A high‐precision, high‐resolution geologic map explicitly documents relationships between tectonic features and large steep‐sided, sulfide‐sulfate‐silica deposits in the vigorously venting Endeavour hydrothermal field near the northern end of the Juan de Fuca Ridge. Water depth in the vent field varies from 2220 to 2200 m. Location of the most massive sulfide structures appears to be controlled by intersections of ridge‐parallel normal faults and other fracture‐fissure sets that trend oblique to, and perpendicular to the overall structural fabric of the axial valley. The fractured basaltic substrate is primarily composed of well‐weathered pillow and lobate flows. As presently mapped, the field is about 200 by 400 m on a side and contains at least 15 large (> 1000 m3) sulfide edifices and many tens of smaller, commonly inactive, sulfide structures. The larger sulfide structures are also the most vigorously venting features in the field; they are commonly more than 30 m in diameter and up to 20 m in height. Actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller. Maximum venting temperatures of 375°C are associated with the smaller structures in the southeastern portion of the field; highest‐temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20°–30°C lower. Hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture‐controlled black smoker activity near the top of the structures. A different type of high temperature venting takes place from the upper sides of the structures in the form of “overflow” from fully exposed, quiescent pools of buoyant 350°C vent water trapped beneath overhanging sulfide‐sulfate‐silica ledges, or flanges. These flanges are attached to the upper, outer walls of the large sulfide edifices. Two types of diffuse venting in the Endeavour field include a lower temperature 8°–15°C output through colonies of large tubeworms and 25°–50°C vent fluid that seems to percolate through the tops of overhanging flanges. The large size and steep‐walled nature of the these structures evidently results from sustained venting in a “mature” hydrothermal system, coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development.

Submersible observations and side‐scan sonar imagery delineate ten large venting sulfide‐sulfate‐silica structures located on an uplifted block of the axial valley floor on the central portion of the Endeavour Segment, Juan de Fuca Ridge. This newly discovered vent field lies 1.8 km north of the main Endeavour vent field. The axial valley, a horst, and faults controlling sulfide deposition trend parallel to the ridge axis. The horst rises 10 to 15 m above the valley floor and offsets a lava lake which extended 500 m in width across the valley. The largest sulfide structures are perched along the major horst‐bounding faults. One structure is the tallest single edifice (45 m) reported in the Pacific Ocean. Smaller, less‐active and inactive sulfide edifices are associated with faulted remnants of the lava lake surface, fissured pressure ridges, and at the edge of collapse pits. Based on the relationship between faults and sulfide structures, the morphology of sulfide edifices, and their venting characteristics, we infer that sulfide structures grow vertically by chimney accumulation and laterally by flange development. Late stage deposition of amorphous silica toughens the large deposits and may be responsible for the unusual size and aspect ratio of Endeavour edifices.

Geophysical and environmental seafloor surveys make an extensive use of sidescan sonar imagery. Most of it is still interpreted visually and qualitatively. We are presenting here a method of textural analysis, with supplements the interpreter with reliable quantitative results. The TexAn technique has been extensively ground-truthed in complex midocean ridge terrains, using submersible and ROV o b s e r v a t i o n s , and in-situ s a m p l i n g . The frequencies used so far range from 6.5 kHz to 500kHz; all applications have been validated and ground-checked. Current applications cover very different environments, from mid-ocean ridges to continental margins and coastal waters. TexAn enables quantitative assesments of sidescan sonar imagery, at all stages of processing and in all conditions. TexAn also reveals details hitherto invisible to the human eye, however experienced.

Recent scientific developments place inquiries about submarine volcanic systems in a broad planetary context. Among these is the discovery that submarine eruptions are intimately linked with massive effusions of microbes and their products from below the sea floor [Holden et al., 1998]. This material includes microbes that only grow at temperatures tens of degrees higher than the temperatures of the vent fluids from which they were sampled. Such results lend support for the existence of a potentially extensive, but currently unexplored sub‐sea floor microbial biosphere associated with active submarine volcanoes [Deming and Baross, 1993; Delaney et al., 1998; Summit and Baross, 1998].

Abshoet-'The volume of data collected by slde-scan sonar during seafloor surveys has become larger and larger as the resolution of th-systems has Improved. As a result, new Image p " i n g techniques need to be developed to partly automate the interpretation d thls increasing wealth of data. "he 6mt two steps In the geological analysis of a new image usually are t h mapplng of W a r structures, and d morphologic units. These maps are then used In tectonk a d geologlcpl Interpretations.