MBARI - 2011 Pacific Northwest Expedition

Although many research papers indicate that they are using high-resolution bathymetry, the meaning of high-resolution has changed dramatically in the past five years. Using the mapping AUV, MBARI has now collected one-meter bathymetry of the Endeavour Ridge, the summit and upper south rift of Axial volcano including full coverage of both 1998 lava flows, the CoAxial site of the 1993 and 1982-1991 eruptions, and the North Cleft 1986 pillow ridge and an extensive sheet flow nearby, creatively named the Young Sheet Flow since it looked young at the time the pillow ridge was explored in the late 1980s and early 1990s.

Over the past few years MBARI researchers have been combining the map data with ROV observations of young/old flow relationships to develop confidence that they can not only map flow boundaries, but identify old/young relationships from the maps. The last piece of the puzzle has been to establish some time constraints, which is done in a way analogous to constraining ages in young lava fields on land (where flows are commonly dated by collecting and radiocarbon dating charcoal from beneath flow edges). Under water, there of course is no charcoal produced and it is not possible to excavate below the flow margins, so they do the next best thing, which is to collect short push cores of the sediment on top of the flows. For the entire Juan de Fuca region MBARI researchers have found that enough sediment concentrates that it can be collected, especially among pillow basalts, for flows older than about 500 years. Foraminifera is extracted from the bottom one centimeter of the core for radiocarbon dating, which yields a minimum age of the flow beneath. This will be a major objective of the North Cleft* and CoAxial dives.

At the same time the research team is collecting cores, they will collect lava samples for geochemistry. As an example, they have found that lavas on the floor of the caldera at Axial volcano and the lavas on the rim from eruptions that flowed away from the summit prior to collapse of the caldera as well as the flows exposed in the caldera walls are chemically and mineralogically distinctive. They can now look at how the lavas change through time.

Another objective is to determine the time of formation of the caldera. The rim is buried under as much as two meters of volcaniclastic debris that the team has tried unsuccessfully to core several times. The goal is to try to build a chemical stratigraphy of the pyroclasts in the section using a series of age dates on foraminifera from the section.

At Endeavour Ridge, the Clague team's goal is similar to that at Axial: to determine when the axial valley containing the extensive hydrothermal deposits began to collapse. As at Axial, the plan is to collect cores of the sediment on top of flows on the valley rims that flowed away from the axial valley. These should provide ages that predate the collapse of the axial valley and development of the extensive faulting that provide the conduits for hydrothermal fluid flow.

At Vance Seamount C, Clague's research team will conduct one dive to further explore thick volcaniclastic units on the rim of a caldera. In this case, the volcaniclastics are lithified, but the team will attempt to collect a section through them and try to determine where they erupted and how they were transported and deposited.

One additional thing Clague's research team is studying is the colonization by benthic animals on the historic flows, and to use this as a means to assess ages of very young flows. Having collected transect data twice previously on the historic flows, they will try to add a few more transects this trip as well. The ROV is flown at a set height above the bottom for 100 meters, and the video is then quantitatively analyzed by counting every animal observed in the field of view.

*The dives at North Cleft may actually occur on Leg 1, for logistical reasons.

The 1993 CoAxial eruption, a collection of papers in Geophysical Research Letters, 22, No. 2 in 1995. These papers describe the mega-plume emitted during the eruption, the seismic swarm that accompanied the eruption, and initial results on the character of the eruption.

The 1998 Axial Seamount eruption, a collection of papers in Geophysical Research Letters, 26, No. 23 in 1999. Same sort of collection for the 1998 Axial eruption.

Chadwick, J., Perfit, M., Ridley, I., Kamenov, G., Chadwick, W.W., Embley, R.W., le Roux, P., and Smith, M., 2005, J. Geophys. Res., 110, doi:10.1029/2003JB002767. Petrology of Axial Seamount, mainly based on samples from the south caldera and south rift zone.

Chadwick, W.W., Jr., and Embley, R.W., 1994, Lava flows from a mid-1980s submarine eruption on the Cleft segment, Juan de Fuca Ridge, J. Geophys. Res. 99, 4761-4776. Describes the 1986 North Cleft eruption distribution and morphology.

Chadwick, W.W., Jr., Scheirer, D.S., Embley, R.W., and Johnson, H.P., 2001, 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, J. Geophys. Res., 106, 16,075-16,099. Describes the North Cleft and CoAxial eruption sites.

Clague, D.A., Paduan, J.B. (2009) Submarine basaltic volcanism, In: Submarine Volcanism and Mineralization: Modern through Ancient, B. Cousens and S.J. Piercey (eds.), Geological Association of Canada, Short Course 39-30 May 2008, Quebec City, Canada, p. 41-60.

Clague, D.A., Paduan, J.B., and Davis, A.S., 2009, Widespread stombolian eruptions of mid-ocean ridge basalt, J. Volcanol. Geotherm. Res., 2009, 180, 171-188. Describes the fragmental eruption products of strombolian bubble-burst activity on Pacific mid-ocean ridges, including Gorda and Juan de Fuca ridges, near-ridge seamounts, and a few back-arc basins.

Clague, D.A., Reynolds, J.R., 2000, Near-ridge seamount chains in the northeastern Pacific Ocean, J. Geophys. Res. 105, 16541-16561. Describes nested calderas on near-ridge seamounts including Vance.

Embley, R.W. Chadwick, W.W., Perfit, M.R., Smith, M.C., and Delaney, J.R., 2000, Recent eruptions on the CoAxial segment of the Juan de Fuca Ridge: Implications for mid-ocean ridge accretion processes, J. Geophys. Res., 105, 16,501-16,525. A synthesis of what was known at that time about the 1993 and an adjacent pillow ridge emplaced between surveys done in 1982 and 1991.

Embley, R.W., Murphy, K.M., and Fox, C.G., 1990, High-resolution studies of the summit of Axial volcano, J. Geophys. Res., 95, 12,785-12,812. Best overall description of the caldera and summit of Axial volcano.

Karsten, J.L., Hammond, S.R., Davis, E.E., and Currie, R.G. 1986, Detailed morphology and neotectonics of the Endeavour Segment, Juan de Fuca Ridge: New results from Seabeam swath mapping, Geol. Soc. Am. Bulletin, 97, 213-221 Describes the tectonics and morphology of the Endeavour ridge.