Large-scale mass wasting on small volcanic islands revealed by the study of Flores Island (Azores)

A Hildenbrand,F O Marques,J Catalão

doi:10.1038/s41598-018-32253-0

Abstract

Small intra-plate volcanic islands (total height above seafloor <2500 m) have been considered gravitationally stable. Topographic, stratigraphic, structural and new K/Ar data show that the small island of Flores (Azores) is strongly asymmetric and made up of nested volcanic successions. Along the northwestern coastline, ca. 1.2 Ma lava flows are in lateral contact with a younger volcanic unit (ca. 0.7 Ma), reflecting the existence of a steep lateral discontinuity. From the general dip of the lava flows, their age and the arcuate geometry of the contact, we infer a major landslide that removed the western flank of the older volcano. Further inland, E-dipping lava flows at the summit of the island are ca. 1.3 Ma, suggesting another landslide structure that displaced the whole western half of the former volcanic edifice. Available offshore data show a large hummocky field west of Flores, here interpreted as voluminous debris-avalanche deposits. Unlike the eastern and central Azores islands, Flores sits on a relatively stable tectonic setting. Therefore, we propose that small-size volcanic islands can be sufficiently gravitationally unstable to experience recurrent episodes of large-scale mass wasting triggered by mechanisms other than tectonic earthquakes and thus represent an under-evaluated potential source of hazard and, therefore, risk.

Highlights

Large-scale mass wasting (LSMW) on big oceanic islands (>5–6 km above the sea floor) can remove catastrophically large volumes of volcanic material and generate mega-tsunamis that may impact the whole surrounding shorelines, up to distances of several thousands of kilometers[1,2]
LSMW in those cases seems closely linked to active tectonics (e.g.32,33) and such islands may represent special cases in which the volcanic edifice would not fail in the absence of tectonic trigger
The main question we address here is : Are small volcanic islands capable of undergoing LSMW even in the absence of triggering mechanisms like active tectonic faults and medium/high magnitude earthquakes? To answer this question, we focused on Flores (Azores), a small volcanic edifice developed in a stable tectonic setting

Summary

Introduction

Large-scale mass wasting (LSMW) on big oceanic islands (>5–6 km above the sea floor) can remove catastrophically large volumes of volcanic material and generate mega-tsunamis that may impact the whole surrounding shorelines, up to distances of several thousands of kilometers[1,2]. From on and offshore studies, giant landslides (>100 km3) have been recognized on large intra-plate volcanic islands, e.g. along the Hawaiian Ridge[3,4], in Polynesia[5,6,7], in the Canaries (e.g.8–14), in the Reunion Island (e.g.15) and in Cape Verde (e.g.16). Such large intra-plate volcanic edifices frequently reach a total height up to 10 000 m from the surrounding seafloor and are the most gravitationally unstable. Small-size volcanic islands and seamounts (total height < 2500 m) are believed to lack LSMW (e.g.17). Geological, geochronological and available bathymetric data, we show that the island is topographically and stratigraphically asymmetric, from which we infer that LSMW has removed at least half of the original volcanic edifice

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