July 24 - August 6, 2006
August 4 update
Tiburon dive T1013, Vance C Seamount
Jenny writes: Today was our next-to-last dive of the series. We dove at the third seamount to be formed in the Vance chain, if you consider where we dove yesterday a "seamount that tried". This one has a deep caldera on the southeastern edge of the cone, a second, less pronounced, caldera wall toward the center of the volcano, and a broad shield for the remaining summit area.
Brian writes: Wow, this was the “Day of the Manganese”. The covering of iron-manganese oxides on most of the lavas that we saw today was so thick that we often were not sure if we recovered pieces of lava or simply the oxide coating over the lava. The oxides also disguise the lavas underneath, so we had a hard time seeing original lava textures. Remarkably, when the ROV returned to the ship we quickly discovered that we actually got a fine haul of lava samples and also samples largely composed of volcanic glass fragments! So this dive was a huge success, and we have more than enough lava samples to determine how Volcano C evolved geologically.
Dave writes: As Brian noted, today we experienced the “revenge of the manganese.” Everything on the bottom, including talus, lava flows, and volcaniclastic rocks, were covered by at least an inch thick layer of hydrothermal manganese crust. This covering made it very difficult to tell talus from pillow lava or sheet flows from layered volcaniclastic rocks. It also made sampling much more challenging as the manganese crusts are surprisingly strong, making it hard to break off even brittle glassy sheet flows. For much of the day, we were unsure if we had seen or sampled volcaniclastite, and thought that our conclusion that volcaniclastite erupted at about the same time as caldera collapses was not going to hold up. However, when the rocks were unloaded from the vehicle, we found that quite a few samples were actually volcaniclastite coated in manganese crusts.
As we had seen during previous dives, the volcaniclastic rocks occurred only at the top of the caldera wall sections and as layers across the caldera floors. These consistent observations tell us the volcaniclastic rocks formed at about the same time the calderas collapsed. We observed an additional clue to the timing today. The inner wall of the younger, southeastern caldera was dominated by talus and draping volcaniclastite above a single thick massive lava flow near the base, with unequivocal pillow lavas only near the top. The drape of volcaniclastite could mean that it slightly postdates formation of the calderas, or that the loose debris that hardens into volcaniclastite was swept over the rim of the caldera by currents and deposited on the talus slopes of the lower caldera wall. In either case, we found that there is very little volcaniclastite on these seamounts, but that it coats much of the surface and is therefore the type of rock preferentially collected by dredging. Only by direct observations is it possible to determine when the volcaniclastite forms and that it is a very minor component of each seamount.
During each dive we have been collecting short sediment cores as well as rock samples. We will be able to separate small bits of volcanic glass from many of these cores that will expand our knowledge of the range of lava compositions present on the seamount. In addition, we hope to determine the ages of the deepest sediment in the cores based on the populations of planktonic microfossils present. The age of these seamounts must be somewhat younger then that of the underlying ocean crust (0.75 million years at the eastern seamount and 2.55 million years at the western end), but we do not know how long the volcanoes were active. We are hoping to gain some insight into the timing of seamount formation adjacent to the ridge axis.
We continued to add to our animal collections today with another enteropnuest, more polychaetes, another squat lobster, and a variety of crinoids. Each day seems to have a different “flavor” and today was definitely crinoid and polychaete day on the Vance Seamounts.
Tomorrow will be our last dive before we head for Newport Oregon to change scientific parties and resupply the ship with fuel and stores. Because we have about 270 nautical miles to go and are due in at 5 pm Sunday, our dive tomorrow will be short so that we can get underway by about 2 pm. We have planned a short dive on the most easterly of the seamounts, a low shield cut by several faults, but completely lacking calderas or other collapse structures.