HR: 08:30h
AN: V11F-03    [Abstracts]
TI: Separated Vs. homogeneous two-phase flow in violent strombolian activity
AU: * Pioli, L
EM: lpioli@uoregon.edu
AF: University of Oregon, Department of Geological Sciences 1272 University of Oregon, Eugene, OR 97403-1272, United States
AU: Cashman, K
EM: cashman@uoregon.edu
AF: University of Oregon, Department of Geological Sciences 1272 University of Oregon, Eugene, OR 97403-1272, United States
AU: Wallace, P
EM: wallace@uoregon.edu
AF: University of Oregon, Department of Geological Sciences 1272 University of Oregon, Eugene, OR 97403-1272, United States
AB: The term ‘violent Strombolian’ was first used to describe mafic eruptions that formed ash-charged columns up to 6 km high, and dispersed material up to a few hundred km from the source (Walker, 1971). These eruptions are often discontinuous and strongly pulsatory and are typically associated with simultaneous effusive activity: they form composite deposits constituted by a cinder cone, tephra blanket, and lava flows spreading from lateral vents. This eruptive regime is typical of water-rich mafic magmas and is characterized by average mass flows (10³-10⁵ kg/s) intermediate between Hawaiian and subplinian regimes. Within this interval, there is a direct correlation between explosivity, as defined by tephra production, and magma flux. When magma flow exceeds 10⁵ kg/s, gas segregation is no longer possible and eruptive activity takes the form of sustained columns (subplinian to plinian activity). At eruption rates below 10³ kg/s passive degassing processes dominate, causing lava effusion and/or mild explosive activity (Strombolian to Hawaiian). We suggest that very shallow gas segregation processes play a fundamental role in violent strombolian dynamics, affecting both explosive and effusive activity. Simultaneous eruption of tephra from the cone and lava flows from lateral vents requires both a gas-rich mixture ascending the central conduit and gas-poor lava flowing in the lateral system. Uneven distribution of liquid and gas phases is possible only when gas and magma are characterized by different momentum, i.e. the flow is separated. At a first approximation, the phase distribution is controlled by the two-phase flow regime (bubbly, slug, churn or annular), both gas and liquid fluxes, and the ratio between conduit and dike diameters. To quantify this process, we analyze in detail the dynamics of a particularly long-lived and well-known eruption of the last century- the Paricutin eruption (1943-1952) of central Mexico. Specific two-phase flow models are then used to evaluate 1) the effect of separated flow in magma splitting between a central conduit and a lateral dike system, 2) the flow regime that can develop in the central conduit 3) the potential effect on the explosive dynamics. Our results indicate that the explosivity of the eruption was strongly increased by segregation processes, promoting shallow fragmentation of the magma, and that the pulsatory dynamics was likely related to unstable, transitional two-phase flow regimes typical of gas fluxes intermediate between slug and annular flow.
DE: 8428 Explosive volcanism
DE: 8434 Magma migration and fragmentation
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting