Experiments were conducted to study the spreading of volcanic constructs. Volcanoes are simulated by a sand cone, and the volcanic substratum is simulated by a sand layer (brittle substratum) overlying a silicone layer (ductile substratum). Similarity conditions between natural volcanoes and experimental prototypes led to the definition of dimensionless &Pgr; numbers. Experiments determine &Pgr; values which predict whether or not spreading takes place. Of particular importance are the ratio between the thickness of the brittle substratum and the height of the volcano (&Pgr;2) and the brittle/ductile ratio of the substratum (&Pgr;3). &Pgr;2 indicates that the volcano must be large enough to ''break'' the substratum before spreading occurs, whereas &Pgr;3 controls the style of deformation. During spreading, these dimensionless numbers change with time, reaching values that tend toward those observed for stable configurations. Experimental values are compared with those from well-constrained natural examples. It is found that an essential requirement for volcanic spreading is the presence of a low-viscosity layer within the substratum. Flow of the weak layer away from the excess load is responsible for the spreading. The overlying edifice displays radial intersecting grabens, due to concentric stretching, dissected summit areas; concentric zones of thrusts and folds form in the substratum around the edifice, and diapirs of the ductile substratum rise within the fault zones. ¿ American Geophysical Union 1996