temperature venting has also been observed at the CASM, east and 

 south vent fields (Fig. 1; Canadian American Seamount Expedition, 

 1985; ASHES Expedition, 1986; and NOAA Alvin Dive Program, 1987, 

 unpublished data). Sulfide deposits, which are evidence of past 

 high- temperature discharge, have been recovered from the CASM and 

 east vent fields. Water column anomalies indicative of high- 

 temperature venting were observed by Massoth et al. (1982) at a 

 depth of about 1600 near the north flank of Axial Volcano in 1981, 

 but were missing during repeat visits to that region in 1983 and 

 1986 (Massoth, unpublished data). The thermal and particulate 

 anomalies normally associated with Juan de Fuca ridgecrest 

 hydrothermal plumes are greatly attenuated within the water column 

 overlying the caldera of Axial Volcano (Baker and Massoth, 

 unpublished data). The anomalies for dissolved Mn and ^He, 

 however, are moderately and greatly elevated, respectively 

 (Massoth and Lupton, unpublished data). 



The only high-temperature venting that has been observed at 

 Axial Volcano occurs within a 60 m diameter area located near the 

 northern end of the ASHES vent field (Fig. 1). The flat and 

 shallow (1542 + 1 m) caldera floor in this region consists of 

 lobate and broken sheet flows that are nearly devoid of sediment 

 cover. Sitting directly on these basalts, i.e., without a 

 supporting basement and surrounding blanket of metalliferous 

 sediment, are four sulfide structures, each of which have multiple 

 chimneys discharging high- temperature fluids. The largest of 

 these structures are the Inferno and Hell vents which are about 

 5 m and 4 m high, respectively, and several meters in diameter. 

 Buoyant plumes of brown and black particulate smoke form above 

 cm-scale orifices located at the top and sides of these vents. 

 Clear, high-temperature fluids flow vigorously through vent 

 orifices located near the base of Inferno vent. These fluids 

 appear to flow laterally after about one meter of vertical rise 

 and form a white particulate smoke beginning a meter or so 

 downstream. The Mushroom and Hi 1 loch vents are sulfide mounds 

 of 1-2 m height and width. High- temperature fluids which form an 

 opaque smoke rise from spires imbedded within a dense covering of 

 tube worms on Mushroom vent. Black smoke was observed to rise 

 from a vent on the side of Hilloch. Two high-temperature vents 

 within the ASHES field are not visibly associated with sulfide 

 structures. Virgin Mound vent is a white, spire-like edifice 

 0.5-0.8 m high composed almost entirely of anhydrite. Clear 

 fluids gush vigorously through its several cm wide orifice and 

 ascend beyond view in a smokeless plume. A cms- thick, 2 m- 

 diameter apron of anhydrite which surrounds the vent is apparently 

 the result of frequent structural failures of the chimney and 

 reflects an equilibrium between venting-induced chimney formation 

 and anhydrite dissolution (Feely et al., 1987). Near Virgin 

 Mound, high temperature fluids issue from a pair of merging cracks 

 approximately 5-10 cm wide and about 10-15 m in overall length 

 which are collectively known as Crack vent. Chunks of anhydrite 

 overflow from these cracks, which, like Virgin Mound, give rise to 



36 



