MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 11, 2005
Tiburon dive 877, South Cleft, Juan de Fuca Ridge.
The sky remains overcast, but the seas are completely calm, the calmest the seasoned Juan de Fuca explorers we have aboard have ever experienced up here. The scenery over 2 kilometers (about 1 1/4 miles) beneath us—in our little ship bobbing on the blue, blue sea surface—is absolutely sensational. Keep in mind that the seafloor at this mid-ocean spreading ridge is very young, and tectonically and volcanically active. The vast majority of the ocean floor is older, "abyssal plain": covered with meters, even kilometers, of sediment.
Bill Chadwick (Chief Scientist of today's dive) writes:
Today’s dive at the south Cleft segment was a continuation of yesterday’s, and our dive track included visits to some spectacular black smoker chimneys and intricate lava pillar formations.
The chimneys are made from sulfide minerals coming out of the hot spring water and they seem to form impossibly tall and narrow towers. One of the chimneys we visited today was 20 meters high and another may have been venting at the boiling point of seawater at this depth (2200 m). A temperature probe at one of the chimneys recorded 340oC. All of the chimneys were covered with abundant tubeworms and other vent animals.
During the transit between vents sites, we were able to view the extraordinary volcanic landscape along the rim of the “cleft”, a deep fissure at the center of the segment axial valley that has acted as an eruptive fissure for voluminous lava flows in the past. Because the cleft is so wide (~100 m) and deep (~20 m), it becomes a huge lava lake during eruptions, often overflowing the rim and feeding lava flows that flood the axial valley floor above. Then at the end of eruptions, the still molten lava drains back into the cleft, leaving a landscape of pillars and walls and drainages, that look almost like the canyonlands of the American southwest. At the end of the dive we were able to recover part of a lava pillar to investigate how these structures may record information about the dynamics of the eruption that formed them. (The work of Bill Chadwick on this cruise is being funded by the West Coast and Polar Regions Undersea Research Center.)
Brian Cousens writes:
After only four dives, this cruise has already included a lot of "firsts" for me. On dive day one, I saw really young lava flows on the sea floor for the first time—only seven years old! Virtually no critters were living on them as yet, meaning that we did not have to scrub the rocks clean when they came on board. And all of the lavas had beautiful, chilled glassy margins on them—the result of cold seawater essentially "freezing" the molten lava before it had time to form crystals. Really cool stuff.
On dive day two, we dove on a huge lava pond with spectacular topography and a huge variety of lava flow types. I saw drained lava flows, lava pillars, thick piles of pillow lavas, and "superhighways" made of flat, ropy sheet lava flows. I was surprised at how quickly we flew from one volcanic flow type to the next—lava flows on the sea floor are really complicated!
Dive day three was marked by my first visit to a high-temperature "black smoker" hydrothermal vent. I felt like I was watching a National Geographic video on black smokers on the East Pacific Rise, except that I was taking geology notes as Bill Chadwick was describing what we were seeing on the monitors. Pinch me, yup, it's not a dream!
Today, on dive day four, we flew down the center of a mid-ocean ridge, the Pacific plate on the west side and the Juan de Fuca plate on the east side. We found a new set of hydrothermal chimneys that had not been documented before, one of which was over twenty meters high. It's amazing that these thin hydrothermal chimneys can stay standing up when they get to be that tall! So what's in store for us for the next seven days? We don't know for sure, but I'll bet it will be fun and include more "firsts".
Laura Karrei writes:
We saw THE neatest thing ever today! Black smokers are a type of hydrothermal vent that typically emit metal sulphides. When sea water percolates through cracks in the ocean crust near the mid-ocean ridge, it gets heated, and dissolves minerals in the rocks as it passes by. When enough pressure builds up, this water with the dissolved minerals escapes at these vents. The moment it reaches the cold seawater, it becomes oversaturated and the dissolved minerals precipitate instantaneously. We could actually see this transition occurring where the clear water from the vent became black as the minerals formed! To see something like this occurring right before my eyes was absolutely A-M-A-Z-I-N-G. I also found it interesting how the abundance of life on these hydrothermal vents is a rough indication of temperature. Some were too hot for "critters" to be living on them, whereas others that were "cooler" had a lot of life on them. I was also surprised at the size of the tube worms.... the animals were up to 40cm long and the tubes in which they live were over twice that length. This was definitely the highlight of my cruise thus far!