Recent studies have shown that hydrothermal alteration in the core of volcanoes may weaken the edifice and trigger sector collapse <Lopez and Williams, 1993; Day, 1996; Voight and Elsworth, 1997; van Wyk de Vries et al., 2000; Reid et al., 2001>. Alteration of volcanic products must take place over long periods of times before collapse can occur, so this process is considered to be less frequent than volcanic failure generated by magmatic intrusion and flank spreading. However, acceleration of this slow process may occur along one direction as a result of a weak lateral confinement following outward gravitational sliding of a flank of the volcano. We present here evidence that the Enclos-Fouqu¿ depression of Piton de la Fournaise volcano results from such a hybrid mechanism. Seaward sliding of the Grand Br¿l¿ trough on top of an ancient hypovolcanic complex led to a lateral unconfined boundary along which collapse of the altered core of the edifice took place. The result is a caldera-like structure breached on the Grand Br¿l¿ slide. Scaled experiments to simulate this process of pseudocaldera formation reveal tectonic features remarkably similar to those encountered in the field and demonstrate the mechanical consistency of the model. Similar processes may have played a role in the evolution of other volcanoes.