March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
May 13, 2001: Leg 4 ;Day 5
Dave Clague writes: Today was an exciting, but very long, day. We completed two dives offshore Niihau and each had numerous highlights. The first dive was launched at 6:30 a.m. on the flank of a large flat-topped volcanic cone, where we found beautiful pillow lavas and a huge tumulus. Tumuli form when lava tubes become over pressured and lift up the lid of the tube, cracking it and sometimes tilting the original lava surface up in the center. Sometimes, with enough uplift, the originally horizontal lava slabs are tilted to vertical, as happened with the tumulus we found at the base of the flat-topped cone.
We proceeded across a narrow stretch of sediment-covered seafloor and then started up a steep slope that led up to the original shoreline of Niihau Island. The bottom of the section consisted of steeply dipping sedimentary rocks, including layers of fine white sediment that were eroded to form small buttes. Deep channels cut through this soft material. As we moved upslope the finer sediment was replaced by coarser sediments and eventually by volcanic breccia. These sedimentary and volcaniclastic rocks form the slope of the island.
As we reached the top of the steep slope and the bottom flattened out, we encountered patch coral reefs that grew soon after the main volcanic shield of Niihau formed about 5 million years ago. The island and its shoreline with the coral reef subsided by 980 meters soon after the main shield stopped forming, so the old shoreline is now 980 meters deep. We continued across a gently sloping sediment-covered plain, with scattered patch reefs and finally reached the lower slopes of another flat-topped volcanic cone of rejuvenated stage lava. On Niihau, the rejuvenated stage lavas, the Kiekie Basalt, are between 2.5 and 0.35 million years old. Because of the relatively young age of these rejuvenated stage lavas, they erupted after the island had completed most of its subsidence, so the cones we sampled erupted at about their present depth.
During the dive we also added to our growing collection of unusual sea stars. We recovered the vehicle during the afternoon and moved to a second site in deep water where we launched again at about 4 p.m. The second dive was done the same day in order to complete our work in the Barking Sands Missile Range before the military commenced missile launches on Monday. The second dive explored two additional flat-topped volcanic cones starting at about 3,850 meters depth. We selected one of the deepest of the cones to explore because we wanted to collect samples that have undergone as little degassing as possible. The greater the pressure (the water depth), the less degassing has taken place and the easier it is to determine the initial volatile complement of the magmas.
The two flat-topped cones were quite different from one another, with the first comprised of pillow lavas and pillow talus, and elongate pillows on the steeper slopes near the rim. The flat top had several more tumuli and some pillow lavas projecting through the thick sediment. The second cone consisted of pillow lavas and lobate flows with thinner sediment than on the first cone. The upper part of the slope and part of the summit consisted of hackly lava flows. The top was quite flat and sediment covered, but large, elongate tumuli were common. These tumuli were oriented towards the summit of the cone and mark the location of at least one tube that transported lava away from the summit. The surfaces of the tilted slabs of the tumuli are sheet flows and folded sheet flows. Such flows are thought to form by rapid emplacement of the lava.
There were very few animals observed during this deep dive so no animals were collected. We collected excellent glassy lava samples from both cones, which can be analyzed for volatiles including water, carbon dioxide, sulfur, chlorine, and fluorine. We started to recover Tiburon at 11:30 at night and it was on deck about 2:30 in the morning. We will start a bit later than usual tomorrow because of the late finish and the long, long day.
Hackly flow, also called jumbled sheet flow, and a gorgonian. The lava surface has been torn up by the surging of a very fast lava flow that poured from the vent. The surface was disturbed into plates, and the plates got shattered, bent, and pushed up against each other, creating a disorganized, wild terrain. (Imagine putting lava in a blender.) Because the flow was so fast and hot, the plates are an integral part of the surface rather than rubble. The scale of this type of flow could be 1-2 meters relief and tens of meters wide. A hackly flow represents an end member in a series of submarine lava flow surface shapes: from bulbous pillows, to lobate pillows, to sheet flows (striated and ropy), to hackly flows.
Looking down the axis of a tumulus on the summit of the first cone of our dives today. The pressure of the lava flowing in a tube underneath broke and pushed up the chilled slabs of the sheet flow. The flow originally would have been horizontal, but the slabs became almost vertical in places.
Another folded, ropy sheet flow, broken and upended to become a tumulus.