depth), (2) two long, linear highs extending northeast and south 

 from the summit area, (3) several smaller ridges extending along 

 similar azimuths and (4) several closed and open basins indenting 

 the edifice on the north and south sides. The total relief of 

 the volcanic edifice is about 700 meters relative to the mean 

 depth of the seafloor spreading axis to the north and south. 



The summit of Axial Volcano is marked by an unusual 

 rectangular shaped 3 km x 8 km caldera that lies in an apparent 

 offset region between the two rift zones (Figs. 2 and 3). The 

 caldera and the proximal portions of the north and south rift 

 zones have been extensively surveyed during NOAA cruises in 1985- 

 86 by Sea Marc I sidescan (5 km, 2 km and 1 km swath widths) and 

 deep-sea cameras navigated within a long-term transponder net. 

 The primary focus of the surveys was to determine the geologic 

 setting of the hydrothermal vent fields within the caldera. The 

 first submersible expedition to the volcano in 1983 discovered 

 warm water vents with associated vent-specific animal communities 

 in the northern end of the caldera (CASM, 1985). In 1984, as 

 part of NOAA's Alvin dive program to investigate suspected 

 active hydrothermal sites, four dives were made in the caldera 

 along the southern wall. During one of these dives (1411), an 

 active high-temperature vent was briefly observed and sampled. 

 These vent fields have been a primary focal point of submersible 

 studies undertaken by the NOAA VENTS program with support from 

 the Office of Undersea Research in 1986 and 1987. 



METHODS 



In anticipation of more submersible dives in 1986 and 

 beyond, the 1985 field season was devoted to beginning a 

 comprehensive geological survey of the caldera and vicinity. In 

 order to facilitate the mapping effort and to ensure accurate 

 intercalibration of the various data sets, a long-baseline 

 transponder net was established and has been maintained to the 

 present. The two primary data sets obtained during the 1985 

 field season were digital side-scan sonar and bottom photography. 

 This combination of precision navigation, side-scan sonar and 

 bottom photography has enabled us to begin to construct an 

 accurate representation of the caldera and rift zone geology. 



A Sea Marc IA, 30 kHz digital side-scan sonar was used in 

 1985, 1986 and 1987 to collect data from the caldera and vicinity 

 as well as the south rift zone and southern Juan de Fuca Ridge. 

 This system has an adjustable scan width ranging from 5 km to 

 0.5 km. The resolution of the digital data is inversely 

 proportional to the scan width. For example, the pixel size of a 

 5 km and a 0.5 km scan is, respectively, 2.5 m and 0.25 m. The 

 higher resolution scans were used in the vicinity of the vent 

 fields. The side-scan sonar system has a built-in processor that 

 corrects the data for both slant range distortion and speed. 

 Post-processing software assigns each pixel an x-y value relative 



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