Seamounts Cruise
April 30, 2004, Day 4
Large solitary cup-corals sieved from sediments collected from the fossil-bearing deposit (right). The increments on the ruler are 1 cm. Chief Scientist Dave Clague examining the fossil coral skeletons collected today (Left).
Another great dive! The weather is very calm and slightly overcast. We left
green, productive waters behind at Rodriguez Seamount. Here at Northeast Bank
(about 275km offshore, slightly south of due west of San Diego) the water is
fairly clear and blue, as it also was at San Juan Seamount. Today the only rocks
we seemed to be able to break free from the outcrops were huge, so we hit the
vehicle's payload limit and filled the sample drawer to the brim with just 33
rocks.
--Jenny Paduan
We steamed from San Juan Seamount to Northeast Bank overnight, arriving in
plenty of time for a 06:30 launch of Tiburon. Northeast Bank is an oval shaped,
flat-topped volcano in the outer California Borderland. Flat-topped is a bit of
an exaggeration since the top is really gently domed towards the center. The
depth of the sharp break-in-slope is about 500 meters and the shallowest point
is charted at 357 meters deep. We dove on the eastern flank of the volcano
starting at about 1130m, and moved obliquely up the steep flank and over the
break-in-slope.
The lower slopes were mostly buried in sandy sediment, but thin outcrops of
volcanic debris flows become more and more abundant upslope. The upper slope is
steeper and the outcrops more spectacular with thick lava flows and layered
volcaniclastic rocks
(image to the right reveals slabby, thin layers of volcaniclastic rock). After
reaching the top, we discovered that the upper domed surface was buried in mud,
with no rocks to be seen. Rather than change the dive objectives to study the
sediment on top of the volcano, we decided to traverse around the rim of the
volcano, about 50 m below the break-in-slope. The idea was to collect the series
of lava flows and volcanic debris flows that had spilled down the flanks of the
volcano. We soon discovered that the margin of the summit was deeply embayed by
landslides and that thick lava flows and volcanic debris flows alternated below
the old shoreline. Dense vertical dikes crosscut the clastic deposits at
several locations. Landslide scarps truncate several flows that cooled slowly
enough that they have rough columnar jointing.
The
image to the left shows volcaniclastic sediments on the flank of the volcano.
The layers are parallel to the slope, indicating that they were deposited by
avalanching debris flows. In one embayment, partly dissolved solitary corals
littered the surface. The fossil-bearing layer was located below volcaniclastic
deposits, so the corals were alive when the volcano was still active. The corals
were also partly dissolved and of a type we did not observe on the dive, leading
us to think that they are fossil corals, perhaps from 5-10 million years ago. At
the end of the dive, we searched for evidence of an ancient shoreline, but found
none.
After we return to shore we will try to determine the age of the volcano using
the volcanic rocks and the solitary corals. We will also analyze any volcanic
glass we find in the clastic samples to see if the glass contains dissolved
gases like sulfur dioxide, water, or carbon dioxide that would indicate that
they erupted below sea level. Low concentrations of these gases would
demonstrate that the lavas erupted above sea level and that Northeast Bank was
indeed an island long ago.
- David Clague
Fossil
cup-corals eroding out of a layer high on the flank of the volcano.
Lonnie preserves some biological specimens after the dive (right).
Eroded volcaniclastic deposit graced by a yellow sponge and a pom-pom anemone.
Blocky basalt flow with columnar jointing.