No abstract

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

[1] The dynamics of separated two-phase flow of basaltic magmas in cylindrical conduits has been explored combining large-scale experiments and theoretical studies. Experiments consisted of the continuous injection of air into water or glucose syrup in a 0.24 m diameter, 6.5 m long bubble column. The model calculates vesicularity and pressure gradient for a range of gas superficial velocities (volume flow rates/pipe area, 10 À2 -10 2 m/s), conduit diameters (10 0-2 m), and magma viscosities (3-300 Pa s). The model is calibrated with the experimental results to extrapolate key flow parameters such as C o (distribution parameter) and Froude number, which control the maximum vesicularity of the magma in the column, and the gas rise speed of gas slugs. It predicts that magma vesicularity increases with increasing gas volume flow rate and decreases with increasing conduit diameter, until a threshold value (45 vol.%), which characterizes churn and annular flow regimes. Transition to annular flow regimes is expected to occur at minimum gas volume flow rates of 10 3 -10 4 m 3 /s. The vertical pressure gradient decreases with increasing gas flow rates and is controlled by magma vesicularity (in bubbly flows) or the length and spacing of gas slugs. This study also shows that until conditions for separated flow are met, increases in magma viscosity favor stability of slug flow over bubbly flow but suggests coexistence between gas slugs and small bubbles, which contribute to a small fraction of the total gas outflux. Gas flow promotes effective convection of the liquid, favoring magma homogeneity and stable conditions.

No abstract

No abstract

No abstract

No abstract

No abstract