THE STYLE OF WATER-MAGMA MIXING AND ITS EFFECT ON THE DYNAMICS OF VOLCANIC PLUMES

At Mount St. Helens on 3/8/2005 and at Fourpeaked Volcano, Alaska on 9/17/2006, volcanic plumes rose to several kilometers elevation from vents that were within or near glaciers that are thought to have contributed some water to the eruption. These ash plumes posed hazards to air traffic and raise the question as to whether external water could have significantly affected plume height. I examine this question using a one-dimensional model for steady, wet volcanic plumes (Mastin, G³, in press). Ash-laden volcanic plumes are initially denser than air and can rise to significant height only if they entrain enough air in the near-vent region to attain buoyancy. As vent radius r and mass flux m increase, the air entrainment rate (proportional to r) decreases relative to m (proportional to r²) and, above a critical mass flux (m_c), the plume collapses before becoming buoyant. Model results suggest, for example, that a dry plume, initially at T=1000 K with 3% gas and an exit velocity of 100 m/s, rises in a Standard Atmosphere to 4.8, 9.2, and 14.6 km for m=10⁵, 10⁶, and 10⁷ kg/s respectively; but above m_c=6x10⁷ kg/s, the plume collapses. As reported by Woods (1993, JGR 98:17627-17636), the addition of water at the vent does not strongly affect the height of buoyant plumes; however I find that m_c depends strongly on the conditions under which water and magma mix. The effect can be illustrated for two idealized end-member cases: (1) pre-eruptive constant-volume subsurface mixing followed by reversible expansion during the eruption; and (2) constant-pressure surface mixing, as might occur with seepage through crater walls into a pyroclastic jet. For case-1 mixing, exit velocities and entrainment rates greatly exceed those for a dry eruption; for case-2 mixing they are lower. For addition of 30% water to the above-mentioned T=1000 K plume, case-1 mixing yields eruptive columns that theoretically remain buoyant up to a remarkable m_c=8x10⁹ kg/s, while case-2 mixing results in column collapse at a modest m_c=1.2x10⁵ kg/s. These considerations constrain the conditions under which high, water-bearing plumes such as those as Fourpeaked and Mount St. Helens may develop.