A two layer model for the formation of dense pyroclastic basal flows from dilute, collapsing volcanic eruption columns is presented. The collapsing dilute current is described by depth averaged, isothermal, continuum conservation equations. The dense basal flow is modelled as a granular avalanche of constant density. Simulations demonstrate that pyroclastic flow formation and behavior is dependent upon slope conditions when the dilute part of the current has lost most of its mass. The dilute current runout increases with decreasing particle size and increasing initial column height. If the dilute current has transferred its mass to the dense basal flow on volcanic slopes with inclination angle greater than the friction angle of the basal flow, then the basal flow will continue to propagate until frictional forces bring it to rest. If the dilute current terminates on lower angled slopes, frictional forces dominate the basal flow preventing further front propagation.