Mike Conway sees us off on Leg #2 of the MBARI Hawaii 2001

View of Diamond Head as we leave the harbor.

Hyaloclastite - "hyalo" glass, "clast" fragment, "ite" rock, layers of ash. This was coating the seafloor in many spots during both of our dives today.

Leg 2 of the Hawaii expedition started soon after we left Honolulu since our first dive site was offshore from Diamond Head about an hour steam from the dock. Diamond Head is one of 37 vents of the Honolulu Volcanics which are the last stage of volcanic activity on Oahu. Diamond Head and several other ash rings near the shoreline in Honolulu formed during phreatomagmatic eruptions (explosive eruptions involving steam). No lava has been found at Diamond Head and the composition and age of the vent are unknown. Our dive found that much of the area offshore is covered in pillow lava, much having a thin discontinuous layer of volcaniclastic rock on top. The pillow lavas have no remaining glass rinds, despite having delicate pillow buds and "elephant trunk" lobes which suggest that the glass did not simply fall off. The volcaniclastic rocks are mostly finely laminated ash that may be the distal ends of surges produced during the explosive eruptions that made Diamond Head crater. The submarine ash layers are finely laminated and sculpted by erosion in much the same way that similar deposits at Hanauma Bay have been sculpted by wind, rain, and surf. The inferred old age of the lavas, based on the altered state of original glass rinds, contrasts with the widely held view that the Honolulu Volcanics might be as young as several tens of thousands of years. We may be able to date our lava samples and establish the age of one of Hawaii's most famous landmarks. At the end of the dive we also found a block of reef limestone that was transported downslope in a landslide. The sample we collected contains coral fragments from a reef on top of the Diamond Head deposits, so its age would establish a minimum age for Diamond Head. We will be diving on other reefs over the next few weeks that grew and died as sea level rose and fell during the Pleistocene. The Oahu reef sample may be an unexpected piece of that puzzle!

We then proceeded a mere 5.8 km farther east and did a second short, shallow dive offshore from Koko Head. The Koko Rift has a string of vents on land belonging to the Honolulu Volcanics, that includes Hanauma Bay. The Koko Rift extends offshore to the southwest with several additional vents that are entirely submarine. Our dive explored two of these volcanic cones and found that they consisted almost entirely of bedded volcaniclastic rocks including laminated ashes, sandstones, and hyaloclastite. These deposits reach at least a meter thick and have been eroded so that they commonly form small stair steps. The explosive eruptions produced steep sided cones with pointed tops and no summit craters. These cones are very different from those produced by steady eruptions of lava that we have previously studied. Pillow lava fragments and a few in-place pillow lavas were found on top of the volcaniclastic rocks and will enable us to establish the composition of these vents, and perhaps their ages. These samples, like those from Diamond Head, had no remaining fresh glass and also appear to be significantly older than previously thought.

-Dave Clague