March 13, 2001 to June 2, 2001
Monterey to Hawaii and back
May 11, 2001: Leg 4; Day 3
Ken Hon writes: We are now west of Niihau, the furthest northwest of the major Hawaiian Islands. The surface of Niihau Island is mostly lavas erupted during the shield stage, when the island was directly over the hotspot. Offshore to the west, there is a huge field of younger submarine volcanoes that erupted underwater. Many of these have steep sides and very flat tops. Today we had two dives, one to a strange seastar-shaped ridge on the top of a flat topped cone and another on what appeared to be a delta-shaped shield with a large shallow depression at its summit that might have been a drained lava lake. On the first dive, #317, we landed on the top of the flat cone 1700 meters below the surface and just south of the seastar-shaped ridge. We found evidence for large shallow channels created by lava flows pouring out of the summit of the flat top cone. The channels are 20-30 meters wide and have vertical margins 2-3 meters high. Small islands of lava crust piled up in-between the channels. We motored along the bottom to the north until we encountered the edge of the ridge. We were expecting to find a lava pond levee, a dam made of stacks of thin crust layers welded together. Instead, the outside of the ridge was a tilted slab of smooth lava sheet flows. These slabs are tilted outward about 20 degrees from the center of the starfish ridge. The ridge is about 20 meters high. In places the surface varies from the smooth form to broken slabs, indicating higher flow velocities, and bulbous pillows, which probably leaked out of the other surfaces after the flow crusted over. When we came to the top of the ridge, we found a sheer cliff on the inside.The surface of the flows was down-dropped about 15 meters into the center of the seastar-shaped ridge. In places the surface is dropped down to a ledge, then into the central area. Exposed in the cliffs is the massive flow interior of what must have been a single, thick ponded lava flow more than 20 meters thick. The flows have a crude "columnar" fracture pattern that shows they cooled slowly. The seastar structure was not a lava lake, but actually formed when a new pulse of lava was trapped beneath the thick ponded flow. This new lava injected into weak layers between older flows and domed up the central part of the volcano. As the central dome grew, the rock fractured and eventually the new lava drained from the dome and it collapsed. In the center of the seastar ridge is a pile of thick pillow lavas that oozed out of the vent after the collapse. This was apparently the last eruption from this large volcano. The real bonus was that the seastar-shaped ridge was covered with seastars too! They evidently like it there a lot. The coolest one we collect was a large bulbous red seastar that looked like a big brain. It was inflated like a balloon and kept floating away, but we finally managed to get it into the sample box. Ed Seidel hopes to bring it back and put it on display at the Monterey Bay Aquarium.
We then moved the Western Flyer a little closer to Niihau and dropped the Tiburon on a region where a shield-shaped volcano had formed against a steep topographic boundary. The boundary appears to be the old coastline of Niihau that submerged as the island slowly subsided. The topographic map showed a small hole that we though might be an old skylight into a lava tube system. However, when we got to the bottom, everything was covered with fine white sand made of foraminifera shells. We couldn't find the skylight, but there we plenty of beautiful red jellyfish floating by that made up for it. We traveled up the slope to the summit and found the large pit that we think may have been a lava lake when this volcano was active. The floor of the pit was covered by sediment, but the walls were stacks of pillow lavas. We sampled the lava flows and then headed upslope across the "old shoreline". Above the delta, the slope was covered by masses of beautiful pillow lavas. We traveled north parallel to the coast and found that this entire steep slope was pillow-covered with small stretches of flat sediment-covered ground. These eruptions were not subaerial as supposed, but submarine. We finally quit for the night around 9 p.m. so we could get the Tiburon up and ready for tomorrow's dives.
Jenny Paduan writes: The weather is continuing to be perfect for holding station, so we've been able to dive where we planned. The lab-work is running smoothly (and we aren't in danger of running out of sample baggies or buckets THIS leg of the cruise, unlike Leg 2 when we collected over 600 rocks in two weeks), the food is great (thanks Derek!), and I haven't misplaced my designated drinking cup (yet)
Periphyllopsis. We saw quite a number of these jellies off Niihau. They drift near the bottom, are deep red and ever-so-slightly transparent, much like another jelly (Voragonema) that is quite abundant in Monterey.