July 24 - August 6, 2006
August 2 update
Tiburon dive 1011, Vance E
Dave Clague writes: We resumed our dive series on the Vance Seamounts today, diving on the second large volcano from the eastern end of the chain. This structure is very complex as it consists of two large volcanoes that have coalesced. The eastern (younger) half, where we dove, has 3 apparent nested calderas, two of which (the oldest and youngest) truncated lava shields on the summit platform when they collapsed. The western half also has three nested calderas and another shield.
The dive began on the floor of the deepest, and most south-easterly, of the three calderas. There we encountered some lava flows that postdate formation of the caldera. Most of the floor was covered in pelagic sediment, but pillow lava projected through the sediment here and there. The wall of the caldera has a long slope with scattered blocks of talus on and embedded in pelagic sediment. Moving upslope, the talus abruptly gives way to a near vertical wall of interlayered thick massive flows and pillowed flow units. One of these thick flows was more than 30 meters thick and had beautiful columnar joints. Others were thinner, but several more also displayed columnar joints. At the very top of the wall, a layered 2-meter section of fine-grained volcaniclastite outcropped and formed the surface on the floor of the next older, middle, caldera. Most of that caldera floor was buried by pelagic sediment. The wall of this middle caldera was a repeat of the wall of the south-eastern one, with interbedded massive, columnar-jointed flows and pillow lavas, again topped by a roughly 2-meter section of volcaniclastite. The similarity of the rocks in these caldera walls suggests that the middle one may not actually be a separate caldera, but a section of the floor of the south-eastern caldera that simply did not subside completely. We continued across the floor of the north-western caldera where we found low outcrops of pillow lava surrounded by and partly covered by the fine-grained, layered volcaniclastite seen at the rim of the caldera to the south-east. These pillow lavas therefore predate the eruption of the clastic unit and the formation of the caldera. The pillows appeared to be unbroken, suggesting that the caldera floor may have subsided as a cylinder, leaving the uppermost flows intact. We finally reached the base of the final, oldest caldera wall and found that it was heavily encrusted in manganese oxide crusts that obscured the underlying rocks. It was very difficult to tell what was talus and what was flows until we reached quite high on the wall. Thick, jointed units were common but it was unclear whether they were ponded lava flows or dikes, and only the upper section appears to be pillow flows. A short distance over the very top of the caldera rim was a coarse-grained volcaniclastite we thought was a sheet-flow until we examined the sample in the lab.
We have been seeing a pattern in which volcaniclastite formed during caldera formation, and that sometimes some subsequent lava flows erupted onto the caldera floors. The final part of today’s dive was somewhat different in that there was no thick volcaniclastic section at the top of the caldera wall but a thinner deposit of volcaniclastite that was coarser grained than what we have collected previously. In each case so far, the surface that was disrupted by caldera collapse is covered in volcaniclastite and is the only location we have found such deposits. We will test this model again during the final dive into a caldera the day after tomorrow.
After a very sparse day yesterday, our biology team has much to look at tonight as we collected 20 animals and also ended up with additional animals attached to the rocks. Several highlights include finding (and collecting) Acesta moreii, a deepsea bivalve related to fileshells, this far north. This animal has been the object of a genetic study at MBARI for several years in an effort to understand their larval dispersal. We collected a large purple urchin we have seen many times, but not collected (due to its 9 inch diameter!) We also collected an unusual 5-armed crinoid, many ophiuroids (brittle stars), several polychaetes, some isopods, another gorgonian, a bamboo coral, and an enteropneust worm. The enteropneust is a hemichordate (thought to be ancient ancestors of chordates, like us) that has only rarely been collected and whose genetic relations to other hemichordates is unresolved.
Brian writes: Another awesome day floating in the Pacific Ocean and getting a ton of work done! For the last two days we focused on collecting a set of sediment cores that would help unravel the history of Axial Volcano. Our first few attempts yesterday to core sediments close to the summit were hugely successful, but as we moved downslope we suddenly found relatively young lava flows with virtually no sediment cover on them at all. So we spent most of the latter half of the dive looking at the lavas. So what do the lavas tell you? On the seafloor, we pay attention to the types of flows. Bulbous pillow lavas form where the rate of extrusion of the lavas is pretty low, and the lava advances as a tube (maybe 1 metre across). Flat sheet flows form when the rate of extrusion is high and the lavas pour out as thin sheets over the seafloor. There are gradations in-between these two types, but these are the kinds of things that we can observe that tell us about the rate at which lavas were erupted. So how do we identify the products of a single eruption or multiple eruptions? In this case you can’t just look at the type of flow, since different eruptions will very often have the same eruption rate and hence flow type. So we need to then look close-up at the individual lava samples once we have them in the lab on the Western Flyer. We carefully describe and photograph each sample, and look for similarities and differences in the detailed rock textures. At Axial we noted a key aspect in trying to decide if we are looking at one or many different flows from different events. We noted that the “older” lavas that we were able to sample from beneath sediments near the volcano summit lacked any mineral crystals, but several of the younger-looking lavas from downslope had a few crystals of a mineral called feldspar. So now we can go back to our dive track and note where we sampled mineral-free vs. feldspar-bearing lava, and map out the distribution of the eruption that produced the feldspar-bearing lava flows.
Kristen writes: Today we got to see more biology on the dive. We collected a total of 20 aminals in total. We saw some really giant (about 60cm long) octopus just lounging around on the ocean floor. We collected a really big sea urchin, the biggest I've ever seen. Its diameter was probably as big as a basketball. Of course we had our usual brittle stars everywhere, but they were still nice to look at. At the end of the dive we came upon a "garden" of Crinoids. They were so pretty, and they seemed to go on forever! In the lab tonight, Gill and I got to disect some clams. Pretty neat stuff, but everything looks the same on the inside. The shells were nice though. And of course I shall go end my day with some well deserved Ben and Jerry's ice cream!