Exploring a caldera

August 12, 2013

Today was our first day at Axial Seamount. The remotely operated vehicle (ROV) went into the water on time and we arrived on the bottom at 7:30 a.m.. We are investigating the northeastern wall of the caldera. The walls are full of information about the old days of Axial Seamount. It grew from countless series of sheets and ponds of molten rock as they piled upon each other, cooled, and formed the base for the next flows, slowly forming a shield volcano. Now that its center has collapsed to form the caldera, many of these layers are once more exposed, and we are granted a view into the core of the volcano. Our first objective today was to map the succession of these lava flows up the crater wall. The first thing that greeted us as we approached the wall, was a massive pile of boulders and blocks that had fallen off the cliffs above into the caldera.

talus blocks that fell from the caldera wall

Large, angular talus blocks have fallen from the caldera wall above. This type of debris lines the base of the caldera. The red dots in the center of the photo are lasers set 29 centimeters apart for scale.

Above the talus, we encountered what we were looking for. Successions of large lava flows are stacked on the inner caldera wall as a series of steep steps, with gravel slopes in between. Some flows are more broken up, some are made of pillow lava which looks like toothpaste, and others were lava sheets that still show their ropy, wrinkled flow surfaces. We took video footage of these rock walls to later feed to photogrammetry software that will evaluate the spatial relationships between the features and create a three-dimensional model of the walls. It is very much like a bathymetric map, only this map covers the vertical walls, can show overhangs, and also has a color texture derived from the imagery. In that way, it is much like a photo-mosaic, only it carries the complete 3D shape of the rock wall. These models can later be geo-referenced and we can do field geology on these models, almost as if we were standing on the bottom of the seafloor.

Spider crab on a lava flow outcrop

A spider crab crawls across a lava flow outcrop on the eastern caldera wall. Above it, there is a sharp boundary with the upper layer. Volcanic sands that have shed from the crest drape the surfaces.

Volcanic sands cover shelves and slopes that are not quite as steep. They guided us up towards our second target—the crest of the wall and the summit plain beyond. The summit is covered by up to two meters of volcaniclastic sediment that tells the story of the eruptions that occurred after the flows exposed in the walls, and perhaps as the caldera was forming. We tried to sample the sediment using a number of coring techniques. The simplest way is to push a plastic tube into the mud and sand using the strong manipulator on the ROV. That usually gives us a core of about 30 centimeters of sediment. Of course we are more ambitious, so we have a vibracorer installed at the front of the ROV. It takes a two-meter-long tube of either aluminum or plastic (which is great: we see right away how much sediment we have recovered) and forces it into the ground. Not only that, it vibrates while the tube gets pushed down, easing the friction of the sediment against it. That way we get much deeper down—or so we thought. For some reason, we did not fill the tubes all the way to the depth at which it stuck in the sediment, so we are losing some of the core at some point. We are still figuring it out but have some more days left to do so!

We found an elegant way around this problem. A number of fissures run across the summit plateau. They opened after the sediments were deposited and cut through the sediment layers. At the rim of many fissures we found exposures of the upper meter or so of sediment, nicely layered. This allowed us to identify the layers we already had sampled in the cores and then take a core into the underlying layers.

a deep fissure

A deep fissure cuts the eastern summit plateau. It is flanked by ochre layers of volcanic sediment that was probably deposited when the caldera developed. Since we have already cored the upper layer, we can bypass it and take a core of the lower layers exposed by the fissure.

At the end of the day we are undertaking the third big sampling campaign of today: wax coring. The first deployment has already been successful and we got some glassy lava chips from a flow in the southern part of Axial’s caldera that erupted in 2011. This is the youngest flow around here, and shows how active this volcano is.

— Tom Kwasnitschka

The good ship R/V Thomas Thompson, operated by the University of Washington, is also out here at Axial. They are laying cables, and deploying cameras and instruments as part of the Ocean Observatories Initiative (see their website here). Today they were diving at the Ashes hydrothermal vent field in the southwestern part of the caldera, while we were diving in the northeastern part of the caldera. It’s fun to have some company out here!

R/V Thomas Thompson

The R/V Thomas Thompson was about four kilometers to the southwest of us, working in a different part of the caldera.

— Jenny Paduan