March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
April 15, 2001: Leg 2; Day 15
Log Entry: Easter Sunday began with some chocolate Easter eggs on the breakfast plates! Today, the weather actually did what the forecast said and the winds subsided enough that we could dive on the Kahoolawe rift zone. Our primary target was a flat-topped volcanic cone with a large crater in its summit. The cone is about 1.5 kilometers across and the crater is about 65 meters deep. The cone is largely covered with sheets of volcanic sandstone and siltstone whose origin we cannot yet determine. It is possible that these fine-grained deposits are volcanic ash from explosive eruptions that took place in shallow water when these vents formed near the shoreline roughly 1.3 million years ago. This material is deposited in sheet-like flows with bedding and appears to have cascaded down the slopes of the cone. Pillow lava and lots of talus consisting of angular lava fragments cover other parts of the cone. We collected numerous samples of the lavas and the volcaniclastic rocks, and proceeded to four additional volcanic cones, which we also sampled. Each of these cones also had a drapery of fine-grained volcaniclastic rocks and abundant lava block talus.
A second dive in the late afternoon and evening was directed at sampling several additional cones farther up the rift zone. The first cone was similar to the ones seen earlier today, but the second one broke the pattern. It had no volcaniclastic rocks-only pillow lava and lava talus. The top of this cone may have been a lava lake since we found an area of large flat pillows, surrounded by a low wall with several bathtub rings of lava. The steep outer slope was covered with elongate pillow lavas that may have formed when the lake overflowed. We ended the dive at a third cone that had abundant volcaniclastic rocks at its base, although we did not observe the rest of the cone. The abundance and fine grain-size of the volcaniclastic rocks was a great surprise since such deposits are unknown from the submarine rift zones of any of the other Hawaiian volcanoes, although fine-grained deposits from explosive eruptions from the summit of Kilauea are well documented. It is surprises like this that make science exciting and lead us back to sea again and again.
We are on our way to Oahu tonight with high hopes for completing our final dive on a submarine cone that probably formed at about the same time as Diamond Head and the submarine cones that we dove on at the very beginning of this cruise. These eruptions occured long after the volcano appeared to be extinct, during a "rejuvenated" eruptive stage, and the lava compositions are very different from the tholeiitic basalts that make up the bulk of the Hawaiian volcanoes.