September 2: Last dive
Today was our last dive of the expedition. This dive sampled another series of large collapsed lava flows, just southwest of yesterday’s dive and also at about 2,200 meters deep on the south rift of Axial Seamount, approximately 25 kilometers south of the summit. During these dives, we explored areas mapped in mid-August by the MBARI mapping AUV to expand our coverage of the deep (over 30 meters) collapsed flows on Axial’s rift zones that are so far unique on the entire mid-ocean ridge system.
As with yesterday’s dive, the flow surfaces were puffy lobate pillows and the walls of the numerous collapse pits were made of layers of solid pillow lavas truncated by the calving of copious amounts of large pillow talus into the pits. Drainback shelves and veneers were not observed, perhaps because veneered surfaces were destroyed when faulting enlarged the pits, or talus of a thick collapsed roof had completely buried them. Or maybe they never were there at all and these pits formed by a different process than ponded sheet flows with level bathtub rings, or the deep perched ponds nearby that overflowed and built levees before the lake level dropped. In addition, we are wondering if all these ponded flows erupted in one extraordinary event, in which case it would be by far the largest single eruption known on the ridge system, or if this behavior has occurred repeatedly through Axial’s history. We also are curious if these large lava flows on the deep rift zones could have triggered the collapse of Axial’s summit caldera. We don’t yet understand the processes involved. Hopefully further examination over the coming months of the high-resolution AUV maps, and determination of the flow chemistries and ages of our samples will help resolve these questions.
At the end of the dive we crossed over the ponded flows, drove over numerous small steep mounds of elongated and unbroken lava pillows, and climbed an older cone 100 meters high. This last cone had a broad flat top with a shallow central crater, large faults offsetting parts of it, and a steep talus slope of fine volcaniclastic debris and angular pillow lava blocks below a vertical cliff of truncated pillows. The sediment on top was thicker than our 60 centimeter long probe so we collected our last piston core at the top. It was time to call it a wrap.