Monterey Bay Aquarium Research Institute

MBARI Ridges 2005 Expedition

Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005

August 8
Tiburon Dive 874, Axial 1998 South Lava Flow:

Dave Clague writes:
We completed our first dive today and explored the southern portion of the 1998 lava flow on Axial Seamount. The day started with our simultaneous arrival at our dive target, a fire drill, and a reboot of all the computers on the ship so that one of the UPS systems could be replaced. All went smoothly and we were diving in less than an hour. The main objectives of the dive were to determine if the 1998 eruption had a mildly explosive component that produced small glass bubble fragments and to determine if animals have colonized a flow so young. These are part of two different studies to determine how common explosive eruptions are in different environments in the deep sea and to determine the rate of colonization and animal growth on young flows to see if we can estimate ages of older flows whose eruption ages are unknown. To accomplish these goals we sampled on and off the flow, and did several measured transects to determine the numbers and types of animals both on and off the 1998 flow. During this we also explored along almost the entire eruptive fissure(image on right) and a collapsed lava pond in the western part of the flow. We were able to map the 1998 lava flow and its eruptive fissure in more detail than previously possible, thanks largely to the high-resolution bathymetric map of the flow and surrounding area collected by NOAA in 2000.

A quick examination of the 20 samples recovered using a small suction sampler (an underwater vacuum cleaner of sorts) established that there were definitely mildly explosive eruptions in the area. However, we will not be certain that these glass fragments were produced during the 1998 eruption until we analyze the chemistry of the grains. The flat to slightly curved bubble-wall fragments, called limu o Pele (Pele’s seaweed in Hawaiian), appear to be most abundant in samples collected from older flows surrounding the 1998 eruption, so they may be from several different eruptions in the area.

After 7.5 years, the 1998 flow was essentially devoid of animals that settle and grow on the surface, and had very few animals that had walk-on roles. Surrounding flows varied in their abundance of animals. Dives later in the program will determine if animals have colonized flows as old as 20-25 years. Hopefully, when we put all these observations together, we will have established a rough chronology of colonization that we can use to estimate ages of flows of unknown eruption ages.

Gillian Clague writes:
I was surprised during our first dive at how few animals there were. I assumed that the megafauna would be more abundant on a lava flow than on a muddy bottom because the flow provides a hard surface which some creatures use as an anchor, while it serves others as protection. On the 1998 flow, there were surprisingly few critters: sea cucumbers, a couple of crabs, and some sea stars. I was also astonished to see that although the older flows had relatively more organisms, including sponges, gorgonians, and stalked crinoids, they were still surprisingly bare of life over the area we covered. I had never thought about how long a new flow would take to be able to support larger animals. I knew that there had to be sediment and marine snow drifting down, however I had never thought about how much. The pillow lava that had sediment caught in the crevices seemed as though it could provide nutrients to larvae, and yet there were few animals on it. I would have expected many small sessile creatures, and yet the few animals we saw were pretty large, including motile ones such as crabs, cucumbers, and sea stars.

Left image: Slime star standing on the tippy toes of its tube feet. The laser dots beyond it are 30cm apart for scale. Right image: Enormous crab walking on jumbled lava flow on the floor of a drained lava pond.

Bill Chadwick writes:
One of the goals of this dive was to explore specific physical features that had been previously mapped on the flow by high-resolution bathymetric sonar, but had not yet been visited on the seafloor. One of these was a large collapse area on the western side of the flow where fluid lava had apparently ponded temporarily before rapidly draining out downslope. We found that the lava morphology within the drained out area was very chaotic, indicating that the drainout had been extremely rapid, and perhaps even violent, since we found glassy spatter-like fragments (image on right)on the surface of a nearby older flow. Another intriguing aspect of this lava flow is that we know it was erupted from a fissure, because in places you can actually see the lava coming out of a crack in the ground. It is very unusual to be able to see this on the seafloor. There are also other older eruptive fissures nearby, so an extended history of similar eruptions is displayed within this small area. Another interesting aspect of the1998 lava flow is that it contains so many different lava morphologies - everything from pillow lava, lobate, ropy, lineated, and jumbled textures in a remarkably small area. These morphologies form under different conditions of supply rate and slope, and can tell us about the sequence of events during the eruption.

Left image:Drainage shelves on the side of a fracture, with pillow lavas from a younger flow draped over the edge. Right image: Drained lobate flow, with a collapsed shelf inside. The shelf was the top of the flow long enough to form a crust that remained when the lava drained. The view is about a meter across.

We all tried on our survival suits during the fire drill.

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