August 31: Two dives in one day

Today we were lucky to have two dives on Axial Seamount. The first dive on the western flank of Axial’s caldera focused on a peculiarly flat area cut up by faults imaged by recent AUV mapping. We suspected that the terrain was comprised of very old and very thick sediment, which filled and covered all of the underlying lava topography and subsequently faulted. It is important to collect the oldest sediment on Axial because it will provide a record of volcanism going far back into Axial’s past. Knowing this will help us understand how deep sea volcanoes evolve through time and what makes them rumble.

As the ROV touched down this morning it was immediately obvious that our suspicions were correct in that the flat terrain was indeed an expansive region of sediment. After using a rod to probe the sediment in various places, it became clear the flat terrain was only covered by a relatively thin 30-50 centimeters thick sediment package. This is enigmatic because the underlying lava elsewhere on the flank is highly irregular with localized differences in height typically greater than 50 centimeters. So we are left wondering how the sediment filled in all the holes and spaces of the underlying lava flows. Lucky for us there were some faults around that exposed the underlying lavas and capping sediment package, which we sampled with the ROV. The base of the sediment package includes a hard crust that is composed of an abundance of bright orange to green muddy sediment. This crust likely filled the local topography and formed a flat surface for more sediment to accumulate onto.

low temperature hydrothermal fluids

Low temperature hydrothermal fluids emanate from a gap under a layer of lava exposed in a fault scarp. The fluids support a population of bright yellow bacterial mat and deposit hydrothermal clays. The irregular topography of the local lava terrain is apparent in the scarp exposures such as this one, but hidden under the flat sediment.


layers of orange hardened sediment cap underlying lavas

Layers of orange hardened sediment cap the underlying lavas and make a hard, smooth surface for subsequent sediment accumulation.


spider crab on pillow lavas

Spider crab (Macroregionia macrochira) at home on large bulbous pillow lavas exposed on a sedimented slope. The sediments are dark with black volcanic glass sand. The crab is about a half meter across.

Our second dive on Axial Seamount today found our intrepid team of explorers doing some detective work on the eruption age, composition and source of a group of lava flows on the western side of the upper reaches of the south rift zone of Axial Volcano. Top on the agenda was to collect data and samples to determine which, if any, of these flows predate the caldera and may have been sourced from the western flank of the caldera, or perhaps in where the caldera is now, before it collapsed. After a highly efficient and well-orchestrated turnaround of the vehicle by the ROV team, and an equally impressive “all-hands” science team processing of the various geo and bio samplers to document and remove all of the specimens collected on the first dive, we were back in the water for an afternoon stroll.

The sinuous dive path took us generally uphill, navigating a series of lava flows and mounds of different shapes, sizes, and extent of sediment cover, which roughly equates to age. We collected 13 rocks and 11 sediment push cores that show the lava flows cover a range of ages and compositions, consistent with the group’s earlier mapping efforts. But the different flows we sampled today are quite closely spaced, packing a nice variety of composition and volcano history into a relatively small space.

The high resolution sea bed map we were using for the dive showed a series of collapse structures in many of the flows, which form when magma stagnates beneath a solidified crust for a time, then rapidly drains out. These include collapsed roof fragments, pillars of lava that held it up, and cathedral-like recesses between arched pillars and uncollapsed roof. We often refer to these collapses as lava pond structures. Midway through the dive we reached a large, complex lava pond structure and were treated to a wide array of beautiful volcanic land forms, such as the lava pillars in these images below.

lava pillars

Closely-spaced two meter tall lava pillars stand like ghostly sentinels in a collapsed lava pond. The horizontal ledges on are remnants of lava that froze as the pond drained, with ones at top forming before ones on the bottom.


hydroids colonizing a hollow lava pillar

Hydroids colonizing the splayed-out top of this hollow lava pillar make for a stunning display of life in this deep sea habitat.


The roof of a ponded and inflated flow collapsed around the lava pillar that could no longer support it when the molten interior of the flow drained.

The roof of a ponded and inflated flow collapsed around the lava pillar that could no longer support it when the molten interior of the flow drained.

—Ryan Portner

The next stop for the lava samples and sediment we collected is the lab, where they, along with samples from the other dives, will be analyzed for a whole range of chemical and physical properties to help us piece together the volcanic history of this place.

bathymetric map

Bathymetric map collected by the MBARI mapping AUV earlier this month (and largely unprocessed so still noisy) of one of the ponded and collapsed flows we explored today. Lava pillars in the complex pond’s interior can be discerned. The pillars were left standing when the lava drained, probably as it continued to flow outward and expand the pillowed flow margins. The depth ranges from 1,565 meters (dark blue) to 1,515 meters (orange). The red box in the inset map shows where this map is located, on the upper south rift zone of Axial Seamount just south of the caldera.

—Ken Rubin

Piston core collecting sediments just in front of 2011 lava flow..

Equipment

Northern 2014 Expedition