March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
April 16, 2001: Leg 2; Day 16
Dave Clague writes:On this, the last day of the leg, we were finally able to dive on the windward side of the islands. We arrived northeast of Oahu this morning in trade winds of only 15 knots and surface current around a quarter knot, and launched our final dive on a volcanic cone that is almost certainly related to the Honolulu Volcanics. The chemical compositions of all examples of the Honolulu volcanics on land have been modified by the excessive amounts of rainwater that percolate through them. Submarine lavas are usually better preserved, despite sitting in sea water for hundreds of thousands or even millions of years, so we hope to collect fresh rocks that can be used to study the source and history of these lavas.
Our target was a remarkably symmetrical cone with a sharp peak rising about 280 meters above the seafloor. The average slopes are steep and the surface is composed of hard materials, as determined from the sonar backscatter intensity in our 1998 sonar mapping survey. We landed on a small ridge north of the cone and collected some angular lava clasts and some chunks of volcaniclastic sedimentary rocks. This small ridge probably formed at the beginning of the eruption when the vents formed a linear fissure, so the lavas should represent this early stage. As the eruption proceeded, lava in the fissure solidified at all but the most vigorous point and the cone then formed above that point.
After crossing a small patch of sedimented seafloor, we reached the base of the main cone and found that it was covered by fine-grained volcaniclastic sands. Along the way, we found an unexploded WWII bomb, which we observed from a cautious distance. As we moved upslope, the volcaniclastic deposits were variably incised by erosion, but we saw no clasts of lava until we had climbed nearly 200 meters up the slope. The upper slopes consisted of a volcanic breccia of lava clasts varying from gravel to boulder size, many lightly cemented into the volcaniclastic units underneath them. The top of the cone was mostly lava rubble, but some in-place lavas were finally found. They were blocky flows analogous to 'a'a flows on land.
The top of the cone was also a rich habitat for animals and we spent a bit of time filming gorgonians, sponges, anemones, shrimp, and urchins. We then traversed down the southern flank of the cone, which was so steep we had to crab down the slope sideways so that we could see the bottom. It was a near mirror image of the northern flank, except the lava clasts extended farther down slope. We continued the dive to the southeast, to a region below submarine canyons on the slope of Oahu where the bathymetric map suggested existence of large sediment waves. These features instead turned out to be a series of descending steps eroded into very gently dipping, unconsolidated sediments.
During the cruise we completed 27 dives with the ROV Tiburon and collected 602 volcanic rock and limestone samples. In addition, we collected 54 push cores of sediment and another 15 mud/gravel scoop samples. Despite the poor weather conditions which precluded our work at nearly all of our planned dive sites, we were able to substitute other dive targets addressing equally important questions. All the members of the scientific party achieved at least some of their objectives, sometimes by studying a different area than planned. (The notable exception was our inability to recover water samplers from the summit of Loihi and collect hydrothermal vent fluids there.) The next leg of the cruise may be able to complete some of the windward dives we had planned to do, since we did a number of dives that addressed some of the main objectives of their leg. We are on our way to Honolulu for a four-day port call. These cruise updates will resume when Leg 3 dives begin on April 21.
April 17, 2001: Leg 2; Day 17
And that's all folks!!
It's total mayhem aboard right now. Rocks and bubble wrap absolutely everywhere. We are trying to make a 1:00 p.m. shipping pick up - Yikes!!