2.1a NeMO 2000 Cruise Summary for Geology and Volcanology- Bill Chadwick and Bob Embley

We started the NeMO2000 cruise at the southern Cleft segment, and during ROPOS dives R541 and R542 we deployed 11 new extensometer instruments in a linear array across the floor of the axial valley. These extensometers will make daily measurements of the distance to their neighbors up and down the 1.2-km long array to within about 1 cm. We are using the instruments to look for horizontal deformation across the axis of spreading that might accompany magmatic or tectonic events. These new extensometers are a modified design from the prototype instruments that we have been using at Axial Volcano with NeMO. The new instruments have 5-year battery lifetimes and are placed into permanent benchmarks on the seafloor (the benchmarks were deployed last summer during the Cleft99 cruise with Jason). The extensometers will need to be revised by an ROV in future years to retrieve their recorded data via an infrared data port. In addition to placing the extensometers in the benchmarks, we also made precision pressure measurements at each benchmark to monitor for vertical deformation across the array, and we recovered and deployed HOBO temperature probes at two active hydrothermal chimneys at Vent 1. This monitoring effort at south Cleft is funded by the NSF/RIDGE Program.

At Axial, we are continuing our instrumental monitoring efforts to measure inflation and deflation of the volcano. We had extraordinary success measuring the deflation that accompanied the 1998 eruption with a combination of rumbleometers and extensometers, and now we are trying to see if Axial is re-inflating between eruptive events. We are employing several strategies to do this. First, we deployed a new Bottom Pressure Recorder (BPR) near the center of the caldera (where the maximum uplift or subsidence presumably occurs), because the old rumbleometer instruments were retired last year (there was no BPR monitoring at Axial between 1999-2000). In addition, we used a precision pressure sensor on the ROPOS vehicle for the first time and made pressure measurements at a series of 5 benchmarks that we deployed in a radial line. The benchmarks are located at the caldera center, Magnesia vent, Marker 33, Bag City vent, and the southern anomaly lava flow (10 km from the caldera center). The pressure sensor can measure the relative depths between benchmarks to with a few cm, so by making annual measurements at these benchmarks and assuming that the outer-most benchmark is stable, we will be able to see if the benchmarks closer to the caldera are moving up or down relative to the outermost one (evidence for volcanic inflation or deflation). The third prong of our instrumental monitoring effort is our prototype extensometer array, that we deployed again on Axial's north rift zone. This array measured part of the deflation signal during the 1998 eruption, but could also potentially measure extension across the north rift zone if a diking event occurred there. These prototype extensometers have to be recovered and re-deployed every year. Eventually we hope to build and deploy some of our new benchmark extensometers at Axial instead.

The other main focus of our work at Axial during NeMO2000 was continued geologic mapping of the 1998 eruption site. As during the last 2 years, this involved a combination of Imagenex sonar mapping (to produce high-resolution bathymetric maps) and visual transects on the seafloor. Imagenex data were collected during ROPOS dives R546 (14.0 hrs), R550 (11.6 hrs), R551 (4.0 hrs), R552 (4.3 + 5.5 hrs), and R554 (11.3 hrs), adding up to a total of 50.7 hrs of Imagenex survey time during the cruise. This mapping was divided between 3 separate survey areas: 1) extending our previous coverage from Marker 113 south to Bag City vent (21.1 hrs), 2) collecting a new survey at the southern anomaly lava flow (25.3 hrs), and 3) extending our previous coverage northwest of Marker 113 (4.3 hrs). The Imagenex data were processed and displayed at sea, and we again found that our seafloor mapping efforts were much more productive when they can be viewed in the context of previously collected high-resolution bathymetry. The southern anomaly survey revealed that this lava flow is transitional in morphology between pillow and more fluid sheets. We had previously seen the pillow flow extending to the east of the axis, but this year we explored the very fluid and drained out parts of the flow near the eruptive vent on the south rift zone. The Bag City and M113 surveys helped finally convince us that the very young lavas extending from M113 southward to Joystick, Bag City and Crevice vents on the western side of what we had previously mapped as all 1998 lava - are definitely in pre-1998 lava. Visually these pre-1998 lavas are indistinguishable from the 1998 lavas, but the collapse features we see in the Imagenex survey between M113 and Crevice can be identified as pre-existing features in high-resolution sidescan sonar collected in 1987. We still think there is 1998 lava on the east side of this young pre-1998 lava, but we have not yet determined how far south the 1998 lava extends. We are still sure that the southern anomaly flow is 1998 lava (because there was a clear SeaBeam depth anomaly there), but some of the young lavas that we mapped last year between the southern anomaly flow and the known 1998 lavas are probably also pre-1998 flows. Examining pre-1998 camera tows and Alvin dives in this area will help us nail down these conclusions.