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

Abstract 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 (Cha 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 Cha 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.