Teacher Logbook - NOAA Ship Ron Brown

Lava pillars (with bathtub rings) at Cloud vent. Imagenex pencil-beam sonar mounted on ROPOS.

Jeff Goodrich's Sealog:
Axial Volcano- 1998 lava flow
July 23, 2001

Pee Yew! What smells? Sulfide. It must be those tubeworm samples again. After ROPOS came up on deck around 1200 the scientists rushed out to the bio box and suction sampler to collect their latest treats from the deep. Dive number R625 brought us back to the Cloud vent area for Sabastian Durand's video/mapping survey with a side scanning sonar called Imagenex. Imagenex is a high frequency (675 kHz) "pencil beam" scanning-sonar used to do high resolution (2-m grid cell size and less) bathymetric surveys. Mounted on ROPOS with the sonar head pointing downward, it sweeps from side to side as ROPOS flies forward resulting in a zig-zag pattern of depth soundings under the ROV track. ROPOS flies at a< slow rate in straight tracklines back and forth, termed "mowing the lawn." After processing the data, a detailed map can be produced on board the ship.

Passing over the 1998 lava flow for Sebastian Durand's survey was like flying in an airplane a couple hundred feet over the Earth's surface. Small fissures, canyons, mounds of broken basalt, and pillars passed underneath. The pillars are spectacular, some rising more than 5 or 6 meters (15-18 ft.) off the bottom. The pillars are created when an eruptive sequence takes place in relatively flat areas on the ocean floor. Rift eruptions start with a dike intrusion along the rift zone. Magma then pours out the ocean floor as a thin sheet flow (10's of cm thick). The flow advances as lobes, with gaps in between where seawater makes its way below the flow. Since the flow comes into contact with the cold seawater, its surface forms a thin crust. As magma continues to flow beneath the thin crust, the lava flow inflates like a balloon and resembles a convex mirror. It now has a thin brittle crust on top of ponded liquid magma.

Water trapped below the lava flow heats and rises through the flow, escaping out its surface. This creates cooled conduits (tubes) that run from the flow's bottom to it's top. Next, the lava drains back, collapsing the thick central portion of the lava flow. Near the edge of the collapsed area, cooled, hardened columns surrounding the conduits are left behind and remain as pillars (most of which are hollow). On the pillars are layered ledges sticking out horizontally, called "bathtub rings". They were formed during the lava drainback stage as the upper, cooling crust attaches to the pillars. As the the deflation continues, the attached portion cools, becomes brittle, and then breaks forming a new ledge. New lava attaches lower on the pillar, cools and becomes brittle, then breaks again forming yet another ledge. This process continues until the drainback is complete. Ledged pillars are left behind. An animated sequence of this process may be found on the NeMO Explorer web site.

Tonight we're doing another Imagenex survey at the fascinating CASM vent field in the northern portion of the caldera. Stay tuned.