Edifice Collapse: Natural Tool to Probe the Roots of a Volcano

The most common cause for structural destabilization of stratovolcanoes and dome complexes is intrusion of volatile-rich, viscous silicic magma prior to the forthcoming eruption. In this situation, rapid landslide-induced decompression provokes a strong explosive eruption. Observational data and stratigraphic relationships show two main scenarios of such eruptions. Scenario 1 includes: moderate pre-climactic explosive activity; edifice failure; catastrophic directed blast; (sub)plinian or vulcanian eruption, and fountain-collapse with generation of pyroclastic density currents. Recent examples are Bezymianny 1956, Mount St.Helens 1980, Soufriere Hills, Montserrat 1996. Scenario 2 includes: edifice failure; moderate phreatic explosion; (sub)plinian eruption and fountain collapse with generation of pyroclastic density currents. Recent examples are Harimkotan 1933 and Shiveluch 1964. The cause of two scenarios relate to different levels of magma intrusion with respect to the detachment surface at failure and hence to magma depressurization conditions. That level, in turn, is related to the intensity of destabilizing effect of the intruding magma, and degree of initial (pre-eruptive) instability of the volcanic edifice. Scenario 1 occurs if the detachment intersects the magma body. A magmatic directed blast immediately follows collapse. Scenario 2 occurs if, at failure, the magma body is notably deeper than the detachment surface(s). In scenario 2, only a moderate phreatic explosion first occurs, due to decompression of the superficial hydrothermal system; a vertical magmatic eruption can follow with delay because rising magma needs time to reach the ground surface inside the landslide scar. Many prehistoric debris avalanches of the Kurile-Kamchatka region (10 studied cases) were associated mostly with Scenario 2, which thus seems more common.