Why do small volcanic ocean islands collapse? Lessons from Santa Maria Island, Azores Triple Junction

Abstract

Abstract Volcanic ocean island collapse is a gravitational process, therefore mass is a key variable. Based on this premise, islands much smaller than Hawaii are believed to be not prone to collapse. Here we show that they can collapse, and more than once, like in Santa Maria Island (Azores), 170 times smaller than Hawaii, as inferred from onshore data and new high-resolution bathymetry. Santa Maria sits on oceanic sediments hundreds of meters thick, the top of which is unconsolidated, water saturated and, therefore, soft. Numerical simulations indicate that, if the volcanic edifice is strong, it does not collapse, even if its base is weak. However, a relatively weak edifice can collapse over a weak base. We conclude that small volcanic islands can collapse when both the edifice and its base are weak. Our numerical simulations also indicate that, if the basal detachment only partially occupies the base of the volcano edifice (outer soft sediment ring less affected by pressure and temperature), the flank will only partially collapse. This could be the case of the Hilina Slump in the Big Island of Hawaii, because the large size of the edifice can produce high-grade metamorphism in the sediments at the core of the edifice’s base.