Monterey Bay Aquarium Research Institute

West Coast Expedition
July 20 - August 30, 2002
West Coast of North America
Logbook

August 17, 2002: Day #29


Figure 1 is a large sample of pillow basalt collected from the Summit Seamount talus ramp near where we tried and failed to create a borehole in basalt.

Debra Stakes writes: We finally got into the water today. In fact we got into the water twice. The first dive ended quickly with a thruster problem. The second dive was fascinating for geology and frustrating for the goal of creating seismometer boreholes. The MBARI coring sled was designed to collect 3.175 cm cores of crystalline rocks such as gabbro, or hydrothermal deposits such as sulfides. Within the Monterey Canyon during the late 1990’s, we discovered that we could obtain really nice cores of granitic basement for geochronological studies. By designing and installing an oversize "reamer" bit, we were able to also create boreholes that were nearly 6.985 cm diameter. These boreholes have served as our deployment sites for the MBARI corehole seismometers.

The highest priority goal for the ongoing Keck dives on the Juan de Fuca Ridge is to establish sites for seismometer deployments scheduled for 2003. The scientific goal of the Keck-supported research project is to constrain the linkages between volcanic eruptions, earthquakes, changes in the chemistry and flow of hydrothermal systems and the waxing and waning of microbial populations. We hoped to have the short-period corehole seismometers placed into boreholes in the mid-ocean ridge basalt. This would provide the cleanest record of seismic signals from the small events linked to seafloor volcanic eruptions and hydrofracture associated with seawater penetration into the oceanic crust. Our experience in 2000 on the southern Juan de Fuca Ridge had shown that the drill was able to recover cores of mid-ocean ridge basalt. But we had never tried a reamer bit on such hard and brittle material. And I had never seen the detailed geology of the Endeavour Ridge - almost all the previous work has focused within the small hydrothermal fields, not the regional geology.


Figure 2 shows the temperature probe measuring the fluids emanating from the Pico Vent in Sasquatch vent fiel

Looking at the map, Summit Seamount appeared to be a perfect spot for a seismometer. The steep contours suggested a clean wall for the ROV to core into. What we found, however, was a young mound of fragmental pillow lavas partially cleaved into extensive piles of angular talus that extended halfway across the valley floor. Though we spent hours looking, we could find no place for the ROV to sit that would permit us to core into the less fractured massive lava units exposed near the base of the scarp. We gave up on this site and moved to the toe of the talus ramp and found a boulder that was larger than the ROV. We chose this as the best we could do for the seismometer in this area. We were wrong. The ROV could not find a stable place to sit and no progress was made with the reamer bits. We gave up, marked the boulder with an acoustic marker, collected a large rock from the area (Fig. 1) and headed across the valley floor to the most northern of the vent sites, called Sasquatch (Fig. 2).


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