THE CALDERA OF AXIAL VOLCANO—REMOTE SENSING AND SUBMERSIBLE 

 STUDIES OF A HYDROTHERMALLY ACTIVE SUBMARINE VOLCANO 



R. W. Embley, S. R. Hammond and K. Murphy 



National Oceanic and Atmospheric Administration 



Pacific Marine Environmental Laboratory 



Hatfield Marine Science Center 



Newport, OR 97365 



INTRODUCTION 



The establishment of the NOAA VENTS program in 1984 provided 

 a focus for several NOAA research groups that had been working on 

 aspects of ocean hydrothermal systems. The evolution of the 

 program during the past few years has led to a focus on the vent 

 systems of the northeast Pacific spreading centers and their 

 effect on the chemistry of the northeast Pacific Ocean. Using 

 the existing data base (as of 1984) consisting of Sea Beam 

 bathymetry, reconnaissance side-scan sonar, bottom photography, 

 water column studies and Alvin dives, a program of systematic 

 geologic and geochemical mapping of vent sites was begun in 1985. 

 The ultimate goal is to understand the hydrothermal "source 

 function" of the ridge system in terms of geographic and temporal 

 variability and to relate this function to the near-, mid-, and 

 far-field water column hydrothermal effects. 



The first site chosen for this systematic approach was Axial 

 Volcano (Fig. 1). This was selected as the initial site because 

 there was a need for a shallow, relatively small vent area to use 

 as a seafloor laboratory to test hypotheses concerning: (1) 

 chemical and physical variability, (2) particulate fallout 

 models, and (3) near-field plume dynamics. The shallow depth of 

 the vent field (1545 m) is within range of both Pisces class 

 submersibles and Alvin . Its small size (suspected on the basis 

 of reconnaissance Alvin dives in 1984) ensured a manageable 

 problem in terms of seafloor instrumentation, flux calculations, 

 etc. 



Axial volcano is the youngest volcanic edifice in the Cobb- 

 Eikelberg seamount chain which extends from the central segment 

 of the Juan de Fuca Ridge several hundred kilometers into the 

 Pacific Plate (Fig. 1). The volcano and its caldera were 

 initially mapped with the Sea Beam sonar during 1981 as part of 

 NOAA's effort to discover and map active hydrothermal venting 

 sites along the entire Juan de Fuca Ridge system (Malahoff, 1985, 

 Crane et al . , 1985). The Sea Beam bathymetry of the central Juan 

 de Fuca Ridge shows that the Axial Volcano Edifice (volcano plus 

 rift zones or AVE) is a morphologically complex structure that is 

 superimposed on the generally linear 020°N seaf loor-spreading 

 fabric of the Juan de Fuca Ridge (Fig. 2). The volcano's major 

 features include: (1) the central shoal area (1450 m minimum 



61 



