MBARI Ridges 2005 Expedition
Juan de Fuca Leg: There are four components to this leg of the expedition. The first component is to survey, map, and sample several historic lava flows on the Juan de Fuca Ridge to determine if pyroclastic particles were produced during their eruption and to ascertain if benthic communities that have colonized the flows since their eruption can be used to estimate flow ages. This component will be joint dives for Dave Clague and Jim Barry, MBARI. The second is to explore, map, and sample an unusually large and deep lava pond complex on the south rift zone of Axial Seamount for Dave Clague. The third is three NURP-funded dives to recover and service sensors deployed at South Cleft and Axial Seamount for Bill Chadwick, Oregon State University. The fourth is a single dive to search for active hydrothermal vents in the Blanco Transform Fault and to collect vent biota for Bob Vrijenhoek, MBARI.
Gorda Leg: The leg has two components. The first is a single dive at the Seacliff hydrothermal site to collect vent biota for Bob Vrijenhoek, MBARI. The remainder is all geologic with the primary objective to quantitatively determine the distribution of volcanic particles produced during two eruptions. The 1996 eruption on the Northern Gorda Ridge will be sampled along the trajectory of the event plume. The NESCA site in Northern Escanaba Trough will be thoroughly sampled in a grid to determine the size and volume distribution of pyroclastic particles produced during a prehistoric eruption (about 300 year-old) in the mud-filled axial valley. The final dive will take place near the epicenter of the June 2005 magnitude 7.2 earthquake on the southeastern Gorda plate with the hope of locating surficial expression of the earthquake. The dive will also sample basalts along fault scarps produced parallel to the Escanaba Trough about 4-5 million years ago.
The monitoring instruments that will be recovered at South Cleft and Axial Seamount are two types of acoustic extensometers, that are designed to measure seafloor spreading events. The tectonic plates are gradually moving apart, but right at the plate boundary they only move during volcanic or tectonic events. We are trying to learn more about these kind of events. The instruments are deployed in a linear array across the ridge axis and measure the distance to their neighbors very precisely by acoustic ranging. The instruments at south Cleft were deployed in 2000 and the ones at Axial were put down in 2003, and have all been recording ever since. However, they need to be recovered periodically in order to download the data and service them. We will also be recovering and replacing some temperature recorders at high-temperature black smoker vents at South Cleft, and deploying a bottom pressure recorder at Axial Seamount. These are all ways of monitoring what is going within the volcanic systems at these active spreading ridge segments.
Mid-ocean ridge eruptions occur infrequently and in remote, deep-sea locations, so are difficult to sample directly. The styles of eruption and their chemical and thermal inputs are poorly understood. We have now found limu o Pele, distinctive curved and folded volcanic glass shards, at mid-ocean ridges with slow (Gorda), medium (Northern East Pacific Rise) and fast (Southern East Pacific Rise) spreading rates. They indicate that mildly explosive eruptions are common on mid-ocean ridges. In each of these locations, the eruption depths were greater than the depth at which water can flash to steam. This raises the intriguing hypothesis that the glass particles are not derived from contact of the lava flows with seawater or wet sediment as previously envisioned, and suggests that gases other than steam are driving the explosive activity. If this is the case, the particles should not be confined to the zone of sediment/lava interaction, but should be entrained into the water column by fire-fountains and the turbulence of the rising, hot eruption plume, and dispersed away from the eruptive vents by near-bottom currents, eventually settling out of the plume. Because their trajectories are dependent on event plume dynamics, they will give another method to evaluate chemical and thermal inputs to the deep ocean from mid-ocean ridge eruptions.
We plan to sieve the glass particles from sediment samples taken during transects radiating from several known eruption sites to measure the particle distributions. We will combine the results with laboratory measurements of particle settling rates to distinguish between two different models of dispersal. The site at NESCA is unique in that the eruption is young, unique in composition, and we have sampled glass shards there previously, so we know that the eruption had an explosive component. We also know that the particles were distributed over a large area, presumably by an event plume, and that we can readily quantify the size and quantity of particles deposited by using push-cores to sample the surrounding sediment-covered region. No other known flow anywhere on the global mid-ocean ridge system will allow a complete quantitative sampling since elsewhere the particles settle on lava flow surfaces where we can qualitatively determine their presence, but cannot quantify their abundance and size distribution.
A large, deep lava pond complex on the Juan de Fuca Ridge is where we can estimate the volume of lava extruded quite accurately and determine where the lava went that drained from the pond. We can estimate the pond lifespan from the thickness of collapsed roof fragments that should be jumbled in the bottom of the drained pond. This site is completely unexplored and is known only from the high-resolution bathymetry MBARI collected in 1998.