July 24 - August 6, 2006
July 30 update
Tiburon dive T1008, Vance A Seamount
Jenny writes: Still good weather up here: very calm, little wind, mostly sunny, with the occasional rain squall.
Dave Clague writes: We completed our second dive on the Vance Seamounts today, but this time at the oldest rather than the youngest seamount in the chain. The northwestern seamount is a circular volcano that appears to be structurally much simpler than the site of yesterday’s dive. This volcano has steep outer slopes and a nearly flat top, but no obvious caldera collapse structures. Instead the summit region is marked by a series of small volcanic cones and shields, and some small faults that parallel the nearby Juan de Fuca Ridge axis. The much older age of the volcano impedes our ability to either sample the rocks or to be sure what we are observing since everything is blanketed by a layer of manganese oxides that precipitated from hydrothermal fluids when the volcano was still active. Subsequently, additional manganese oxides have been added that simply precipitate from seawater, but these tend to be quite slow accumulating and so are not the primary source of the oxide layers. Many samples we thought were flat layers of volcaniclastic rock turned out to be lava or talus fragments of lava with a thick (1”) layer of manganese obscuring the shape of the underlying rocks. The manganese also cements loose rocks together and makes sample collection both challenging and frustrating. In addition, the seamount has accumulated abundant sediment in the several million years since volcanic activity probably ceased. These factors all combined to make the dive much more difficult and the track covered much shorter, so we did not get to see as much today.
The story that we can decipher about the history of this seamount is quite different from the rather simple story yesterday, although some themes are consistent for the two volcanoes. The main thing that is similar is that both have at least thin deposits of volcaniclastic rocks draping the summits and that both had some eruptions that occurred after the formation of the clastic deposits. The outer slopes of the old volcano are mostly talus rather than lava flows draping the slopes. Today’s dive also observed numerous fault scarps that offset portions of the summit, but these are too small to be seen in the existing bathymetry. The sediment trapped in the deep calderas observed yesterday was almost entirely pelagic microfossils that had accumulated over time whereas the sediment observed today on the summit of the old volcano had been winnowed and redistributed by strong currents and included a significant component of lava fragments in sand and gravel sizes. We have sampled these gravelly sediments with the hope of determining the full range of lava compositions erupted near the end of volcanic activity. We also collected similar samples on the outer slopes that may provide a glimpse at the range of compositions of lavas during construction of the bulk of the volcano.
During the transit across the summit, we encountered a small, unusual orange-colored outcrop that consisted of highly altered hydrothermal sulfide. This is probably the oldest such deposit yet found on the seafloor since it certainly formed when the volcano was active, perhaps 2 million years ago.
One surprise was that the animal communities were not as prolific as we expected, but again consisted dominantly of gorgonians, sponges, and tunicates. Two new additions were anthomastus (mushroom corals) that reached sizes up to about 40 cm in diameter with their polyps extended, and several species of bamboo corals. We added to our collections of gorgonians, including several very tiny ones (about 2-5 cm tall). The suction sampler was also used to collect an array of small animals including baby crabs; polychaete, scale, and annelid worms; cup corals, brittle stars; a sea pen; and brachiopods.
Gillian Clague writes: At one point in the dive today, we came across an outcrop of rock that had hundreds of small cup corals, white and purple anemones, all gathered in crevices in the overhanging ledge. The abundance of these few animals, in distinct areas from each other, was an amazing sight. This projecting piece of rock was the only part that supported a dense population of animals, so the difference was striking. Why that one chunk and not any of the surrounding ones? We concluded that a strong current passed by and under the ledge, whereas the surrounding areas are most likely relatively still. With this exception, there were not as many animals on today's dive, although we did see black corals, polychaetes, and a number of enormous soft corals, called Anthomastus. The flows were very old and logically should have had more life, but had a thick manganese crust that is toxic to sessile animals.
Brian Cousens writes: It’s been a very interesting day to be a geologist looking at fairly old volcanic rocks on the sea floor. Vance seamount “A” is probably at least a million years older than Vance seamount “F”, the relatively young target of our first dive yesterday. The older lavas on Seamount A have become heavily coated on manganese-iron oxide, a black substance that precipitates out of seawater onto rocks on the seafloor. These causes two problems for us. First, it’s hard to collect rock samples! The manganese oxides tend to cement the rocks together, and it’s very tough to break rock fragments out to keep as samples. Second, it becomes harder to decide exactly what the surface in front of the ROV actually is: is it a flat lava flow, or a thick sheet of manganese oxides coating the rocks underneath? Several times today, we thought that we were sampling pillow lava fragments that instead turned out to be thick, curved manganese crusts! This is very frustrating when you want lava samples but don’t want manganese oxide samples. We also sampled what we thought were thick crusts of basaltic glass-rich sedimentary rock, and after unloading the sample from the ROV found out that it was a large fragment of almost pristine basaltic lava with a thin manganese oxide coating! In this case, we were thrilled to have been wrong in our initial guess as to what we were sampling. Like yesterday, we saw a wide variety of lava types, including pillow lavas, lobate collapsed lavas, sheet flows, and a few massive lavas, all indicative of different eruption rates and emplacement styles. Maybe our view today of how these seamounts have formed is not as clear as our view yesterday at seamount “F”, but we are making progress!