional areas, some of which have simply been eroded by wave abrasion. The interpretation of collapse structures appears to have originated partly from a misreading of the volcano-stratigraphy and tectonic structures. Overall, wave erosion coupled with subaerial erosion and tectonic activity can more easily explain the onshore observations of Sibrant et al. (2014), providing a less catastrophic explanation for the evolution of Graciosa Island.