The insular shelves of the Faial‐Pico Ridge (Azores archipelago): A morphological record of its evolution

Abstract

Shelves surrounding reefless volcanic ocean islands are formed by surf erosion of their slopes during changing sea levels. Posterosional lava flows, if abundant, can cross the coastal cliffs and fill partially or completely the accommodation space left by erosion. In this study, multibeam bathymetry, highresolution seismic reflection profiles, and sediment samples are used to characterize the morphology of the insular shelves adjacent to Pico Island. The data show offshore fresh lava flow morphologies, as well as an irregular basement beneath shelf sedimentary bodies and reduced shelf width adjacent to older volcanic edifices in Pico. These observations suggest that these shelves have been significantly filled by volcanic progradation and can thus be classified as ''rejuvenated.'' Despite the general volcanic infilling of the shelves around Pico, most of their edges are below the depth of the Last Glacial Maximum, revealing that at least parts of the island have subsided after the shelves formed by surf erosion. Prograding lava deltas reached the shelf edge in some areas triggering small slope failures, locally decreasing the shelf width and depth of their edges. These areas can represent a significant risk for the local population; hence, their identification can be useful for hazard assessment and contribute to wiser land use planning. Shelf and subaerial geomorphology, magnetic anomalies and crustal structure data of the two islands were also interpreted to reconstruct the long-term combined onshore and offshore evolution of the Faial-Pico ridge. The subaerial emergence of this ridge is apparently older than previously thought, i.e., before 850 ka.

Soil CO2 Degassing Path along Volcano-Tectonic Structures in the Pico-Faial-São Jorge Islands (Azores Archipelago, Portugal)

“…Several authors consider that Faial and Pico are the emerged parts of a single main volcanic ridge, the Pico-Faial ridge (Hildenbrand et al, 2012 and references therein;Quartau et al, 2015). In the present study we will analyze the degassing patterns associated to each island individually.…”

Comment on “The insular shelves of the Faial‐Pico Ridge (Azores archipelago): A morphological record of its evolution” by Quartau et al.

“…In the opening of the Abstract of their paper, Quartau et al . [] state that “Shelves surrounding reefless volcanic ocean islands are formed by surf erosion of their slopes during changing sea levels.”; in the second paragraph of the Introduction they say that “The shelves surrounding volcanic ocean islands are formed by a competition between wave erosion, which forms and enlarges them, and volcanic progradation which narrows them”; and in section 4.1, dedicated to the conceptual model, they say that “The analysis of the data shows shelves that have been formed by wave erosion and whose accommodation space has been continually diminished by volcanic progradation, although some subsidence has occurred.” This means that, although recognizing the role of processes like subsidence, they actually did not estimate its rate, and consider mostly two dominant processes in their analysis: wave erosion and volcanic progradation. This might be true for stable islands (null vertical isostatic or tectonic movements), but not for subsiding islands as we argue in this comment.…”

“…Quartau et al . [] concluded that “… insular shelves were formed by wave erosion of the flanks of the volcanic edifices of Faial and Pico Islands. Pico insular shelves have been significantly modified by later volcanic progradation,” thus not taking into proper account the major effects of large‐scale flank collapses in shaping a great part of Pico Island.…”

“…Despite the likely dominant effect of island subsidence, Quartau et al . [] recognized its existence in Pico but did not estimate its magnitude or use the subsidence rate to discuss the wave erosion rates reported in the paper. Therefore, the wave erosion rates reported in Quartau et al .…”

“…Therefore, the wave erosion rates reported in Quartau et al . [] may be greatly overestimated by incorporating indiscriminately two rates, island subsidence and wave erosion. The effects of subsidence on shaping insular shelves as discussed here can potentially be applied to other subsiding islands mostly made of shield volcanoes, like the Big Island in Hawaii.…”