March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
May 15, 2001: Leg 4; Day 7
Dave Clague writes: This morning we returned to the site where we had been unable to dive on the first day of this leg. The target is a large flat-topped cone with variable backscatter on its summit plateau. We chose the site to dive on based on the presence of a water column anomaly encountered during a survey conducted last December. The anomaly interfered with the 3.5 kilohertz seismic system and with the swath bathymetry/sidescan data collection. We had thought that it might be caused by either warm water or bubbles in the water column, and either would be consistent with active or recent volcanic/hydrothermal activity.
The dive landed on the plateau at about 715 m depth and proceeded towards the center of the anomaly, where a small hill is located. The bottom consisted of rounded beach cobbles, a few strands of lithified beach sand, and some large lava blocks. All were clearly old and cold. The hill consisted of thick subaerial lava flows and an area of white carbonate sediment that was mostly skeletons of recent deep water corals. We collected a number of basalt cobbles, several of the thick flows, the lithified beach sand, and some unconsolidated dark sand/gravel that turned out to consist of basalt fragments, sea urchin spines, and many hundreds of shark teeth. The entire area we surveyed was at one time subaerial and has progressively subsided below sea level, probably soon after Kaena Ridge formed some 3-4 million years ago. It is also clear that the single survey line that suggested the form of a flat-topped volcanic cone was really mapping a small part of a larger subaerially-formed platform. We were dissappointed that the dive did not find young volcanic rocks nor any evidence for recent activity, but did manage to collect the first suite of samples from Kaena Ridge. We also confirmed that Oahu has subsided more than 700 meters since it formed.
We cut the first dive short when we were convinced that there was no young lava to be found. After recovering the vehicle, we steamed a few hours south and did a dive on a large block in the Waianae landslide. The only survey data available were several old SeaBeam transit lines that showed a scarp facing the shoreline that rose from 2550 to 1920 meters depth. The traverse started slowly with a sea of mud that blanketed the lowermost 150 meters of the steep slope. We then encountered loose blocks of basalt talus scattered in the mud and finally outcrops of volcanic breccia and hyaloclastite. The outcrops turned out to be exceedingly difficult to sample due to the presence of thick (0.5-1 centimeter thick) manganese crusts that cemented all the rocks together. Many of the outcrops were vertical with no ledges or handholds for the manipulator to grab onto. Despite the difficulties in sampling, we managed to collect 31 samples from the section of outcropping volcaniclastic rocks. The samples include numerous basalt clasts and many fine hyaloclastite samples, which were easier to sample since they formed bedded deposits and small ledges that we could break samples from. We hope to learn more about the construction of the flanks of Hawaiian volcanoes from these samples, as well as clues about why they fail as giant landslides.
We are currently steaming to the north flank of Oahu where we will spend the next two days conducting heat flow surveys using the ROV to collect temperature gradient data in sediments and then to collect short vibracores to measure the thermal conductivity of the sediment.