HR: 0830h
AN: V21D-0546    [PDF]
TI: Lava-seawater vapor interaction at the mid-ocean ridge crest: an important volcanic process to explain lava transport and flow morphology on the deep sea floor
AU: * Ridley, W I
EM: iridley@usgs.gov
AF: US Geological Survey, Box 25046, MS 973, Denver, CO 80225 United States
AU: Perfit, M
EM: perfit@geology.ufl.edu
AF: Univ. of Florida, Dept. of Geological Sciences, Gainesville, FL 32611 United States
AU: Fornari, D
EM: dfornari@whoi.edu
AF: Woods Hole Oceanographic Inst., Geology and Geophysics Dept., Woods Hole, MA 02543 United States
AU: Cann, J
EM: joe@earth.leeds.ac.uk
AF: Univ. of Leeds, School of Earth Sciences, Leeds, LS2 9JT United Kingdom
AU: Smith, D
EM: dsmith@whoi.edu
AF: Woods Hole Oceanographic Inst., Geology and Geophysics Dept., Woods Hole, MA 02543 United States
AB: Eruption of lava from seafloor vents at the mid-ocean ridge (MOR) crest remains a poorly understood phenomena, despite the fact that it is the dominant volcanic process on earth. During the last decade only a handful of MOR eruptions have been documented using either NOAA-PMEL hydrophone detected events or serendipity, and observations of seafloor manifestations of those effusive events did not capture the actual interaction between erupted lava and near-freezing ambient seawater. Because of the great physical and technological obstacles to actually observing volcanic eruption processes in the deep sea, we must rely on the physical and chemical evidence left behind in the cooled seafloor lava flows to deduce the likely processes that occurred. Based on observations and sampling of numerous lava flows from slow to fast-spreading MORs we find a plethora of delicate macroscopic features preserved on the crusts of lava flows and in lava pillars that suggest intense and extensive interactions between hot magma and seawater during seafloor eruptions resulting in a briny vapor phase. Undersides of many lobate and sheet lava crusts have glassy drips (lava stalactites) and flanges (relict bubble walls) that could only have formed in cavities initially filled with a hot vapor at magmatic temperatures as lava was transported across the seafloor. Detailed petrologic observations of the surfaces of drips and flanges, including the presence of molten salt, exotic Cl- and S-bearing secondary silicates, secondary sulfates and almost pure forsterite, suggest that the vapor phase was flashed seawater. This vapor phase is a key to understanding delicate drip structures formed on lava crusts and the mechanisms by which lava is distributed far from eruptive fissures on the deep sea floor. We suggest that vaporized seawater is incorporated at the flow front as lava moves over the seafloor. The vapor rises as streams of bubbles through the lava behind the flow front and then collects beneath the rapidly chilled, insulating upper crust. The vapor exists at near magmatic temperatures beneath the crust and facilitates the formation of delicate drip structures and complex surface morphologies of the flow, especially when the effusion rate is fast and a variety of sheet and lobate flow surfaces form. Subsequent, cooling of the vapor and cracking of the crust leads to collapse of the crust into the underlying void. Entrapment of vapor beneath submarine lava flows may also have significant impact on the ability of flows to travel long distances (km to 10s of km) across pre-eruption seafloor by reducing the effective friction between the flow and the water-saturated seafloor. Recent results of SEM and a variety of microscopic analytical techniques on drip structures in seafloor lavas will be presented.
DE: 1749 Volcanology, geochemistry, and petrology
DE: 3035 Midocean ridge processes
DE: 8414 Eruption mechanisms
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting