MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
Map of the lava pond on the rift zone south of Axial Seamount, showing the track for ROV Tiburon dive 875
Dave Clague writes: Today’s dive explored an unusual, deep lava pond on the south rift zone of Axial Seamount. The pond is simply sensational! It is actually a complex of 5 ponds separated from one another by levees that rise about 90 meters above the adjacent seafloor. The tops of all the levees are at the same depth, which suggests that all the ponds were erupted at the same time.
The levees that we crossed several times consist of pillow talus and pillow lava on the outer flanks, with lobate flows on the flat levee top. The interior walls are nearly vertical cliffs, overhanging in places, of truncated lobate flows. Roughly horizontal shelves were left like bathtub rings as the lava inside the pond drained away. The pond floors are covered with talus and sheet flows. We crossed a breach in the levee and traversed a delta-like feature built from lava that flowed through the gap. The delta had a tube system that formed secondary vents on top, and these vents collapsed to form large pits.
We confirmed that the structures we had identified from bathymetric data were indeed large and deep lava ponds surrounded by levees, and that the ponds drained, leaving a sequence of bath-tub-ring-like lava shelves (image on left) from near the floor of the pond, all the way to the top of the levee. We also learned that the flow is not young since it has both significant sediment cover and has been lightly colonized by sessile animals. A rough guess is that it erupted perhaps 500 years ago. From our push-core and glass suction samples, we found that lava bubble fragments were common at this site, as they were at the 1998 eruption site we explored yesterday, implying that this eruption also had a mildly explosive component. All the lava samples had similar, and rather unusual mineralogy with abundant large feldspar and rarer olivine crystals, suggesting that all are from the same eruption. Apparently, this eruption began with sheet flows that extended beyond the limits of the ponds and, as the eruption rate declined, the lava did not flow as far and built up the ponds and levees.
The ponds alone had a volume of more than 0.6 cubic kilometers. The extent and volume of the earliest sheet flows remains unknown, and will be assessed during two dives we are planning for this site later in the cruise. In any case, this was a large volume eruption relative to others at the Juan de Fuca Ridge. We do not know where the lava that drained out of the ponds went - it certainly did not simply overflow the levees since the lake drained to levels roughly 60 m below the tops of the levees. Did it drain out to erupt farther down the rift zone, as commonly occurs on Hawaiian volcanoes? One of our later dives will try to determine if this unusual lava occurs several kilometers downslope, farther south along the rift zone.
Nick and Bill prepping the elevator, which will be deployed at the dive site before the ROV is launched tomorrow morning. The dive plan is to recover instruments using the ROV, which will place them in the black plastic quivers of the elevator one by one during the dive. After all the instruments are retrieved, the anchor for the elevator will be released, and flotation spheres (yellow, in cage at right) will provide bouyancy to bring the kit and kaboodle to the surface.