MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 14, 2005
T880, Axial Lava Pond, Juan de Fuca Ridge.
The sun finally came out! It was pleasant outside (for the few minutes we weren't in the darkened control room, at least), partly cloudy with moderate wind and seas.
Dave Clague writes:
Our dive today was back in the area of the large deep lava pond on the south rift zone of Axial Seamount. We wanted to determine where the lava that formed the ponds erupted and to examine the walls and floor of several more of the drained ponds. The dive was set up to start east of the pond complex to determine if any overflows had covered the region east of the pond. We landed on jumbled sheet flows and crossed several fault scarps on our way to the levee of one of the smaller ponds. The outer slope was entirely elongate pillow lavas that flowed down the steep slope. The rim was lobate and drained lobate flows with many collapses and the inside wall had horizontal shelves of lava that mark former surfaces of the lava pond as it drained. The floor of the first small pond was entirely talus blocks tumbled down the steep slopes on the inside of the drained pond. However, the second larger pond we entered had a complex floor with sheet flows, pillow lava, and jumbled sheet flows.
We found several vertical shafts lined with lava that may have been vents as well as a small (several meter tall) spatter deposit. The spatter was extremely delicate and difficult to sample, but we did recover several pieces that proved to be agglutinated spatter (spatter stuck together when still molten). Such deposits form during low spattering or fire fountains and demonstrate that the eruptions in the pond were mildly explosive. The abundant limu o Pele (bubble wall) fragments in all the sediment samples around the ponds also show that the lava was quite gas rich and that mild bubbling occurred during the eruption.
We continued the dive to the west of the large ponds to see if the lava that formed the ponds could have erupted from a fissure that formed a small N-S ridge. In between we crossed a region that was severely tectonized, with numerous faults and gaping fissures. Almost all the lava in this region was shattered. We proceeded to the northwest and explored several low lava shields surmounted with craters. We had thought that these would also turn out to be drained lava ponds, but both were collapse pits that truncated pre-existing flows. The dive ended on the small ridge where we followed an open fissure to the north until it was buried by a younger flow that filled the fissure and flowed south along it.
During the dive we learned that not all depressions are drained ponds, although the pit crater-like depressions must also form due to magma withdrawal, but from beneath the surface rather than from within a surface pond. The discovery of spatter formed during a mid-ocean ridge eruption is novel and important in that it confirms our view that essentially all deep sea basaltic eruptions have a significant magmatic gas component and erupt more energetically than most researchers think. We will be keeping an eye out for similar deposits on our remaining dives.
Fred Pleijel writes:
A bit of technical info about the worm pictures. The photographs were taken with a digital SLR camera Canon EOS S20, either with a 60 mm macro or with an extreme 65 mm macro lens that gives up to five times magnification. This set up is mounted on a microscope stand in order to be able to focus properly. The animals are in filtered seawater in a clean petri dish and I use two flashes, one from each side at a perpendicular angle, camera settings to manual (time 1/250 s, aperture usually 13 or 11, sometimes larger for the 60 mm lens) so the actual exposure is determined by the flash system (E-TTL). And then hoping for luck as this is done on a moving ship...