Ridge Crest
Studies of mid-ocean ridges are motivated by three overarching goals of understanding:
(1) the geological processes that form and age the oceanic crust;
(2) the role of mid-ocean ridge volcanoes in fostering diverse and productive biological communities above and below the seafloor; and
(3) the exchange of heat, chemicals, and biomass between the oceanic crust and overlying ocean. Investigations to address these goals are highly multidisciplinary because they seek to unravel the complex linkages among myriad of physical, chemical and biological processes. The also require coordinated long-term high-resolution observations because the systems are very dynamic and many processes of most interest occur during short-lived volcanic and tectonic events.
Cabled observatories will play a critical role in ridge studies. Arrays of seismometers and hydrophones will record the seismicity associated with volcanic events and large extensional earthquakes, and will monitor the ongoing seismicity that creates and maintains the crack networks necessary for vigorous hydrothermal circulation. Geodetic experiments will measure the character and timing of strain that accompanies spreading events and the deformation that occurs in such events as the result of processes such as magma recharge and faults creep.
Extensive networks of sensors, samplers and cameras will be deployed in hydrothermal vent fields to characterize changes in the chemical and microbiological composition of hydrothermal fluids and observe the evolution of the biological communities that inhabit vent ecosystems. Instrumentation will also be deployed on moorings in the plumes of hydrothermal fluids that rise above the vent fields to monitor the fluxes of heat and chemicals and their influence on the physical, chemical, and biological structure of the overlying ocean. Autonomous underwater vehicles will be kept on site to conduct routine surveys of the water column and the seafloor and to respond to volcanic eruptions, tectonic events, and other perturbations.
Endeavour Segment
Ridge studies will initially focus on two sites. The Endeavour Segment is located toward the northern end of the Juan de Fuca Ridge and will be a node on the NEPTUNE Canada cabled observatory. From a hydrothermal perspective is one of the best-studied ridge sites in the world. It is one of three initial sites selected for integrated studies by the RIDGE2000 program and is the location of an experiment funded by the Keck Foundation.
The central third of the segment hosts at least five large black smoker hydrothermal vent fields that are driven by the heat from a crustal magma chamber located about 2 km below the seafloor. Each high-temperature field extends over several hundred meters along the axis and hosts multiple sulfide structures that are large in comparison to those found on most other intermediate and fast-spreading ridges. Fluid chemistry shows well-defined gradients in salinity, carbon and temperature from north to south and sharp, well-defined geologically controlled gradients in temperature and fluid composition within the Main Endeavour Field. In contrast to segments to the south, the Endeavour segment is seismically very active and many of the earthquakes appear to be associated with hydrothermal circulation.
Axial Volcano
Axial Volcano is a large volcanically active seamount with a summit caldera located near the center of the Juan de Fuca Ridge and will be a node on the US OOI observatory. Since 1996, it has been the site of NOAA’s New Millenium Observatory, the first multidisciplinary observatory established on a mid-ocean ridge. The rim of the caldera hosts multiple hydrothermal fields that are characterized by active boiling and magmatic degassing. The caldera is underlain by an extensive magma chamber that erupted in 1998. Continued monitoring has taken place since the eruption, documenting geodetic, chemical, and micro- and macrobiological changes. Based on the inflation rate and total re-inflation since the eruption, there could be another eruption in 5-10 years.
Cleft and Middle Valley
In addition to the Endeavour Segment and Axial Seamount, at least two more ridge sites will likely be developed over the 30-year lifetime of the OOI cabled observatory. The Cleft segment at the southern end of the Juan de Fuca is a ~50-km-long segment with a fairly simple linear structure comprising a 1- to 2-km-wide axial valley that is underlain by an axial magma chamber along most of its length. The presence of the Blanco transform fault just to the south of the Cleft segment provides the opportunity to document perturbations to the ridge-crest system from nearby large-magnitude earthquakes. Middle Valley is a sediment-filled rift that, until recently, was the primary axis of spreading at the northern end of the Juan de Fuca Ridge. The rift valley is filled with turbidite sediment and hosts both active and inactive hydrothermal mounds. In 1991, two Ocean Drilling Program boreholes within the Dead Dog field were instrumented with CORKs (circulation obviation retrofit kits), which are designed to seal the borehole monitor down-hole temperatures, pressures, and allow sampling of fluids. The Middle Valley site is an ideal location to develop down-hole experiments to study the subseafloor biosphere and the response of ridge hydrothermal systems to anthropogenic perturbations.
