Keck Expedition 2004
September 7, 2004 Day 9

Update for Tuesday September 7 — Endeavour Exposed - By Debra Stakes

Today was Dive T741 and the last dive for the Endeavour Segment by us for this season. It was also the last dive of our Leg 4 and the last dive of the MBARI 2004 Northeast Pacific Expedition. While many of the people on the ship were gazing longingly toward the east with thoughts of going home, we had one last chance to put together the pieces of our Endeavour structural model. When I was a geology post-doc, Professor Cliff Hopson from UC Santa Barbara told me how learning geology had completely changed the way he saw the Sierra Nevada mountain range in California. When he was young he would look at the mountain peaks and see the greens of the forests and the shadows of the summits. After he became a climber he would look at the same peak and see routes, bivouacs and toe-holds. But once you learn some geology, you look at a peak and see millions of years of Earth history with oceans that once brimmed with life now fossilized within the layers of an extinct ocean bulged upward by the most powerful forces on the surface of our planet. What was once a pretty view becomes a humbling experience.

And thus it was with changed eyes we began this dive. We now have a-half dozen different types of pillow shapes to search for and some suspicion of what these may represent for the ongoing construction and destruction of the Endeavour axial geology. We use all the tools of a land geologist to unravel the temporal relationships between these different events. We carefully document the rock samples we collect to represent each of these events. Accurate description and bookkeeping is a huge component of science, to permit us to correlate the rocks back to the exact setting from where they were collected. The dive plan for today is a mirror image of what we did yesterday. We will drop down to the western toe of the west flank of the spreading axis just west of the Main Field vent site to climb east to the summit’s highest peak.

west_flank_base.jpg (54255 bytes)We had thought that the western deep was completely covered wit h sediment, but our ROV pilots placed us right on top of a small mound of unbroken sheet flows. The bottom of the scarp includes both round pillow talus broken from overlying pillow tubes as well as in-place ribbon flows disgorged from the centers of large collapsed pillows. All of this is on top of the sediment showing to us that it had to be a pretty recent event unburied by the continual rain of sediment particles. We drove to the east straight up the wall and observed the same steep walls of pillowtubes hanging on the west flank that were so abundant on the east flank. The same variety of pillows was apparent with very fluid looking ribbon flows on the bottom, drained oblong pillows on top of the ribbons, and spectacularly elongate pillows clinging to the steep outward facing west flank wallvertical_pillow.jpg (53473 bytes). folds_closeup.jpg (64202 bytes) This pile of pillows has become diagnostic of the outer flank eruptions, which thicken the upper crust. As the ROV neared the summit of the western flank ridge we began to search for the sheet flows that are diagnostic of the fissure eruptions within the axial valley. We need more field evidence for the relationship between the pillow forms and these flat or folded lavas. The ridge top is cross-cut by fissures that are somewhat parallel to the orientation of axis of the ridge and suggest to us these fractures and faults are caused by the regional extension related to seafloor spreading. The outer flanks did not have such cracks or fissures – why not? Could they have been completely covered by the pillows? If you look down into some of the open cracks pillows_over_sheets.jpg (68709 bytes) you can just make out flat lavas down below on the wall capped by pillows. The low saddle between the pillowed highs revealed even more of the elusive sheet flows, now partially covered by the stringy pillows. The inward facing walls appear to be made of steps of the sheet flows from the inner flow neatly sliced and pushed up as if on an up escalator. These slices must be patiently waiting to sheet_slice.jpg (49986 bytes) get their cap of pillow lavas before they become part of the ridge flank.

During all of our Juan de Fuca dives, we have diligently collected samples from each lithology, from each type of outcrop, from each of these envisioned scenarios. We have collected, in total for this leg, 82 samples from south Cleft and 191 samples from the Endeavour Segment. As soon as we get back to our home laboratories the analytical work will begin on these array of rocks to see if the major and trace element chemistry provide credibility to our field inspired story.

We completed our last dive at 4 pm and by 6 pm the ship is traveling toward Newport where they will offload the scientists and the ROV pilots. Then the ship and its crew finally get to go back home to Moss Landing and recover from the remarkable and exhausting work here on the Juan de Fuca Ridge.

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