Gorda Ridge Cruise
August 5 - 21, 2002
August 10, 2000: Day #6
Log Entry: Dive 190 met all of its objectives and then some. We began with a traverse up a 200-m ridge-parallel fault at the eastern end of the President Jackson seamount chain. The scarp at the location we began was draped by younger lavas and hyaloclastite, apparently erupted from a small cone about 1.5 km to the west. We dropped back down the scarp and tried a second traverse about 400 m along the fault where it was steeper. This traverse encountered the volcanics erupted at the Gorda Ridge about 1.9 million years ago that we were seeking. The bottom of the scarp was large talus blocks and the upper part was truncated pillow lavas. We had great difficulty finding samples that were small enough to collect, but did collect about a dozen samples from the scarp.
We found several landslide chutes filled with finer sediments and small basalt fragments and collected samples of basaltic glass in both a push core and using the biology "fry basket" to sieve the glass from the sediments. This was extremely successful and yielded a large collection of glass fragments large enough to analyze. We then proceeded to the northwest and sampled 3 small- to medium-sized volcanic cones. The smallest was draped with sheets of bedded hyaloclastite, suggesting vigorous, high-volume eruption. We now have a collection of samples from 12 of these small cones that are precursors to formation of the large President Jackson Seamounts. These volcanic structures have very sparse biota, but along the way we saw several specimens of a predatory tunicate that looks like the one found on the Taney Seamounts, although one specimen was so large that it would have filled the bio-box. We also collected some barnacles, pink and orange soft corals, a gorgonian, and a bright yellow encrusting sponge.
Tomorrow, we are returning once again to the "Seacliff" hydrothermal site at the northern Gorda Ridge to collect a more complete suite of water samples and measure the temperatures of additional vents, and to collect additional biological specimens, pushcores, and volcanic rock samples.