MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 13, 2005
Tiburon dive 879, North Cleft Flows, Juan de Fuca Ridge.
The seas have picked up. There is a hurricane that originated in the Gulf of California and headed west, and is now somewhere south of us in the Central Pacific. Swells generated by it are traveling at us from the south against the usual swells from the north. The seas are confused, making a rougher ride for us. The conditions are still well within the operating range for the Tiburon and are not expected to get worse, so we anticipate good diving for the next several days at least.
In contrast to the surrounding older lavas, the 1986 lava
flow is darker and has much less sediment cover. A close up view shows that it still has its
glassy outer surface, making it shiny, although not as shiny as right
after the eruption, when it was observed from camera tows and an Alvin dive.
Bill Chadwick writes:
Today we made a dive at the northern end of the Cleft segment at a site
that we know erupted in the mid-1980’s. In fact, this was the first
site in the world where we were able to document a historical eruption
on the mid-ocean ridge system. Before then lots of young-looking lavas
had been found, but we had no way of telling how young they were. The
age of the lava flows at north Cleft could be determined because they
are so thick (pillow lava ridge up to 45 m high!) that they show up as
areas of significant depth change between bathymetric surveys before and
after the eruption. Camera tows showed that young glassy lavas were
located exactly where the depth changes were found. In addition, a
giant plume of warm water, called a “megaplume”, was found by chance
over this site in 1986, and was probably produced by the eruption.
A stalked crinoid has already colonized the 1986 lava surface (laser dots are 30 cm apart for scale).
Our dive today showed that the answer is YES! Colonization of the north Cleft lava flow by sessile species is already happening. We found sparse stalked crinoids up to 30 cm long and small spherical sponges growing on the 1986 flow. In a few days, we will visit another historical lava flow that erupted in 1993 on the CoAxial segment, and this may help us to further refine the pattern and rate of biological colonization on young lava flows.
Montage of animals seen today (see Linda's writeup).
Linda Kuhnz writes:
While traversing both older flows and newer lavas that covered the seafloor in the mid-1980’s, we saw a variety of animals living on the bottom. We find long-legged crabs on nearly every dive. There were also some large red shrimp bouncing along the lava pillows and beautiful frilly worms swimming from rock to rock. A new sight was a long worm, nearly translucent with a large lobed hood near its head. These animals feed on the nutrients in the sediment that settles in between the big pillow lavas over time. Fish are rare; there are two types swimming above the bottom (rattails and ophidids), and today we found a bottom-dwelling form called a zoarcid.
There is not much color variation as we fly along the seafloor here,
so the shade of an occasional pink or orange anemone looks very
bright. We find sessile (attached) white sponges and soft corals as
well. All of these organisms feed by filtering or catching small
particles from the water. Sticking up, out, or growing a long stalk
helps the animal reach into the water column where small bits of food move by.
The crust of this pillow cracked open while lava still flowed in the tube, and it drained to form another pillow downslope. A feather star (crinoid) is clinging to the rim. It is capable of swimming. Mobile species like this can't reliably be used to help determine the age of a flow.
The older lavas surrounding the 1986 lava flow at north Cleft have sediment accumulated between the pillows and large sessile animals growing on them, such as this Gorgonian coral.
Laura is sieving sediments collected with a push-core. We sampled the sediments on and off the young lavas for volcanic glass fragments that will tell us if pillow eruptions show evidence for the mild explosive activity that we have found associated with more fluid sheet flow eruptions elsewhere.
Glass fragments sieved from push-core samples are drying under heat lamps in the lab. A board from one of the high-temperature probes, which flooded when we broke it free from the black-smoker chimney at South Cleft, is also drying. We are confident that we'll be able to get the data off it!