Monterey Bay Aquarium Research Institute

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

July 28, 2002: Day #9

Rob (Z), Jenny, and Jim sorting rocks taken from the ROV's sample drawer with the aid of images taken of the collecting events.

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.

DrawerChaos-sm.jpg (118793 bytes)
After the dive, the partitioned sample drawer on the vehicle was full of rocks, biological specimens, two designs of water samplers, and the singed temperature probe.

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

Tubeworm meadow in a low temperature, diffuse flow hydrothermal vent site. Vestimentiferan tubeworms (Lamellibrachia spp.) cluster among basalt talus and small barite chimneys densely covered with limpets.
Two generations of lava? This figure shows lava flows that lie on the backside (the east side) of a linear ridge. The lower pillows may have formed before the ridge was faulted and tilted to its present position. The upper pillows are different in texture and appear to have been draped at a steeper angle over the top of the earlier lavas after the ridge tilted.
Our suction sampler, developed for sampling tiny volcanic glass, is also useful for collecting biological specimens like this blue mat (a folliculinid protozoan).
Capture of water from a hydrothermal vent in the Sea Cliff hydrothermal field. The nozzle of the sampling bottle will be placed into the most vigorous flow of hot water. These samples will be analyzed by Dr. Karen Von Damm of the University of New Hampshire and Dr. Marvin Lilley and Eric Olson at the University of Washington.


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