9Violent strombolian (transitional) eruptions are common in mafic arc settings and are 10 characterized by simultaneous explosive activity from scoria cone vents and lava effusion 11 from lateral vents. This dual activity requires magma from the feeder conduit to split into 12 vertical and lateral branches somewhere near the base of the scoria cone. Additionally, if 13 the flow is separated, gas and liquid (+ crystals) components of the magma may be 14 partitioned unevenly between the two branches. Because flow separation requires bubbles 15 to move independently of the liquid over time scales of magma ascent separation is 16 promoted by low magma viscosities and by high magma H 2 O content (i.e. sufficiently 17 deep bubble nucleation to allow organization of the gas and liquid phases during magma 18 ascent). Numerical modeling shows that magma and gas distribution between vertical 19 and horizontal branches of a T-junction is controlled by the mass flow rate and the 20 geometry of the system, as well as by magma viscosity. Specifically, we find that mass 21 eruption rates (MER) between 10 3 and 10 5 kg/s allow the gas phase to concentrate within 22 the central conduit, significantly increasing explosivity of the eruption. Lower MERs 23 produce either strombolian or effusive eruption styles, while MER > 10 5 kg/s prohibit 24 both gas segregation and lateral magma transport, creating explosive eruptions that are 25