22 February, 1999
Ken Rubin (transcript), Scott White (transcript), Matt Heintz
Summary: The objectives of dive 3361 were to map the across-axis extent of the youngest lavas erupted at the axial high, between about 17 31.5'S and 17 29.50'S, to search for possible contacts at the southern end of the so-called Aldo Lake lavas, and to examine lava morphologies within individual lava flows. We located 7 contacts between very fresh lavas (possibly of the Aldo Lake lavas) and older volcanic units to the west and east of the axis, as well as a single contact with even older lavas on the east side of the axis. Relative age was determined by a number of factors, including sediment cover and glass luster. During this dive we also investigated and sampled two very-near axial lava mounds occurring just west of the youngest lavas. In all, 14 lava samples were taken (6 from the youngest unit, 7 from somewhat older units, and 1 from a very old unit encountered at the start of the dive). The width of the youngest lava flow appeared to increase from south to north, being 200m wide on the first traverse and 500m wide on the last.
The dive started 800m east of the topographic summit of the ridge on a jumbled-folded sheet flow that had accumulated deep pockets of sediment. Driving west the seafloor began to shoal as we passed into lobates and pillows of similar sediment accumulation age. Along this traverse up the ridge flank we observed several elongate structures reminiscent of pressure ridges found in subaerial pahoehoe flows. Much younger lobate lavas tentatively identified as part of the Aldo Lake lavas were encountered on top of the axis platform in a swath about 200m wide. These younger lavas appeared here (and throughout the dive) as extremely glassy surfaces mottled with small (<10cm diameter) patches of sediment. The transition out of the young lava was a contact with a pillow flow that gradually became less lustrous, and had fewer pillow buds and more interpillow sediment. These pillows formed a mound that is composed entirely of pillow lavas up to it's summit. Sediment cover ages may have been slightly less than older material found on the east side of the axis.
On our next traverse, we encountered the youngest lavas again about 2m east of a 3m wide fissure that was crosscutting older lobate lavas. This fissure was the largest in a series of small ridge parallel fissures localized within a 50m wide zone. The contact was easily discernable in this case and was characterized by small apple-sized balls of glassy lava over older lobate lavas followed by a 1-2m swath of small pillows that then graded into very glassy lobate lavas. The flow is mostly lobate here and we encountered a few several-meter-deep collapses ringed by spaghetti worms and thicker than usual orange-colored sediment, probably hydrothermal in origin. The eastern edge of the flow is marked by a change from glassy lobates to more sedimented pillows. Our only strong evidence of high temperature venting was the discovery of a ~3m high extinct hydrothermal chimney on the western margin of the youngest lava flow at ~17 30.7'S.
During the middle period of the dive we headed essentially due north through an area of collapsed lobate lavas. Some collapses were large enough for the submarine to enter and maneuver in, and were floored with a combination of rubble, broken lobate crust and some folded sheet flows. Collapses also contained lava pillars, void spaces into the hollow subsurface of the lobate flow and bathtub rings on the collapse walls. We eventually obliquely crossed a contact out of the Aldo Lake flow lobates into an older pillow flow. And then turned to head 075 and cross back into the younger lobates. On this traverse we also encountered several small collapse pits, but unlike previous traverses these appeared to be closer to the eastern boundary of the Aldo Lake lava, rather than up the center. The collapses had a thick coating of orange hydrothermal sediment around them. Immediately after crossing the young lobate graded into a pillow flow on a small steep slope (~6m high) and then we encountered a contact with older pillows. We turned to head 295, and crossed again from pillows into younger, more elongate pillows as we climbed back up to the topographic summit of the ridge. This was the contact with the Aldo Lake lava. At the topographic summit the flow turned to lobates. Within a few meters after crossing into the lobates, we were within a zone of several meter deep collapse pits surrounded by clumps of spaghetti worms and hydrothermal sediment pockets. After about 100m, the lobates were un-collapsed, and after another 200m, we were at the contact between the Aldo Lake lavas and older pillows. The seafloor started to shoal, and we crossed from the older pillows into lobates with a continuous sediment coating. Upon reaching the summit of the axial lava mound, we found a pit crater that had been previously identified from the 1996 DSL-120 sidescan record and sampled the lobate lava there. We then headed east and passed back over essentially the same sequence of lava morphology as on the previous traverse, eventually reaching the contact back into older lavas off of the axis. On this northernmost traverse the young lava flow was 500m wide.