Seamounts Cruise
May 5, 2004, Day 9
We continue to have both good weather and good dives, and have now completed 8 of 10 planned dives and all we will do at San Juan Seamount, Northeast Bank, Patton Escarpment, and Little Joe Seamount. Tomorrow is our one dive on San Marcos Seamount, and then on Friday we return to Rodriguez Seamount for the final dive before steaming to Moss Landing starting late Friday for arrival Saturday late afternoon. Today's dive on Little Joe (yes, named for the character in the old TV series Bonanza by Jim Hein in 1987) was deeper than we had been previously - down to 2650 m depth. Nearly the entire dive was pillow lavas or talus, except for a small area of volcaniclastic deposits in thin sheets on the surface. The volcaniclastitic deposits contain fine glass sand that probably formed during explosive eruptions at about 2500 m depth. Many of
the pillows were large and bulbous, as expected for
cooler, pasty lava. Other areas were blocky flows similar to a'a flows
on land. These are very difficult to distinguish from talus, but occur
on gentle slopes whereas the talus is on steep slopes. One of the cones
we crossed today had several small perched lava ponds on its flat top.
The flat tops also had shallower rims (by about 20 meters) with inward
facing talus slopes below truncated pillow lava, suggesting that the
cones had collapse pits (small calderas) that had been mostly refilled
by subsequent lava flows. Such features could be mapped in detail using
high-resolution mapping systems, but the low-resolution maps we have
available do not show such detail.
We had an unusual dive today in terms
of biology since the most
abundant animals we saw were a chiton and a cup coral, followed by a
pink cucumber and two anemones we see everywhere, and a stalked mushroom
coral (Anthomastus)(see photo) that we have only seen about 3 of previously--all in
Hawaii! There were very few sponges and almost no corals or crinoids. It
looked like a different planet than where we have been diving
previously.
For the first time since our initial dive on Rodriguez Seamount we had
a few vehicle problems. That first dive, which had numerous challenges,
was really a test dive since the vehicle had not done but a few dives in
several months while some ship maintenance and repairs were completed.
About 2 hours from the planned end of today's dive, the drawer would not
work right. It would open, but you had to push it closed with the
manipulator. This turned out to be a sheared bolt that connects the
drawer to a hydraulic ram that drives it back and forth. We finished the
dive dropping samples in the front edge of the drawer and into a few
push core holders that I had left empty to save some weight and increase
our payload (great planning, better luck!) About a half hour later, the
zoom and focus controls on the science camera failed because a power
supply burned out, probably because we were trying to zoom and focus at
the same time and overloaded the power supply. We switched and recorded
the pilot's camera output since it was in better focus, but with these
two problems it was difficult to collect samples or document what we
were seeing. Fortunately, we had already had a successful dive. We
simply reduced the length of the dive by an hour to give the pilots time
to repair these two problems.
We also learned something really bizarre today. Our favorite little
seamount clam has some tricks we did not suspect. We had been trying to
figure out how they hang under rocks and why we sometimes find them out
in the open, even on flat ground. One that we have been keeping alive in
an ice chest in the walk-in refrigerator climbed up the wall and
attached itself to the side of the ice chest and is filter feeding.
Turns out they can climb using small tube-feet-like structures that
extend through the crack between the valves! They then attach themselves
to the wall with hair-like filaments of sticky mucous. This is certainly
unexpected behavior!--Dave Clague
Slabs of volcaniclastic rock. The samples we collected here consisted mainly of sand-size grains of volcanic glass.
View of the open moon pool through which the ROV is deployed. On the deck beyond the moon pool is the nodding-head crane that dampens the transfer of ship motion down the tether. The ship's port pontoon can be seen in the clear water through the moon pool. Some 2500m (1.5 miles) directly below is a seamount and the ROV.
Talus of pillow lava fragments derived from the faulted exposure uphill.
Truncated pillow lava. Cross-section exposed by faulting shows typical radial jointing caused by contraction of the pillow while cooling.
Small haystack-like vent or hornito (secondary vent above a breached lava tube).
Tumulus (pressure ridge) formed above a lava tube when excess pressure of flowing lava buckled the tube and cracked the lava crust above.
A white crinoid we've never seen before; we saw two today.
The rock samples we are collecting on this cruise will be analyzed for
chemical and isotopic compositions and radiometric ages will be
determined. The data will expand our knowledge of volcanic processes
that we have been studying on seamounts offshore central California
(e.g. Davidson, Guide, Pioneer seamounts) to seamounts farther south.
Like their northern counterparts, these seamounts are north-east
trending ridges, built on ocean crust millions of years after sea floor
spreading stopped. Our studies of the northern seamounts have shown that
the volcanoes repeatedly erupted small volumes of viscous, gas-rich,
alkalic lavas over a great range of depths. Eruptions occurred
repeatedly over a time span of millions of years. When the volcanoes
reached shallow water the eruptions became highly explosive, creating
fragmental volcaniclastic deposits. Lava and volcaniclastic samples
collected on this cruise suggest similar compositions and processes but
we suspect the rocks will be younger and we are certain that Rodriguez
Seamount and Northeast Bank were islands.
Despite the proximity of the seamounts to the continental margin, data
on their compositions and ages are sparse and our studies will help
develop better understanding of the complex tectonic history of the
continental margin of California.
--Alicé Davis
ROV pilot Paul operates the robotic manipulator arm to collect a sample from the sea floor. The master unit in the control room on the ship simulates the joints in the arm on the vehicle.