MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 16, 2005
Tiburon dive 882, CoAxial Lava Flows, Juan de Fuca Ridge.
Jenny Paduan writes:
The dive today traversed the 1993 CoAxial flow, which Bob writes about discovering below, another flow that erupted sometime between 1982 and 1991, and at least one additional older flow whose age is unknown. The purposes were several-fold:
Watching the seafloor unfold today on the plasma screens in Western Flyer’s control room as Tiburon drove across the seafloor 2400 meters below was like stepping back over a decade to one of the most exciting periods in my career as a marine geologist.
It was almost too good to be true, but sometimes you get lucky. In June 1993, our group at the NOAA Pacific Environmental Laboratory had only a week earlier begun to receive real-time data from the U.S. Navy’s hydrophone arrays located on the floor of the Northeast Pacific when a swarm of earthquakes from an eruption on the northern Juan de Fuca Ridge appeared on the “sonograms”. The extraordinary swarm of earthquakes detected by SoSUS in 1993 was located on the north side of Axial Volcano in a poorly known area of the ridge that became known as the CoAxial segment.
We had realized several years earlier that the high sensitivity afforded by the hydrophones was critical for detecting volcanic eruptions on the seafloor spreading ridges lying in the NE Pacific. In 1986, we had serendipitously discovered the site of a seafloor eruption when a large plume of warm, chemically laden water was discovered above the southern part of the Juan de Fuca Ridge (which became known as the “Megaplume”). However, there were no earthquakes large enough to be detected on seismometers on land, so we knew that a more sensitive detection system was needed to detect these events. Hydrophones placed in the upper sound channel of the oceans could detect signals about 100 times lower in amplitude than seismometers and the Navy’s array of such hydrophones (called SoSUS) were optimally placed for detecting and locating volcanic earthquakes on the floor of the NE Pacific.
We had begun a cooperative program the previous year with Canadian colleagues to develop the ROPOS remotely operated vehicle (this is a deeper version of MBARI's Ventana) for use on the offshore ridges and fortunately had an expedition already scheduled for the second week of July. Meanwhile, a Canadian research vessel working nearby was able to divert to the area and discovered a large hydrothermal plume similar in character to the megaplume.
The first dive on the site was one of those rare discovery experiences that are coveted by scientists. As the ROPOS traversed the older lavas east of the earthquake epicenters, the anticipation was almost electric. Suddenly the video monitor began showing much darker,almost unsedimented pillow lava. We thought at first that this was “the” eruption, but were surprised not to see some evidence of hydrothermal venting associated with the cooling lava. We continued heading west and passed over the contact of the young lava back into the older more sedimented lava. After traversing some 100’s of meters of the older lava we crossed a sharp contact into pristine, glassy black lavas.
After climbing up the side of the mound of pillow lavas the seafloor turned into a wonderland of vivid yellow microbial filaments bathed in warm, shimmering water emerging from numerous hollows between the pillow lavas. This was the site of the eruption we had first heard on the hydrophones two weeks earlier—it was breathtaking to realize that we now had the capability to visit the site of recently “birthed” seafloor. Over the next five years we were able to repeat this experience two more times, once on the Gorda Ridge in 1996 (to be revisited on the next leg of Western Flyer) and again on Axial Volcano in 1998 (revisited on the first dive of this expedition).
One puzzle took a little longer to solve—the first “young” lava flow we crossed was later found to have erupted between 1981 and 1991 using a technique that compared bathymetric surveys from 1982 and 1991—old enough to have cooled off but still with a very young geologic appearance.
Today the 1993 lava has “aged” a bit, with small amounts of sediment collected on the pillow surfaces. The vivid yellow microbial mats died off as the lava cooled within about a year of the eruption—today the former venting sites are still visible as a dull yellow surficial coating. Some life has returned to the lava surface and detailed analyses of the video data by MBARI scientists will reveal to what extent the colonization of this surface is taking place. This study is unique because there are only a few places on the deep seafloor whose birthday we know.
A brachyuan crab and two crinoids were spotted on this pillow at the edge of a large fracture. Note the heavy sediments that fill the cracks between pillows, a sign that this flow is older than the others we traversed today. Except where the sediments are derived locally such as hydrothermally active areas, or near land where turbidity flows can dump large volumes of material here and there, the rate of the gentle rain of pelagic detritus is relatively constant in any given region and can be used to estimate relative ages of surfaces.
From Jenny again: By the way, data was downloaded successfully from the high-temperature recorder that flooded during recovery from the hydrothermal vent at South Cleft! (See the August 13 update.)
Linda and Brian have been at sea a little too long now and are seeing things. They found this elephant (a new species to be sure!) among the pillow lavas we traversed today. Eat your hearts out, vent kitty fans!