Day 8: Exploring the 2011 lava flow
August 17, 2013
After two full days cruising along the lava flow that was probably erupted in 2011 along the South Rift Zone of Axial Volcano, we are back to Axial Volcano’s summit, chasing lava flows that definitely came from the 2011 eruption. Today's dive was on the caldera floor and east flank of the volcano. This dive turned out to be one of the most diversified dives we have done. Unlike its southern sister, this flow obviously created all sorts of possible flow features. Not to say yesterday’s dive wasn’t informative or didn’t have its highlights (exploded pillows and lavacicles among them), but today's dive surely paid off compared to the rather uniform-appearing long pillow ridges on the South Rift Zone.
We started out in the caldera floor making our way toward the caldera wall. Following jumbled and lineated sheet flows (these are the highways among the various lava flow types), we would pass by some of the wildest architectures the seafloor has to offer, collapsed lava lakes. If thick inflated lava flows start to drain outwards or back down the vent, their roofs become gravitationally unstable and collapse, leaving behind structures of beautiful pillars and arches.
Near an eruptive fissure of the 2011 event, this collapsed lobate flow records two levels to which the lava flow rose before it drained away. It was dramatic and awe-inspiring, like the soaring arches of a cathedral in the deep sea.
Although collapsed terrain is not uncommon in this environment, we still were really surprised to find one site that had pillars and arches gorgeously painted in bright orange and cinnabar red by bacterial mats. Michelangelo at work in architecture and painting were my first thoughts. These patches of bacteria also occur elsewhere on the 2011 flow and are testimony of hydrothermal activity, in other words, release of heat still associated with the eruption from two years ago. However, none of us had stumbled across the red bacterial mats before.
Red bacterial mat on the partially collapsed roof of an inflated lobate flow supported by lava pillars. The orange clays are the result of hot fluids flowing through the 2011 lava flow as it cooled, and may perhaps include bacterial mat.
Arriving at the caldera wall, we continued flying along the base of the wall. The wall itself is made up of sequences of older lava flows, and truncated lava pillows within the wall tell a story of rapid collapse of the caldera floor along this fault scar long ago (likely over 31,000 years). Parts of the 2011 flow erupted on the flank of the caldera and have spilled over the rim and nearly vertical wall to produce unique lava formations. In many places, lava had cascaded down as tubes or elephant trunk-like objects, sometimes breaking up halfway leaving the lava free-falling through the water and ending up on the ground as lavacicles. At other sites, several fingers of lava tubes merged or were interwoven, reminiscent of a network of tree roots.
At the margin of the lava cascade, these thin elongate pillows flowed over the near-vertical rim of the caldera.
Lava cascade, where lavas from the 2011 event flowed over the caldera wall to the floor. From left to right, it left talus, bulbous pillows, and unbroken sheet flow. The differences probably reflect flow rate.
Outside the caldera, we continued diving over the 2011 flow, mapping its contact with lava flows that had erupted earlier in the history of the volcano.
How can one tell old and young flows apart? Obviously lava flows do not have a date imprinted upon them. It’s actually, in most cases, rather simple: Old flows are covered by a fair amount of sediment. Young flows, by contrast, still have their juvenile and often glassy look. Along the contact, lava pillows had partially burrowed under the loose sediment, and further within the flow we came across more exploded pillows, similar to the ones we had seen yesterday, and the new fissure that fed the recent lava. The 2011 lava flow clearly re-used flow paths and channels from its predecessor, the 1998 flow. Flow margins often turn out to be rather diffuse, with “fingers” of the recent flow reaching far into the older flow field.
Exploded lobate pillow, where 2011 lava flowed over an older, heavily sedimented flow.
A fracture in an older, deeply sedimented flow is filled with lavas from the 2011 eruption. This is the northern-most extent of the nearly eight-kilometer-long eruptive fissure system of this event.
Back close to the rim, where sediment cover is usually thickest, we finally tried our latest coring innovation, a piston core designed on board by our engineer François Cazenave. Retrieving cores representing the full thickness of sediments is utterly important. The sediment provides a depositional record over time and shall help to constrain the eruptive history of Axial Volcano. Making sure we had chosen a good site with thick sediment cover, we gave it a trial with a standard one-meter push core first. The push core was inserted 80 centimeters into the sediment before it hit the underlying lava flow, but only 30 centimeters was recovered within the core. The piston core was then inserted to the same depth and recovered 70 centimeters of sediment, almost the full sequence. Success! This surely made the day. More about this in tomorrow's log.
This dive turned out to be our last dive on this leg, and despite some unforeseen obstacles, we all had a splendid time during the past dives.
— Christoph Helo
Young male Amblyraja skate lies on the thick sediment near the contact with the 2011 flow on the east flank of the Axial caldera.