West Coast Expedition
July 20 - August 30, 2002
West Coast of North America

August 8, 2002: Day #20

Fig. 1 Feather star on top of large pillow basalt on southern Juan de Fuca Ridge.

Debra Stakes writes: Dive 460 was placed at the southern end of the Cleft Segment to produce a geological survey of the mid-ocean ridge that is supposed to be the mirror image of what we saw in 2000 on Dive 179. We hoped to find more evidence of off-axis eruptions that would be systematically thickening the upper part of the ocean crust (Layer 2A). I was also hoping to find more evidence of off-axis fluid flow in the form of vents or mounds.

The ROV navigation system has been having a few problems so we carried down two extra beacons to conduct some tests during the dive that might help us to refine our sample locations. Because this was an all geology dive, we only had empty spaces for rocks, the magnetometer, the low-temperature manifold water sampler, and the temperature probe.

The flow of the cleft was covered with young ropy pahoehoe, but as soon as we got to the top of the valley, all we saw were big pillows (Fig. 1) and increasing amounts of sediment.

Just outside of the young pillows, however, we found a remarkable area of collapse features including lava pillars that reminded us of the gateway to the lost city of Atlantis (Fig. 2)

After the lava lake area, the sediment ponds increased but every once in while, we would find an eruptive fissure with pillows on either side (Fig 3). We collected samples to decide if these are younger lavas erupted off axis.

Near the very end of our dive, we did find a large mound of altered rock and low temperature minerals. Our push core from here (Fig. 4) will tell us more about the fluid flow many kilometers from the mid-ocean ridge.

Fig. 2 Lava pillars left standing after a lava lake drained away. The pillars are still holding up one lobate flow from the top of the lake.

Fig. 3 This is an eruptive fissure that was found several kilometers from the mid-ocean ridge. Do you think that the little pillow lavas erupted from here??

Fig. 4. Push core from a low-temperature mound of altered rock and diagenetic minerals. The conspicuous change in color marks the boundary between the upper oxidized zone and the lower reducing zone.


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