Axial Volcano summit caldera
August 29, 2013
Today’s dive focused on exploring two of the youngest known eruptions from the summit caldera of Axial Volcano (1998 and 2011). A major goal of the dive was to locate and trace the contact between these two relatively young eruptions and to determine how far south the 2011 flow extends. While these two eruptions occurred 13 years apart, differentiating between them on the seafloor is quite challenging. Typically, we use characteristics such as variations in sediment cover, the presence or absence of a glassy rind on the lava, the color of the lava, and/or a change in the density and diversity of marine organisms to differentiate between lava flows on the seafloor. However, because both eruptions are relatively young, they have similar low levels of biological diversity and both still have lavas with glassy surfaces. Therefore, we had to rely on lava color and slight variations in sediment cover to discriminate between eruptions. The lavas erupted in 2011 had a blacker color and slightly less sediment cover compared to the 1998 lava flows, allowing us to identify and map the contact.
Earlier interpretations of this flow contact were based primarily on bathymetric data. During our dive, we were able to extend the length of the 2011 flows by 112 meters compared to previous interpretations. In one location, the 2011 flows filled a wide, drained channel created during the 1998 eruption. We collected several samples from these eruptions for geochemical analyses, which will help confirm our visual observations.
In addition to mapping the young eruptions near the summit, we were also able to examine several older lava flows of unknown age. Based on a few rock core samples from previous cruises, we know that this area contains some of the most primitive (high magnesium) lavas erupted near the summit of Axial Volcano, but their extent and age are unknown. In hopes of determining the age of these eruptions, we collected several sediment cores from the top of the lava flows. If we are lucky, there will be foraminifera in the sediment that can be dated, providing a minimum age for lava emplacement. Rock samples were also collected from several locations within the older lava flows to determine if the entire region is composed of primitive, high magnesium lavas or if there are multiple eruptions in the region with variable lava compositions.
Unfortunately, our dive was cut short by high winds from the southwest. A map of the wind in our area is shown below. Due to the difficulty involved with recovering the ROV during high seas, the dive is stopped if there are sustained winds of greater than 25 knots. Good call—the maximum wind gusts today reached 38 knots.
— Dorsey Wanless