9 Feb 1999:
Observers: Ken Rubin (transcript) and Eric Bergmanis (transcript)
Pilot: Bob Waters
Summary: This dive investigated the EPR axis between 18°40' S and 18°41.5'S. Most of it was too far south to be covered by the Animal Farm area transponder net, so a combination of dead reckoning and GPS fixes from the ship were used to construct the dive track.
The dive began west of the axis in pillow lavas transitional to lobates, which appear to be similar to the Animal Farm lava flow in age and phenocryst content. After sampling at the landing site, We traversed up slope over the western of two highs that characterize the ridge axis in this area. This was an unfissured, step-walled pillow ridge. We descended this ridge and then passed over the second (wider) axial high. This one was heavily fissured on its west side and up through the bathymetric high; the lavas themselves were primarily lobates of apparent Animal Farm age. Once over the high point, we encountered a significantly younger, thin lobate lava flow, which we sampled three times. We passed in and out of it a number of times on a heading of 070, observing low temperature diffuse flow venting and extensive mussel and clam beds in at least one large area. We continued in this flow on a northwesterly heading, encountering both high temperature hydrothermal vents and low temperature diffuse flow vents. The flow is locally comprised of lineated and jumbled sheets, but is primarily lobate. On this transect, the flow was filling a topographic low of ~5 m. We passed back out of this flow into fissured lobate and pillow lavas while still east of the bathymetric high of the ridge axis. An extinct high temperature hydrothermal vent site was encountered past this contact. We continued on this heading over the rise axis, on a long transect that culminated on hill west of the ridge axis. We encountered heavily fissured lobate and then pillow lava terrain accross the entire area after leaving the young flow. The fissuring became more extensive (with large and deeper fissures) on the wester part of the axis. The lavas appeared to be Animal Farm flow age.
The second half of the dive consisted of three traverses of the axis for which there was limited transponder navigation. 7 samples were taken during this part of the dive. The geology on each transect agrees remarkably well amongst them: The western half of the axis is comprised of heavily fissured lobate lavas (with some pillows interspersed); the eastern half of the axis is comprised of the young lava flow described previously. The young lava flow is characterized near its western extreme (which occurs just east of the topographic rise axis) by a series of low temperature diffuse flow vents, heavily colonized with biota, that are aligned roughly parallel to the local orientation of the axis. Further east, the young lava is not colonized by biota. It is also a relatively thin lava flow in this area and a number of kipukas of older lobate and pillow lavas within it. The lava flow is generally lobate, transitional to sheet flows as one moves east (downslope) through it. We hypothesize that the diffuse flow sites are aligned along the eruptive fissure for this presumably post-Animal Farm lava flow.
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