 This same modelling shows how this central bulge is dependent on the ration between rift zone length and depth of the magma sources, with longer fissures over shallower sources being more positively associated with very elongated topographies of the associated flanks.Occasionally, fissure eruptions associated with rift zones can actually evolve into new vents along the volcanic edifice, generating lava flows lasting for months or longer.These lava flows add surface materials to the slopes of the volcano, extending the slopes outward in a general flattening of the morphology of the flank.