High‐resolution bathymetric surveys using scanning sonars: Lava flow morphology, hydrothermal vents, and geologic structure at recent eruption sites on the Juan de Fuca Ridge

The CoAxial and Cleft segments of the Juan de Fuca Ridge have isolated, chronic, high‐temperature, and focused hydrothermal vent sites. Both segments also have experienced recent volcanic eruptions which produced extensive, ephemeral, low‐temperature, and diffuse hydrothermal venting. To study the geologic setting of these sites, high‐resolution bathymetric surveys at eight locations on the CoAxial and Cleft segments were collected between 1993 and 1999. Two 675‐kHz scanning sonar systems were used, Mesotech on the submersible Alvin and Imagenex on the remotely operated vehicle Jason . The bathymetry from these surveys can be gridded at a scale of 2–4 m and contoured at 1 m and thus can resolve many fine‐scale features on the seafloor that are indistinguishable in multibeam bathymetry collected at the sea surface. Bathymetric data at this resolution are particularly useful for identifying geologic features related to diking, faulting, and lava flow emplacement. For example, the high‐resolution bathymetric maps show that submarine fissure eruptions that form pillow lavas last long enough to become localized and to produce point source constructs along their length, and their extrusion rate is low enough that no significant drainback occurs. In contrast, lobate sheet flows are formed by short‐lived, high‐effusion rate eruptions in which no localization of output occurs along the eruptive fissure, and inflation is quickly followed by drainback, resulting in extensive collapse features. However, if the process of submarine lava flow inflation occurs at a slower rate and over a longer period of time, it can create lava rises up to 25 m high with distinctive structure and morphology. The scanning sonar data also show that fissures and grabens have formed or reactivated where dikes approach the surface adjacent to recent eruptive sites. The fine‐scale bathymetry establishes that all the hydrothermal vent sites studied at the CoAxial and Cleft segments are located along prominent volcanic or tectonic extensional structures which provide the physical pathway for fluids from the subsurface to the seafloor. Furthermore, the fine‐scale morphology of recent lava flows can be used as a qualitative indication of eruption duration.