to the transponder grid. In addition, various image enhancement 

 schemes are presently being applied to the data at a special 

 image processing facility established at the Newport lab. 



The side-scan sonar provides an image of seafloor texture 

 derived form acoustic backscatter. However, the relationship of 

 the backscatter to the seafloor geology can only be determined by 

 direct observation, either from towed cameras or from 

 submersibles. During the 1985 and subsequent field seasons, more 

 than 27,000 35 mm photographs of the seafloor were taken in and 

 near the caldera. The photographic coverage in the vicinity of 

 the ASHES vent field is shown (Fig. 4). To integrate and 

 manipulate the photographic data, a data base scheme was 

 developed that allows key parameters (such as lava type, 

 hydrothermal indicators etc.) to be plotted along the track. Two 

 sets of data can be plotted along track at its actual scaled size 

 (see Fox et al., 1988 for details). Superimposition of these 

 data on the sidescan image allows identification of geological 

 associations with the major backscatter patterns. 



In 1986 and 1987, 28 submersible dives were made in Axial 

 Caldera. Of these 28 dives, 24 were located in and around the 

 southwest vent field, primarily in the ASHES high temperature 

 field, 1 dive revisited the CASM area, and 3 dives mapped and 

 sampled the south rift vent fields. Some of the methods and 

 initial results from the water chemistry program (the major 

 emphasis of the dives) are reported in Massoth et al . (this 

 volume). Other major experiments and data collected on these 

 dives include: (1) a time lapse camera deployed for 28 days in 

 1986 (Johnson and Tunnicliffe, 1986) and for a year beginning in 

 September, 1987, (2) sulfide samples, (3) basalt collections, (4) 

 biological collection, (5) photographic and video surveys, and 

 (6) near-bottom heat-flow surveys. 



RESULTS 



The caldera is defined on three sides by a steep wall with 

 up to 150 m relief. The caldera wall generally marks a sharp 

 backscatter boundary on the side-scan records; a low amplitude 

 return characterizes the more heavily sedimented area outside the 

 caldera. Sidescan, camera, and submersible observation show that 

 the wall is a fault structure with extensive outcrop of truncated 

 lava flows. Talus piles have developed in some places along the 

 base of the wall, whereas in other places lavas directly abut the 

 wall. These relationships reflect an interplay between mass 

 wasting and faulting along the wall as well as volcanic episodes 

 within the caldera. The interior of the caldera (Fig. 3) is 

 covered by lava flows ranging from young, glassy and/or thinly 

 sedimented jumbled sheet and lobate flows, which characterize the 

 central high of the caldera. The side-scan sonar records resolve 

 differences in backscatter between at least three types of lava 



66 



