West Coast Expedition
July 20 - August 30, 2002
West Coast of North America
Logbook
July 29, 2002: Day#10
Log Entry: Today's dive was to explore the deepest section of the Gorda Ridge axis at the southern end of the northern segment of the ridge. The main objective was to determine if lava bubble-wall fragments have formed as deep as 3900 m where the mildly explosive activity that produces the bubbles is almost certainly due to release of magmatic gases rather than seawater boiling to produce steam. We chose the deepest part of the ridge axis for this experiment because the volume expansion of heated seawater gets smaller with increasing pressure. This means that the formation of the lava bubbles is less and less likely to be caused by heating seawater as depth increases. The dive began on a small flat-topped volcanic cone on the ridge axis to determine the morphology and lava morphology on the cone. This particular cone has a small crater in its center, where we landed. The floor of the crater was pillow lava whereas the wall consisted entirely of talus fragments of angular lava fragments derived from thick flows. The flat top was mostly sediment covered, but some pillow lava projected through the sediment. The outer slope of the cone consisted of elongate pillow flows showing that they flowed down the outer steep slope. These observations are consistent with a model we developed to explain the formation of similarly shaped cones around Hawaii. We think these cones form above a point source eruption by steady, moderate effusion of lava that forms a lava pond that gradually grows upward as the pond overflows. The crater is formed when the pond is drained. The dive continued along the middle of the ridge axis where the bottom was mostly buried by sediment. Some open fissures were observed, but are surprisingly rare considering the rather old age implied by the thick sediment cover. Many of the fissures are not parallel to the main trend of the ridge axis and may reflect a recent change in the direction of spreading on this ridge axis. We sampled a series of lava flows exposed in small fault scarps and also took push-cores of the sediment to search for lava bubble-wall fragments, which are usually less than 1/8 inch across. The dive ended on a second small flat-topped cone, where we observed lava structures similar to those at the beginning of the dive. After recovering the vehicle, we sieved the sediment in the push cores and found that most of the cores did indeed contain the sought after lava bubble-wall fragments. With this find, we have established that mildly explosive eruptions can occur along mid-ocean ridges at depths as great as 3900 m. The dive accomplished all the major objectives despite continuing strong winds and large seas. At depths as great as this dive, the ROV has to drag more tether through the water, which slows the dive progress and also makes it difficult to stay on the bottom. In addition, the pan control of the main camera, which had been causing problems for several days, failed early in the dive and negatively impacted our ability to video the desired lava structures and fauna along the way as well as slowed the process of sampling. We terminated the dive about 6:15 pm in order to have time to repair the camera and the manipulator, which continues to operate only with limited range.
