interpretation show the caldera to have well-defined east, west, 

 and north walls that from SeaBeam bathymetry appear to be about 

 100 m high in the northeastern corner of the caldera and about 

 30 m high along the eastern wall. The northern end of the 

 caldera is marked by two fissures. The CASM vent site is 

 associated with the eastern one, which is the major fissure of 

 the northern rift of the volcano. The southern end of the 

 caldera has a poorly defined margin, presumably because the 

 caldera floor was tilted down toward the southeast during the 

 phase of magma withdrawl that developed the current morphology of 

 the caldera. 



Although it straddles the Juan de Fuca Ridge, Axial Volcano 

 is a volcano with a hot-spot source (Hammond and Delaney, 1985). 

 The morphology and structural setting for the hydrothermal 

 activity observed in the summit caldera of Axial Volcano have 

 striking similarities to the geothermal systems active in 

 Hawaiian calderas that were formed by the Hawaiian Hotspot, such 

 as Mauna Loa and Kilauea. Geothermal activity in the Hawaiian 

 calderas is associated with normal faults and talus cones along 

 the margin of the caldera floor, much the same as what has been 

 observed and mapped for Axial Volcano. 



Photographs taken by a tethered camera system using the R/V 

 Discoverer in 1983 show the floor of the caldera to be covered 

 largely with sheet and lobate flows. Fresh pillow mounds in 

 lobate flow terrain were observed in photographs collected during 

 the detailed photo survey of Ashes Vent Field conducted in 1984. 

 The base of the caldera wall is covered with talus and marked by 

 a shallow depression 1 to 2 m deep. The high-temperature vents 

 are located within a small hydrothermal field on top of lobate 

 lava flows intermixed with broken sheet flows and pillows. 



SULFIDE ZONATION 



A 20-cm-long section of the top of one of the high 

 temperature chimneys, venting both black and white "smoke" at a 

 maximum measured temperature of 270°C, was sampled with Alvin for 

 analysis. This sample was cut into seven horizontal sections 

 through the chimney (Fig. 4). Results of ^°°U-series gamma-ray 

 spectrometry on Slab #1 yield an estimated rate for the 

 horizontal (i.e., away from the vent orifice) growth of the 

 chimney of 3.9 mm per year (Kim and McMurtry, in review). Using 

 this growth rate, the 55 mm radius of the chimney corresponds to 

 approximately 14 years of growth. 



Slab #1 of the chimney was selected for mineralogical and 

 chemical analyses. Optical and electron microscopy of the 

 section revealed seven concentric mineralogical bands that were 

 then subsampled for chemical analysis (Fig. 4). A thorough 

 interpretation of these results is in preparation. Semi- 

 quantitative x-ray diffraction shows that pyrite and sphalerite 



240 



