Keck Expedition 2004
September 6, 2004 Day 8
Update for September 6—Searching for clues on the east flank.
After yesterday’s dive in the Endeavour axial valley, we had some vision of the volcanic processes frozen in rock on the rift floor. We saw artifacts of great lava lakes rising and falling within the steep boundary walls. The walls themselves are more challenging to interpret because the pieces of ocean crust get very disrupted as they are sliced apart and pushed up into the high walls. Dive T738 provided us with a vocabulary and an inventory of potential types of volcanic constructs and dismembering faults. The last two dives for this expedition are focused on gathering field observations and rock samples to constrain the distribution of these volcanic rock types and their relationship to off-axis volcanism. Dive T740 was focused on the eastern flank of the ridge and Dive T741 will focus on the opposite western flank. We decided to examine the flanks adjacent to the central axis because this is thought to be the most volcanically robust section of the spreading segment. Dive T740 begins in the deep, sediment covered sheet flow east of the axis flanks. This type of flow is characteristic of the spreading axis and yet we find it way out here, completely barren of any cover of younger lavas. This remains an enigma to us.
The rest of the Endeavour Ridge however becomes more explicable with each dive. The outward facing ridge flanks themselves are continuous pillow mounds covering very steep walls. We look at the direction of elongation of the pillow tubes to decide the downhill direction when the pillows were erupted (see image top left). Some of the pillow tubes actually flow nearly vertically downhill clinging to the steep fault face wall. The hornitos on the outer flanks are unique. There are lower pillows that have cracked open and poured out smooth thin flows that remind me of a roll of ribbon (see image to right). At the next level, pillows are oval and covered with glassy buds where the interior lava popped through the crust and leaked away. The last gasp of the vent is a single incredibly inflated pillow shape (see image lower left). This succession likely has something to do with the temperature and rate of effusion of the lava, combined with the vent becoming filled with time. Clearly this orderly stack of volcanic structures must have erupted exactly in the spot that we find it—off-axis building up the outer flanks of the axis. On this dive we wander off our trackline for a few hundred meters parallel to the spreading axis to determine whether these volcanic features are continuous along the sides of the ridge axis. There are alignments of the giant pillows (see image lower left) and pillows pouring into cracks (see center image below) that suggest some underlying linkage between the faults and the volcanic activity. We complete our dive by driving over the top of the ridge and dropping into the axial valley floor. The inward facing wall was mostly great slopes of fault debris and avalanche chutes of pillow talus rolling downhill. BUT, every once in a while, we would get a peek at an outcrop of layered rocks, like the valley floor, topped by pillows, like the outward flanks (see image lower right). Hmm—Is the Endeavour Segment revealing its history to us in bits and pieces on these faulted slopes? Tomorrow is our last dive and we must put together the clues by the close of the day.