Why and how do small volcanic ocean islands collapse? Lessons from Santa Maria Island, Azores

Abstract Volcanic ocean island collapse is a gravitational process, which means that mass is a key variable. Here we show that small volcanic islands (small mass) usually considered stable can undergo full flank collapse (i.e. full flank plus summit), and we also evaluate the most likely collapse mechanisms by numerical modelling. Santa Maria Island (Azores) is ca. 170 times smaller than Hawaii; however, it has collapsed more than once, as inferred from onshore data and new high-resolution bathymetry and seismic reflection data. Santa Maria is less than 8 Ma old and lies on an oceanic crust ca. 40 Ma old; therefore, several hundred meters of marine sediments lie between the oceanic crust and the base of the volcanic edifice of Santa Maria, which could behave as a weak layer (detachment) under shear. The 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, from which we conclude that small volcanic islands can collapse when both the edifice and its base are weak. The proximity of Santa Maria to the East Azores Fault, active during the lifetime of Santa Maria, may point to the trigger mechanism of flank collapses. Confirmation that the collapses have actually occurred increases both hazard and risk, because the number of recognised collapses increases, and so does the frequency, thus bringing closer the time scales of flank collapses and human life.