West Coast Expedition
July 20 - August 30, 2002
West Coast of North America
July 28, 2002: Day #9
Log Entry: On Sunday, July 28, 2002, we undertook the first of three dives at GR-14, the Sea Cliff Hydrothermal Field on the northern Gorda Ridge. This site was first discovered in 1988, and was first visited by the Tiburon in 2000. It is an unusual site because it is located off axis, and it sits along the west-facing wall of a linear ridge that rises 300 meters above the rift axis. The dive included several goals: First, we hoped to characterize the hydrothermal field more completely, from a physiographic, chemical, and biological perspective. Second, we wanted to establish if the field is evolving over time. Finally, we hoped to map structures that could be associated with the hydrothermal site.
Today's observations revealed that the field is very vigorous. Many of the chimneys we observed and sampled in 2000 remain active two years later. Furthermore, we discovered several new chimneys, that hadn't been active before. We collected water samples and measured temperatures at two of the vents. Also, we made a large collection of vent fauna, and recovered some 15 geological samples. In addition, we deployed 3 "HOBOs", temperature probes that will record vent temperatures for several days.
At the end of today's dive we made a lengthy transect to the east from the hydrothermal field. The transect was approximately 1.5 kilometers long and crossed several faults. None of these revealed any evidence of hydrothermal activity. The offsets on these faults were far less than those at Sea Cliff, and they do not reveal the same level of faulting intensity. This suggests that hydrothermal vents such as those seen at GR-14 require deep, more active faults to accommodate fluid flow.
Another of the observations made today was that the linear ridge exhibits distinct episodes of volcanic activity. Some of the lavas appear to have been faulted and tilted after they erupted. Overlying them are more recent pillow lavas that appear to have flowed down the side of the tilted block. It is not known if this later activity may have influenced the present hydrothermal system. However, both generations of lava appear to be relatively old.
-- Jim McClain