Monterey Bay Aquarium Research Institute

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

August 9, 2002: Day #21


Figure 1. The manipulator arm gently places a sample of porous amorphous silica chimney material into a biotube mounted on the front of the drawer.

Debra Stakes writes: Our last dive of the Cleft series already! Although it is tiring to do geology for so many hours each day, we will be sorry to leave so soon. Our last dive is planned as the mirror image of Dive T183 from July 2000. This transect will take us from the axial valley across the southern flank of the near-axis seamount and out to a series of major block faults that mark the beginning of abyssal hill morphology.

The vehicle drawer for Dive 461 is maximized for the collection of rocks. There are no cables in the way. The entire drawer is filled with numbered partitions to help keep the sample numbers straight. Small canisters will lids held on with elastic are mounted on the front of the drawer. We think that these will be especially good for small fragile samples such as the glassy sheet flows (Figure 1).

The center of the Cleft segment has a much lower volume of magmatism compare to areas just north and south. The center is marked by large lobate pillows bounded by pahoehoe lavas. Near the edge of the axial valley we found inactive chimneys with sulfide (?) on the bottom that was too hard to recover. The white tops are silicic but the green staining suggested Cu-rich fluids nearby (Figure 1).

The seamount proved to be an enormous pile of pillow tubes flowing down in every direction. (Figure 2). We saw large fissures on the western flanks of the seamount. These basalt flows were populated by some very beautiful sea fans (Figure 4). The vehicle made it out to one of the major east-facing boundary faults before we filled up the drawer and decided to return to the surface (Figure 5).


Figure 2. Continuous flow front of pillow tubes that form the small eruptive center about 3 km off-axis on the western flank of the center cleft area.

Figure 3. Large fissure partially covered by flows.


Figure 4. Sea fans on a mound of pillow basalts.

Figure 5. Truncated pillows exposed on the wall of a major fault. These faults frequently have talus ramps at the bottom.

 

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