HR: 0800h
AN: V51C-1511    [Abstracts]
TI: Large Lava Pond Complex on the Juan de Fuca Ridge: an Effusive, Energetic Eruption that Drained Away
AU: * Paduan, J B
EM: paje@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 United States
AU: Clague, D A
EM: clague@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 United States
AU: Davis, A S
EM: davisa@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 United States
AU: Chadwick, W
EM: bill.chadwick@noaa.gov
AF: Oregon State University, Hatfield Marine Science Center 2115 SE OSU Drive, Newport, OR 97365 United States
AU: Cousens, B L
EM: brian_cousens@carleton.ca
AF: Carleton University, Department of Earth Sciences 1125 Colonel By Drive, Ottawa, ON K1S 5B6 Canada
AU: Embley, R W
EM: robert.w.embley@noaa.gov
AF: NOAA, Pacific Marine Environmental Laboratory 2115 SE OSU Drive, Newport, OR 97365 United States
AB: We explored an unusually large, deep, drained lava lake complex on the south rift of Axial Seamount on the Juan de Fuca Ridge during three dives with the ROV Tiburon in August 2005. The complex of five large ponds, first identified from EM300 multibeam bathymetry, is 5 km long and more than 1 km wide. The ponds are separated from one another by narrow levees that rise about 90 meters above the pond floors. The levees are all about the same depth, which suggests that the ponds formed at the same time. The volume of the lake, prior to draining, was 0.2-0.4 km³, making it the largest lava lake known along the ridge system. The outer slopes of the pond levees are constructed of elongate pillows that flowed down the steep slopes. The rims are narrow, level plateaus of lobate flows with many collapses. The inner walls are vertical cliffs, overhanging in places, with horizontal shelves from the top of the levees down to the floors of the ponds. Left like bathtub rings, these shelves mark former surfaces of the lava pond as it drained away while the lava was still molten. In many places, this veneer has collapsed to reveal truncated lobate flows and pillows. The floor of one small pond was entirely talus blocks. However, the floors of the other, larger ponds had little talus and, instead, were vast expanses of thin broken crusts, lobate flows, and very fluid, chaotic, folded and jumbled sheet flows. The lavas from each pond have abundant large feldspar and rarer olivine crystals, suggesting that all were from the same eruption. This eruption apparently began with sheet flows whose advance was limited by topography. It then ponded and built up the levees that were left when the lava drained away. On the floor of one pond we found a deposit several meters tall that was delicate and difficult to sample, and turned out to be agglutinated spatter. Limu o Pele (lava bubble wall fragments) was abundant in all the sediment samples in and around the ponds. The spatter and limu demonstrate that the eruptions were magmatic-gas-rich and mildly explosive to the end, with strombolian-like bursts and even fire fountains, though such activity had been presumed to be impossible at 2300m depth. We did not find obvious signs where the lavas went that drained from the ponds. A delta-like fan of partially drained and collapsed lobate flows extended from breaches in two adjacent levees, but since the pond floors are considerably deeper than the breaches and delta surface, the lake must have drained elsewhere after it breached the levees. The ridge axis outside the pond complex is severely tectonized, with numerous faults, gaping fissures, and shattered lavas of similar, unusually feldspar-rich composition, so there is no evidence that the ponds drained down-rift. We propose that the drained lava was recycled back into a crustal magma chamber below the ponds.
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
DE: 8425 Effusive volcanism
DE: 8427 Subaqueous volcanism
DE: 8428 Explosive volcanism
DE: 8429 Lava rheology and morphology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005