High-temperature “black smoker” hydrothermal plumes occur when seawater that has penetrated into the oceanic crust and assimilated heat from magma is discharged from vents located at the axis of a mid-ocean ridge. The acidic, metal-rich, discharge mixes with alkaline, oxidizing seawater, and a fine suspension of sulfide particles is precipitated and convected by the flow. Vents fields have now been found at both fast and slow seafloor spreading centers and may be a ubiquitous feature of mid-ocean ridges. A review of the progress made in using underwater acoustics to study black smoker plumes is presented. Both active and passive techniques are being investigated. Active techniques involve a high-frequency monostatic sonar mounted on a submersible. Analysis of the amplitude and phase of the signal backscattered from the plume provides information about the three-dimensional shape of the plume as well as estimates of the flow-velocity field of the discharging fluid. Passive techniques use bottom-mounted hydrophones to listen to the very low-frequency, hydrodynamic noise generated by a plume. These noise signatures have potential use in locating, characterizing, and monitoring plume sites and in determining the contribution plume noise makes to the overall ambient noise field in the ocean.
A mechanically scanned sonar operating at 330 kHz has been used to image hydrothermal flows on ocean ridges using two different techniques. Scattering from particulates is used to image smoker plumes [Rona et al., Geophys. Res. Lett. 18, 2233–2236 (1991)], and scintillation of seafloor backscatter is used to image diffuse flows. Results will be presented from cruises on the East Pacific Rise and the northern Cleft segment of the Juan de Fuca Ridge. Plume images have been analyzed to extract physical parameters relevant to plume theory. Observations of diffuse flow employ cross correlation of ping doublets. Scattering theory is used to relate the correlation levels to the variance of temperature fluctuations. [Work supported by the NOAA National Undersea Research Program through the West Coast Undersea Research Center.]
Research Article| August 01, 1980 Structural behavior of fracture zones symmetric and asymmetric about a spreading axis: Mid-Atlantic Ridge (latitude 23°N to 27°N) PETER A. RONA; PETER A. RONA 1National Oceanic and Atmospheric Administration, Atlantic Oceanographic and Meteorological Laboratories, 15 Rickenbacker Causeway, Miami, Florida 33149 Search for other works by this author on: GSW Google Scholar DALE F. GRAY DALE F. GRAY 2Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1980) 91 (8): 485–494. https://doi.org/10.1130/0016-7606(1980)91<485:SBOFZS>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation PETER A. RONA, DALE F. GRAY; Structural behavior of fracture zones symmetric and asymmetric about a spreading axis: Mid-Atlantic Ridge (latitude 23°N to 27°N). GSA Bulletin 1980;; 91 (8): 485–494. doi: https://doi.org/10.1130/0016-7606(1980)91<485:SBOFZS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Two classes of fracture zones are distinguished on the basis of their orientations with respect to a spreading axis and length of associated ridge-ridge transform offset. (1) Minor fracture zones associated with transform faults of short offset (<30 km; minitransforms); the minor fracture zones may exhibit an asymmetric V-shaped configuration with respect to a spreading axis at variance with small circles about poles of plate rotation. (2) Major fracture zones associated with transform faults of long offset (>50 km); the major fracture zones exhibit a symmetric configuration with respect to a spreading axis following small circles about poles of plate rotation. Recognition of the coexistence of the two classes of fracture zones with different orientations on a slow-spreading oceanic ridge raises questions regarding the significance of the different orientations and nature of the intervening structural transition.A systematic narrow-beam bathymetric and magnetic investigation was performed to answer these questions in an area encompassing the change from minor fracture zones asymmetric with respect to the axis of the rift valley of the Mid-Atlantic Ridge at lat 26°N between major fracture zones, and a major fracture zone, the Kane, symmetric with respect to the axis of the rift valley at lat 24°N. The investigation delineated an intervening transitional region where structural features have continuously undergone geometric adjustments that have accommodated the discrepancy in orientation between the two classes of fracture zones for at least the past 6 m.y.A hypothesis of differential structural stability determined by thickness of lithosphere within transform offsets is advanced to explain the observed differences in behavior of the two classes of fracture zones. The orientation of a major fracture zone is constrained to follow small circles along a trajectory of relative plate motion by a long section of thick lithosphere in the associated transform fault. The orientation of a minor fracture zone is susceptible to reorientation in the short section of thin lithosphere in the associated transform fault; the reorientation reflects response to intra-plate and interplate stresses. The geometric adjustments that occur as a consequence of differential structural stability continuously accommodate any discrepancy in orientation that may develop between coexistent major and minor fracture zones, so that an ocean basin such as the Atlantic can open symmetrically. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
The Cabled Observatory Vent Imaging Sonar (COVIS) was deployed at the Main Endeavour Field node of the Canadian NEPTUNE cabled observatory in September 2010 and has acquired long time series on plume and diffuse hydrothermal flows. This talk will focus on recent efforts by the Rutgers-APL collaboration to invert sonar data to determine heat flux from the Grotto plume complex. Inversion employs plume theory to relate velocity as determined by Doppler shift to buoyancy flux, hence heat flux. The primary uncertainties have to do with plume bending due to ambient current and short sampling times relative to dynamic changes in plume shape. These uncertainties have been quantified by means of special high-statistics experiments using COVIS. Time series for heat flux will be compared with ground truth obtained by thermometry using an ROV. [Work supported by NSF Grants OCE-0824612 and OCE-0825088.]
