MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 25 update - from Larry Mastin
Tiburon dive 886 - 1996 lava flow on Gorda Ridge
As with yesterday, our objective today was to examine features in the lava flow field that erupted along the Gorda Ridge in 1996. This eruption had been detected in 1996 by its seismic activity, and the extent of lava flows estimated by comparing bathymetry obtained before and after the eruption. Our transect started from east to west across the middle of the flow field, at roughly 42°40.650'N latitude.
Surprisingly, we saw fresh-looking pillow lavas over only a small fraction of the approximately one-kilometer width that had been previously mapped. These flows were clearly more than nine years old as evidenced by the thin sediment layer that covered them and, more importantly, the number and variety of marine organisms that had colonized them. We were fascinated by crinoids, sponges that looked vaguely like ferns with large white balls instead of fronds, white, single-stalked, meter-long fragile gorgonians, and more exotic pink, translucent worm-shaped creatures known as enteropneusts.
For geologists unfamiliar with these deep-sea organisms, they appeared exotic and otherworldly; but their presence betrayed the age of this landscape and we found ourselves exploring farther south in search of the fresh 1996 lavas. We found them again near the south end of their previously mapped extent, leading us to believe that the eruption may have come to the surface as a few isolated vents with little or no lava effusion between, rather than pouring out along the full length of a continuous fissure system.
The geology consisted mostly of basalt pillows which ranged in shape from elongate downhill-trending tubes on sloping surfaces to more equant pillow shapes on flat ground. Fields of pillows were occasionally interrupted by talus fields, by north-trending fissures or, in one or two places, by circular drained depressions a few meters in diameter that may represent collapsed lava ponds. Most importantly, today’s exploration has shown us how much is yet to be learned about the extent of recent volcanism on the sea floor.
Kim Fulton-Bennett writes: During the days, we sit in the darkened ROV control room and try to make sense of the strange geologic forms and even stranger animals we see on the video screens. Occasionally we take samples, which will give us a more concrete, "hand's on" idea of what is down there. After the ROV returns to the surface, the real work begins. Since we may collect over a dozen pieces of black lava during a dive, it is critical that we keep our samples organized during both collection and processing.
When the ROV comes back up to the surface, the process of removing the samples from the ROV and processing is like a highly choreographed dance. Each sample is matched against a numbered note sheet, and in questionable cases checked against video images taken during the collection process. The idea is that, ten years from now, we will be able to tell exactly where each sample was taken, and how it fits into the regional geology, as revealed by the video record.
In the wet lab, we set out a series of stations where each rock sample is processed. Each sample is first given it's own identifying sheet and plastic storage bag, which is sometimes used to hold little fragments that break off during handling. Next, any living animals attached to the rock are examined by a biologist and if appropriate, removed for later classification, preservation, and sometimes DNA analysis on shore.
After the biologists have had their turn, the sample is washed carefully with a soft brush to remove any loose sediment (not uncommon with all the mud drifting around the sea floor, not to mention the mud we stir up with the ROV). The trick here is not to remove any alteration or crust that actually forms part of the sample itself.
After washing, the rock is carefully patted dry, and like a Hollywood star, placed under the bright lights for a photo session. These photos will be added to the sample database (along with the video frame grab taken while the sample was being collected), to provide another method for visually identifying the rock.
The next step is arguably the most important, and is usually performed by the more experienced geologists in the science party. One person examines the rock with the naked eye or hand lens, and describes it out loud in precise scientific terms. This description includes the rock's size, shape, color, and surface texture (including rinds, or bubbles, if any), and information on the geologic setting in which the rock was found. Another characteristic that is particularly important in identifying volcanic rocks is the size and mineral composition of any crystals (phenocrysts) embedded in the lava, as well as a description of the lava itself. A second scientist writes down these descriptions and serves as a second opinion in questionable cases.
After this exhausting process, the rock is laid out along with all the other samples and allowed to rest in a safe place where everyone can admire it and compare it with the other samples. The next day, when it is nice and dry, glass is chipped for later geochemical analysis, and the rock will be bagged up and carefully stowed in plastic buckets for shipment back to MBARI, where it becomes part of a priceless scientific record. However, a "priceless" rock can instantly become a worthless rock if we can't tell where it came from. Thus, the attention to detail throughout this process.
We don't usually finish processing all the rock samples until after 9:30 or 10pm. But even after we're done with the grab samples, the folks processing sediment cores are still hard at work. I'll describe this process in a later expedition log, since sediment samples will be the "core" of our scientific work for the next five days...