March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
May 12, 2001: Leg 4; Day 4
Dave Clague writes: We have finally had a day with decreasing winds that by the end of the day were below 10 knots - a first for the three legs here in Hawaii! We took advantage of the improved conditions to complete three dives (dives T319, T320, and T321), the last truncated when a thruster shorted out and we had to recover the vehicle.
The first dive was on an adjacent pair of steep pointed volcanic cones at about 1800 meters depth with the goal of discovering why these steep-sided cones have such smooth bathymetry and even backscatter on the sidescan data. Only two cones in Hawaii with similar shapes have been previously sampled, one on Mahukona and one on Haleakala. Both turned out to be made of postshield alkalic lavas, the lavas that cap the main shield-building tholeiitic lavas. The cones consisted almost entirely of low-relief pillow lavas and pillow fragments, which we collected at 14 sites. The lower slopes are sediment draped and have gravel of volcanic fragments, which we also sampled.
The second dive explored a somewhat shallower cone at about 1300 m depth that has similar smoothness and backscatter characteristics to the first one. This cone has apparently been modified by a large slump that removed the southwest sector of the cone, leaving a partial ring with a large amphitheater. This cone consisted almost entirely of volcaniclastic rocks that formed bedded deposits that draped the slopes. The slopes were steep, probably about 30 degrees, and erosional gulleys were cut into the volcaniclastic rocks that exposed the layers of volcaniclastic rocks. Only near the top of the cone did we finally find some pillow lava to sample for geochemical analysis.
The third dive was on a series of cones and volcanic ridges shallower than the break-in-slope at about 1100 meters depth that marks the shoreline of Niihau at the end of voluminous shield-stage volcanism that built the the island. The dive began on a small steep cone that consisted on pillow lavas and a small amount of volcaniclastic rock, suggesting that it formed after the island had subsided so that this site was submarine. The second ridge appeared much older and had thicker sediment cover and the lava fragments appeared to be rounded boulders rather than pillow lavas. Most of the rocks were heavily coated with Mn-oxides that cemented the fragments together. At the end of the dive we found a near-vertical cliff of massive lava that looked like a subaerial `a`a flow. The top surface was similar to shoreline-eroded `a`a, the only `a`a you can walk on barefoot. This ridge almost certainly erupted subaerially and has since subsided below sea level. It must have erupted soon after the main shield stage was completed nearly 5 million years ago. These two volcanic features do not appear to be closely related, despite their close proximity, with one formed as a submarine rejuvenated-stage vent and the other formed as a subaerial postshield-stage vent. We had completed only about half of the planned dive when the vehicle had to be recovered.
We collected a number of animals for the Monterey Bay Aquarium, including several Anthomastus species and three more seastars. We found a large predatory tunicate which we tried, unsuccessfully, to collect. We also spent some time trying to capture a star that was almost exactly neutrally buoyant-which we discovered when it floated away from the vehicle as we tried to stow it in the bio-box. We are currently steaming west to collect a gravity core near the base of the slope of Niihau at a depth of about 3800 meters. Tomorrow, we will continue to work northwest of Niihau, mainly on the large flat-topped volcanoes, but also on the section exposed below the old shoreline feature at about 1100 meters depth.
Putting a seastar into the "bio-box" in the sample drawer of the ROV. This box will seal to keep the animal surrounded by cold water during the ascent. This animal was collected on an earlier dive and is being maintained by Ed Seidel (Monterey Bay Aquarium) for potential display at the Aquarium. The scientists have graciously provided valuable sample space (for the biobox) and some dive time for collections.