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, chr onic, high-temperature, and focused hydrothermal vent sites. Both segments also have experienced recent volcanic eruptions which produced extensive, e phemeral, low-temperature, and diffuse hydrothermal venting. To study the g eologic setting of these sites, high-resolution bathymetric surveys at eigh t locations on the CoAxial and Cleft segments were collected between 1993 a nd 1999. Two 675-kHz scanning sonar systems were used, Mesotech on the subm ersible Alvin and Imagenex on the remotely operated vehicle Jason. The bath ymetry from these surveys can be gridded at a scale of 2-4 rn and contoured at I 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 identifyin g geologic features related to diking, faulting, and lava flow emplacement. For example, the high-resolution bathymetric maps show that submarine fiss ure eruptions that form pillow lavas last long enough to become localized a nd to produce point source constructs along their length, and their extrusi on rate is low enough that no significant drainback occurs. In contrast, lo bate sheet flows are formed by short-lived, high-effusion rate eruptions in which no localization of output occurs along the eruptive fissure, and inf lation is quickly followed by drainback, resulting in extensive collapse fe atures. 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 in high with distinctive structure and morphology. The scanning sona r data also show that fissures and grabens have formed or reactivated where dikes approach the surface adjacent to recent eruptive sites. The fine-sca le bathymetry establishes that all the hydrothermal vent sites studied at t he CoAxial and Cleft segments are located along prominent volcanic or tecto nic extensional structures which provide the physical pathway for fluids fr om the subsurface to the seafloor. Furthermore, the fine-scale morphology o f recent lava flows can be used as a qualitative indication of eruption dur ation.