ALBERT

Description: Volcanoes are often associated with vertical collapse, due to deeper magma withdrawal. Calderas are the most notable type of vertical collapse, on the summit of volcanoes. Caldera collapse has been observed and monitored only at Miyakejima (Japan; 2000), Dolomieu (Reunion; 2007) and Fernandina (Galapagos; 1968), highlighting our limited knowledge on its kinematic behavior. Here we use experimental models to investigate the kinematic evolution of calderas and vertical collapses in general. We extract velocity and strain fields using the Particle Image Velocimetry (PIV) technique, generating time series. Experimental vertical collapses undergoing constant subsidence velocity show three main kinematic behaviors: (1) continuous collapse, whose velocity is similar to the source subsidence velocity; (2) incremental collapse, with episodic (stepwise) accelerations along pre-existing ring structures; (3) sudden collapse, resulting from the upward migration of a cavity, only for T/D 〉 2 (T and D are the depth and width of the magma chamber, respectively) and without ring structures. The velocity in the collapsing column may increase up to four orders of magnitude with regard to the constant subsidence velocity of the source. Comparison to nature suggests that: (1) there are close kinematic similarities with monitored collapse calderas, explaining their incremental subsidence after the development of ring structures; (2) sudden pit crater formation is induced by the upward propagation of cavities, due to magma removal at depth and in absence of ring structures; (3) all these types of vertical collapses have a consistent mechanism of formation and kinematic behavior, function of T/D and the presence/absence of ring structures.