Caldera or flank collapse in the Fogo volcano? What age? Consequences for risk assessment in volcanic islands

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The target of this study is the Fogo Island in Cape Verde, where the hazards (caldera and/or flank collapse) and the timing (120 ka or younger?) are still controversial. Using high-resolution DEM, field geological and structural analysis, and high-precision K-Ar dating, we produced an age-calibrated volcanic stratigraphy of Fogo’s summit (Chã das Caldeiras, for the last ca. 220 ka). From this, we infer the following evolution and associated processes: (1) the Fogo Volcano formed during seven stages of construction and partial destruction; (2) three flank collapses can be recognised, the biggest of which occurred between 60 and 43 ka; (3) this collapse occurred retrogressively, producing at least two distinct collapse blocks; (4) the innermost collapse was only partial, forming a flat step where a new volcano (Pico do Fogo) grew and formed the Chã das Caldeiras (literally Flat of the Calderas); (5) the removal of the buttressing eastern flank by the outermost collapse can be responsible for the seemingly “caldera” structure; (6) the growing load of the young volcano can produce a new flank failure following the innermost fault; (7) the young and closely spaced collapse ages here reported indicate a significant risk for the inhabitants of Fogo, in particular, and for the whole Cape Verde archipelago (and Atlantic seaboard) if a tsunami is produced by a future catastrophic flank collapse.