Dive 4602 Summary
Pilot: Mark Spear; Port Observer: Mark Behn; Stbd Observer: Alice Colman
The main objective of dive 4602 was to investigate the small mounds and ridges
north of el buho and northwest of la herradura. A secondary objective was to observe and
sample the older rocks that make up the nearly vertical northern wall of the axial graben
after an attempt at retrieving one of the unresponsive transponders.
Over the course of the dive, we observed three
distinct seafloor morphologies. Early in the dive, both at
the top of the northern rift valley wall and at the base of
the wall within the axial graben, the ground was thickly
blanketed with sediment. The underlying lava
morphology was completely obscured except in fault
faces; the seafloor was featureless except for bioturbation
and rare blocks of pillow talus covered to varying
degrees with sediment. Fault walls exposed primarily pillows, though one sheet flow was
observed and sampled (sample 2) in the rift valley wall.
After traversing south across the graben floor for almost 0.5 km over thickly
sedimented seafloor and talus, we reached intact, in-place flows. These were heavily
ornamented pillows, meters across. Many pillow roofs had collapsed, exposing deep,
sediment-blanketed interiors. Pillows were coated with a
thin layer of sediment, and pockets in between pillows
were entirely filled. At times, as much as half of the
seafloor was blanketed with sediment. The first large
mound that we visited was largely covered with these
highly ornamented pillows (sample 5), as was the ridge
that we climbed at the end of the dive (samples 7 and 8).
The southwestern segment of our dive track took us over much smaller,
unornamented pillows (sample 6). Sediment cover
appeared to be thinner here, but there were still thick
accumulations between pillows, and to a lesser extent on
the surfaces of the pillows themselves. We did not see the
contact between the two lava morphologies; at the north,
there was a topographic low between the two, and at the
south, a steep, talus-covered slope.
Throughout the dive, we encountered near-vertical walls where faults or smaller
fissures cut the seafloor. These were generally E-W trending, and provided the
opportunity to sample pillows (samples 1, 3, 4) and a sheet flow (sample 2) in areas that
were otherwise blanketed with sediment. Several of the faults in the northern half of our
dive were outward facing (two on the plateau outside the graben, and one on the graben
floor).