2.1 VOLCANOLOGY - Robert W. Embley and William W. Chadwick

Volcanology studies during NeMO'99 focused on, 1) continuing our instrumental monitoring efforts, 2) expanding our Imagenex sonar bathymetry of the 1998 eruption site, and 3) extending our mapping of the 1998 lava flow south to the area of the southern SeaBeam depth anomaly at 45 52' (confirmed as a new lava flow during dive R465 last year).

The extensometers that were recovered from Axial's north rift zone during NeMO'98 yielded an interesting data set. The instruments had been in place during the 1998 eruption and recorded a 4 cm contraction at the time of the eruption. This represents deflation of the entire summit area of Axial as magma intruded into the south rift zone, and is consistent with the rumbleometer instruments that recorded dramatic subsidence in Axial caldera. The extensometer and rumbleometer data can be modeled together to estimate the depth of the magma reservoir beneath the caldera and the volume of magma removed during the eruption/intrusion. These results were published in the December 1999 issue of Geophysical Research Letters. After the data was downloaded from the extensometers in 1998, they were re-deployed in the same area, and we recovered these again in 1999. Unfortunately, all 4 instruments failed during the 1999 deployment due to connector leaks and failure of components on the circuit boards. The status of these instruments for future deployments is currently being assessed.

The high-resolution bathymetry that can be collected with the Imagenex sonar was one of the highlights of NeMO'98. We were able to double the area of Imagenex coverage in 1999, mainly to the west of the area mapped in 1998. This helped define the western contact of the 1998 lava flow, and also showed many drain-out channels in the older lava to the west, suggesting that the previous eruptions from this area were more voluminous that the 1998 eruption. The Imagenex map fills a critical observational gap between multibeam bathymetry and bottom observations, and helps us understand what we see from ROPOS. It helps us map out the 1998 lava flow in unprecedented detail, including collapse areas and flow morphologies, puts the locations of vents sites and samples in a meaningful spatial context, and helps us interpret the sequence of events during the eruption. This is the first high-resolution map of a mid-ocean ridge eruption site and we will continue to expand the Imagenex coverage in future years.

During NeMO'98, our geologic mapping efforts were concentrated in the 3 km north of Marker 113. However, we know that the 1998 eruption extended at least 7 km south of Marker 113, so during NeMO'99, we extended this mapping to the south. Observations during dive R495 showed that even the large pillow mound that erupted at 45 52' was fed from a narrow fissure. Within a few hundred meters south of the pillow mound, 1998 lava can be seen filling, but not overflowing, a fissure that is 1-1.5 m wide. This extraordinary observation is essentially the surface outcrop of the 1998 dike that fed the eruption at this location, and the width of the fissure we saw is the minimum amount of seafloor spreading that occurred during this event. Between the pillow mound at 45 52' and Marker 113, the story is more complicated because there are other very young lava flows that are difficult to distinguish from the 1998 lava, except that locally they host clearly pre-1998 vent communities (mainly large, dead tubeworms). We were able to map out the distribution of lavas of at least 4 different ages during dives R492, R493, R494, R495, and R501. However, these dives only covered limited parts of this large area, and additional mapping will be necessary in future years to complete this mapping effort. In collaboration with M. Perfit and his students at the University of Florida, a careful collection of basalt samples was made during these mapping dives to determine if there are distinctive chemical differences between the 1998 lava flow and the surrounding older lavas. These results might help in the mapping effort and have implications for the evolution Axial's magmatic system.

The geologic results from this project also aid in putting into context the remarkable changes observed in the chemical and biologic systems at Axial Volcano since the 1998 eruption. The mapping provides the geologic context for the along-strike variations in the chemical and biologic characteristics of the hydrothermal system. For example, we were able to compare the general pattern of hydrothermal flow from the new eruption with that of the pre-eruptive system (mapped by previous camera tows and dives), suggesting that the conduit system was not dramatically changed by the dike injection and eruption. The north-south pattern appears to be similar, but there was apparently a small East-West displacement between the pre- and post-eruptive hydrothermal systems. The center of maximum hydrothermal flow appears to have remained in roughly the same place. This is also apparently where the maximum diversity in the microbial population is. The "snow blower" vents occur primarily at the northern portion of the system, where the lowest flow rates are located.