March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
April 8, 2001: Leg 2; Day 8
Jenny Paduan writes:In the photo on the left, we are using a sediment scoop to collect gravel next to a basalt flow, seen in the image on the right.
In the gravel we hope to find basalt glass that has shattered off the lava (there is hardly any remaining on these old rocks anymore). The outer rind of a lava flow chills quickly to a glassy texture when it contacts the cold sea water. It doesn't have time to form crystals or degas, so it retains the chemical composition of the melt plus we can tell if it erupted under water or on land.
This image shows a view of a massive, eroded reef. Some portion of this material will be the skeletons of reef-building corals, such as staghorn corals, that contain aragonite and can be dated. A larger portion will be coralline algae, sandy sediment, and coral that was once aragonite but that has remineralized to calcite, none of which can be dated. We have been lucky to have found some pieces containing corals, so we may be able to date the reefs and constrain the ages of the nearby lava flows.
Dave Clague writes: After completing two dives on the landslides on the Kona coast that Juli Morgan told you about over the last two days, we returned to Mahukona and the stairstep coral terraces. The trade winds continue to blow hard and make work on the windward side impossible. Even at Mahukona, in the lee of Hawaii, the strong winds have contributed to surface currents in excess of 1.5 knots, eliminating some of this area for dives as well. We steamed to a site on the outer part of the Mahukona terraces and found that the currents were too strong to dive. We then steamed to an alternate site on the 950-meter-deep terrace and found the conditions acceptable. Dive 282 drove up the terrace face where we found no reef limestone at all. The entire terrace face was lava flows draping the steep slope. Some consolidated reef sands formed drifts around the lavas. The steep slope that led us to select the site is apparently preserved by the draping lava flows, whereas adjacent sections of the terrace have more gentle slopes presumably because the limestone is more easily eroded. The dive continued to the south and sampled two volcanic vents aligned along fissures that may be part of a single long chain of vents, or may be from separate adjacent fissure eruptions. Analyses of the recovered lavas should tell us which is the case. We were somewhat frustrated that the corals from the 950 meter terrace had eluded us, but also learned that the steep slopes of certain reefs may reflect lava armoring rather than exposed reef limestone. The morphology of the two vents will be used in a comparative study of submarine vent structures.
A second dive began in mid-afternoon on the 585-meter reef, a 100-meter tall structure that is only apparent in certain areas. The lower talus slopes at the base of the steep slope consisted mostly of loose lava fragments, but gradually more and more limestone fragments appeared, suggesting that we would find limestone outcrops upslope. Indeed we did. The limestone formed a steeply dipping massive unit that would have been impossible to sample had there not been some cracks that broke up the rock in some places. We collected a number of carbonate samples, although none had the visible reef-building coral fragments we had encountered on previous dives. The dive continued upslope and found a 30-meter thick lava sequence that was nearly vertical in outcrop. It forms the edge of the terrace and is overlain by perhaps a few meters of limestone. We proceeded to survey to the north, and found a young lava flow that erupted from submarine vents upslope near the 400-meter terrace (and is therefore younger than 130 thousand years old) and that is apparent in the bathymetric and sidescan data. A quick examination of the samples after we recovered the vehicle suggests that some corals were recovered and that we should be able to determine the age of this reef.
Models for the formation of the reefs are highly dependent on the timing of glacial and interglacial low and high stands of the sea, which is in turn dependent on many assumptions about the causes of glaciations. Our study of reefs in Hawaii may provide a truly independent test of the timing of the glacial/interglacial global climate cycles over the last half million years.