Observers: Tracy Gregg (P-O) (transcript) and Scott White (S-O) (transcript)
Pilot: Bob Waters
Summary: We landed just on the west side of the axis and motored south, trying to identify a single lava channel that we could follow to the west. We picked out 2 channels (which both ended abruptly) before concluding that we needed to follow the entire region of jumbled lavas. We followed the jumbled lavas until they abutted with age 2.5 pillows. We then turned south, and eventually east, in an attempt to map the extent of the jumbled "lava distributary system" (LDS).
The LDS, which has low sonar reflectivity, is dominantly a system of anastamosing channels within a field of jumbled lavas. Within the jumbled lavas, we observed patches of smooth, lineated, and/or folded lavas that appeared to be channeled. Whenever we attempted to follow a channel, however, we observed that the channels were only ~50 m long at most. We conclude, therefore, that these smooth sheet flows may be analogous to similar features found within platy or transitional a'a flows on land. Smooth, channeled lava "tongues" are observed within these rough terrestrial flows, and form due to local variations in emplacement rates an styles. These subaerial tongues are also small and discontinuous.
Vertical relief of up to 8 m locally, and 2 m generally, is common within the jumbled lava flow. We interpret the topography to be the result of anastamosing channels within the jumbled flow. The bathymetric highs represent channel levees or islands; the lows are locations where lava flow was concentrated.
The origin of the jumbled lavas is within approximately 100 m west of the axial high, and is most commonly marked by a 2-5 m high scarp within the lobate flows. The jumbled flows appear, fully formed, from the base of these scarps. As one approaches the scarps from the east, one observes broad lobate flows with increasingly common and increasingly larger collapse pits. The scarp walls are most commonly covered with talus, although rarely, in situ "bathtub ring" structures can be observed on the walls. At one location, a jumbled flow was observed to be pouring directly out of a broad (~5 m wide) lava lobe. Within the jumbled flows, near the scarps, channeled regions of smooth lava are common. These are typically characterized by lineated folds at the margins, grading into flow parallel lineations and finally to flat, smooth sheet flows at the channel centers. We attempted to follow 3 such channels, and each time they originated at the edge of a collapse or a topographic high. We interpret these narrow (<20 m wide) channels to be levee overflows.
We followed the jumbled LDS to the west until it lapped up against an age 2.5-3.0 pillow mound, approximately 1.5km from the axis. At the distal margin of the jumbled flow, we were forced to climb upward approximately 6m to stay on top of the flow. The northern, southern, and western margins of the flow were marked by an interfingering (both laterally and vertically) of lobes and pillows with the jumbled terrain until ultimately there were only lobes and pillows. The LDS we identified was approximately 500m wide near its source.
Assuming an eruption temperature of 1200° and an eruption viscosity of 100 Pa-s, this LDS may have required eruption rates on the order of 103 104 m3/s.
Dive 3345 Sample Locations
Sample # Lat Long 3345-1 18°37.42'S 113°24.28'W 3345-2 18°37.55' 113°24.29' 3345-3 18°37.57' 113°24.30' 3345-4 18°37.59' 113°24.61' 3345-5 18°37.62' 113°24.69' 3345-6 18°37.58' 113°24.98' 3345-7 18°37.59' 113°25.27' 3345-8 18°37.63' 113°25.19' 3345-9 18°37.65' 113°25.15' 3345-10 18°37.66' 113°25.12' 3345-11 18°37.72' 113°24.54'