HR: 0800h
AN: V51A-02 INVITED     [Abstracts]
TI: Gas segregation and two-phase flow in basaltic explosive activity
AU: * Pioli, L
EM: lpioli@uoregon.edu
AF: University of Oregon, Department of Geological Sciences University of Oregon, Eugene, OR 97403-1272, United States
AU: Cashman, K
EM: cashman@uoregon.edu
AF: University of Oregon, Department of Geological Sciences University of Oregon, Eugene, OR 97403-1272, United States
AU: Wallace, P
EM: wallace@uoregon.edu
AF: University of Oregon, Department of Geological Sciences University of Oregon, Eugene, OR 97403-1272, United States
AB: Basaltic explosive activity is highly variable in intensity, ranging from less energetic fire fountaining and intermittent strombolian explosions, to more energetic ash-forming violent strombolian, subplinian and plinian activity. Moreover, unlike silicic volcanism, there is no direct relationship between explosivity and magma flux, due to the complex interplay between gas segregation and initial gas content of the magma, ascent rate, and gas segregation. Highly explosive activity is particularly common in mafic arc volcanoes, where magmas contain abundant water and higher gas fluxes are expected. Gas segregation and two-phase flow processes play a fundamental role in the explosive dynamics of basaltic magma. Passive degassing and bubble bursts are common in lava lakes or lava-filled vents, that is, in nearly static lava ponds. This style of activity indicates the rise of discrete bubbles through the low viscosity liquid. With an increase in the magma supply rate and initial water content, activity changes to that of contemporaneous lava emission and explosive activity, as is typical in many cinder cone eruptions. This paired activity illustrates preferential segregation of gas into the vertical conduit with respect to a lateral dyke system; the result is eruptive activity that is referred to as either transitional or violent strombolian. When magma rise rate exceeds values of the order of 10^4-5 kg/s, gas segregation is no longer possible and eruptive activity takes the form of sustained columns (subplinian to plinian activity). This summary illustrates the role of liquid and gas fluxes on the development of two-phase flow patterns in the conduit, which, in turn affects the eruption dynamics. For example, discrete explosions are generated when the pattern is periodic (characterized by regular temporal and spatial fluctuations), due to formation of gas slugs or void fraction waves, whereas strong fluctuations in the eruptive dynamics may be related to flow instability during transitions and in churn flow. Sustained flow, in turn, occurs when the flow regime has a regular geometry, such as in annular and homogenous bubbly flow. Using theoretical and experimental models, we explore the possible two-phase flow patterns (bubbly, slug, churn, annular and dispersed) that can develop in vertical conduits for different liquid and gas fluxes (Taitel et al., 1980; McQuillan and Whalley 1986; Lucas et al., 2005). We then discuss their expected stability in magmatic systems and their potential effects on the explosive eruption dynamics. Finally, we compare the theoretical results with natural examples.
DE: 8428 Explosive volcanism
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly