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
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
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.