A volcanic eruption occurs when a magma-filled fracture, a dike or an inclined sheet, is able to propagate from its source (a magma chamber) to the surface. In this talk I explain how dikes/sheets choose among, theoretically, an infinite number of paths. I also explain how and why most dikes and sheets become arrested, that is, stop their propagation at depth in the crustal segment/volcano and never reach the surface to supply magma to an eruption. Being able to forecast the paths of dikes/sheets is of fundamental importance for hazard assessment during unrest periods and for understanding how volcanoes work.
