flows (ropy sheet, jumbled sheet, and lobate) but resolution of 

 age relationships between lava flows is more difficult. 



Within the caldera, areas of lowest amplitude return signal 

 (not including shadow zones caused by volcanic topography) are 

 usually associated with lava pond characterized by ropy sheet 

 flows. The relatively low microrelief of these lavas (<0.25 m) 

 and associated evenly distributed sediments gives rise to, 

 respectively, low backscatter and high acoustic absorption 

 relative to the other lava types. The highest signal return 

 (excluding reflections off tectonic scarps or flow fronts) is 

 associated with the jumbled sheet flows. The presence of 

 numerous sharp ridges and the absence of any continuous sediment 

 cover provides a combination of high microrelief (0.5 m - 1.0 m) 

 and low acoustic absorption. The lobate flows are associated 

 with intermediate return signal strength but tend to grade into 

 the other two types. Pillow lavas are not common in the 

 caldera, but where they do occur (predominantly in the middle of 

 the caldera) they are associated with either discrete volcanic 

 cones (northwestern caldera) or "lumpy" appearing forms. Since 

 extensive fields of pillow lavas tend to be associated with 

 larger edifices, their appearance on the sidescan is not strictly 

 comparable with the other lava types, which occur in flatter 

 areas. 



The hydrothermal vents are associated with primary 

 structural features of the caldera, i.e., the rift zones and 

 fault-bounded wall. The low-temperature vent areas are 

 characterized by a diverse vent fauna and patches of 

 silica/nontronite exhalite. The CASM vent field (north end of 

 caldera) is primarily located within a discrete rift zone, 

 although some extinct sulfide chimneys occur about 100 m to the 

 east of the rift zone. The ASHES vent field, which is probably 

 structurally controlled by fracturing associated with the base of 

 the southwestern caldera wall, consists of a 1000 m by 200 m wide 

 area of semicontinuous low temperature ( <35°C water) exhalite and 

 patchy vent fauna (Fig. 5). At the northern end of the vent 

 field there is a 100 m-diameter area characterized by high- 

 temperature venting. This was the site of comprehensive 

 interdisciplinary studies using the Pisces IV submersible in 1986 

 and Alvin in 1987. Active low-temperature venting also occurs on 

 the wall adjacent to the high- temperature area. All of the high- 

 temperature (both active and extinct) vents within the caldera 

 appear to be immature, that is, there are no basal sulfide mounds 

 present; the chimneys protrude directly from the basalt flows. 

 Water temperatures of up to 330°C were measured in clear fluids 

 exiting from chimney structures apparently made predominantly of 

 anhydrite. 



Pervasive low-temperature venting was also discovered along 

 the southeastern side of the caldera (Fig. 3). These vents were 

 initially found by towed-camera surveys in 1985 and 1986 and were 



68 



