MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 12, 2005
Tiburon dive 878, East Blanco Depression, Blanco Fracture Zone
Update by Bob Embley; photos and movie by Bill Chadwick.
Bob Embley writes:
Today’s dive was in a deep basin at the western end
of the Blanco Fracture Zone, where the ocean floor generated by the
Juan de Fuca Ridge to the north slides by the Pacific plate born on the
Gorda Ridge to the south. The primary purpose for the dive was to
search for hydrothermal activity in this basin and to collect the
chemosynthetic fauna that inhabits them. Such fauna could explain some
anomalous discontinuities in the genetic lineage of the animals
inhabiting hydrothermal vents on the Gorda Ridge and those of the Juan
de Fuca Ridge.
The Blanco Fracture Zone is the undersea equivalent of the San Andreas Fault. The sliding motion, known as “strike-slip”, can move plates for hundreds of kilometers past one another over time. These long fault zones can also “leak” lavas in places where the fault zone bends or is offset. This creates the strange undersea terrain we saw on today’s dive. We explored volcanic hills whose lavas have been broken and ground up by numerous earthquakes along faults that criss-cross the basin. The lavas are old and in stark contrast to the young lavas we saw on yesterday’s dive.
We knew from dives made almost a decade ago that there was hydrothermal
activity in this basin so our first goal was to return to that site. To
our delight, we found the warm springs early in the dive and we were
able to take samples of the warm fluid being emitted through the
iron-rich yellow mounds that made small fairy-castle structures growing
out of the side of the slope. The image to the left is a yellow chimney at a warm spring. The red laser dots are 30cm apart for
scale.
As a bonus we relocated the “Party Hat”
vent fluid collector we had deployed almost 9 years ago and recovered
it and the temperature recorder attracted to it. Maybe we’ll be able to
recover the temperature record from it when we return to shore! Although there was no sign of the larger chemosynthetic fauna we were
hoping to see on this dive there is a lot more to explore here and
other similar places along the fracture zone and this dive was a great
start! 
Image to the right: "Party hat" sampler deployed in 1996. It is a plastic dome to isolate diffusely-venting fluids from the surrounding seawater, with a thermistor inside recording temperature. The funnel on top allows the fluids to escape. The white square is a marker for the site. A similar, brand new, sampler was deployed at a different location today.
Left image:
The volcanic seafloor hosted a large variety of sponges, tunicates, and
soft corals like this unusual one above. The red laser dots are 30cm
apart for scale. Right image: This gorgonian (soft coral) looks very much like stalked crinoids we
have seen elsewhere. It is attached to talus of pillow lava fragments