August 27, 2014
We arrived at Axial Seamount at about 1:15 p.m. local time after our transit from Newport, Oregon. The sun was shining brightly early in the day, but clear skies up here means it can be really windy. Luckily, it became partly cloudy by the time we got to our destination. The ROV Doc Ricketts was in the water at 1:30 p.m. headed for a dive on the eastern flank of the volcano. We traversed west from a cone recently mapped with the AUV up the flank to where 2011 eruption lavas had flowed down the channel of a relatively recent, little sedimented, sheet flow and ramped against the base of a much older cone, where the lavas interacted with the thick sediments there.
This was my first time on a mid-ocean ridge (virtually), and I was struck by the similarities and differences between these seafloor lavas and my familiar lava flows of Kilauea volcano. Some similarities I expected; after all, they are all chemically about the same—your basic basalt. The differences were a bit of a surprise. Essentially it is the effect of the ocean—it dramatically changes the way lava can move, and thus what it ends up looking like.
Seafloor geologists also have a whole new descriptive terminology for lava flow features that occasionally matches our landlubber terminology, but might be used to mean something a little different. For example, what we call an “inflated sheet flow” on Kilauea is a very specific type of smooth-surfaced, inflated pahoehoe flow that forms on flat ground, far from the vent. The top of the flow freezes into a solid crust, but the molten lava underneath it continues to flow and if there is enough pressure from the upslope, pushes the crust upward as a relatively level surface. Once cooled, it is normally solid throughout, except perhaps where lava tubes are located. Underwater, basically everything is inflated—that’s the effect of the water making instant thick frozen crust that the lava has to push outward or upward to get anywhere. But the lava could still later drain away from underneath the crust and leave huge collapsed areas and other strange features, and it might still be called an “inflated flow.” This could take me a while to get used to.
The lava pillars are just wonderful. I think they are the equivalent of our lava trees, only the flowing lava here cooled around something ephemeral, like a stream of hot water, rather than a solid object. The horizontal ridges along the sides formed as the lava level fell, as we see in lava tubes. That would be a sight to see.
The 2011 flow was quite interesting—black, shiny, fresh and easy to tell from the older, more sedimented flow it covered. I was surprised by how much discoloration was already showing up on it, though it seems to be mostly biological. In places along the flow boundary, there was sediment actually on top of the new lava, but I’ll let Ryan talk about that.
On today’s dive we investigated the interaction between a lava flow and a thick water-saturated sediment pile. As shown in the image below, the lava appears to burrow under the sediment and locally protrude through it, like a large gopher. Some of the sediment also seems to be churned up during the process. The lava flow erupted in 2011 so the interaction area between lava and sediment is fresh and unobscured. Our knowledge of this type of interaction has so far been limited to much older lava flows or on-land exposures where rocks are typically altered and/or the depth of eruption is poorly constrained. Depth is a very important constraint. On Axial Seamount we know that the eruption occurred approximately 1,500 meters deep so the pressure is much higher than at sea level. At these high pressures the ability of liquid water to expand to steam becomes highly suppressed. So even though lava erupted on the seafloor may be greater than 1000° Celsius and liquid water within tiny pore spaces between sediment grains will heat up and turn to steam, the steam will not expand like it does at sea level.
This is important because there has been much debate over the last 20 years on whether or not the limited ability of steam expansion during lava-sediment interaction in the deep sea can cause explosive fragmentation of the lava. Our initial observations from today would suggest that such explosivity did not occur at the depth of Axial Seamount. At shallower depths, like that found in lakes or under glaciers like on Iceland, explosive volcanism during the interaction between lava and water-saturated sediment is more likely and can cause significant hazards for regional air travel. Our observations and samples collected at Axial Seamount today will provide a key component for understanding the critical depth/pressure where explosive volcanism and associated risks during lava-sediment interaction can occur.