Determining the nature of volcanic eruption
August 24, 2013
After leaving Newport around 7:00 a.m. on Friday we arrived at Axial Seamount just after lunch today and began our first ROV dive on the northern extension of this recently active submarine volcano. The main edifice of the volcano, where the active caldera sits, is bounded by elongate ridges that extend to the north and south. Although we think these so called “rift zones” have largely been built by volcanic intrusions that came from the magma chamber beneath Axial, there are some small cones a few hunded meters high that have formed along the ridges. From our previous rock-core sampling we know that some are made of higher temperature lavas that may have bypassed the magma chamber to erupt directly along the rift zones. Our bathymetric maps showed that they have smooth debris slopes on the flanks, so might be made of submarine pyroclastic rocks–fragmental volcanics that form from highly unusual explosive eruptions beneath the sea surface. Pyroclastics are commonly formed during eruptions on land where they form features like cinder cones. This might happen if these high-temperature “primitive” lavas contained significant amounts of gases that expanded as the magmas rose from great depths and ultimately exploded when reaching the seafloor.
Our focus today was to find out if, in fact, the cone we selected consisted of pyroclastic rocks and therefore likely erupted explosively. We also wanted to find out if the rocks had compositions suggesting they came from a magma source different than that which feeds Axial volcano. Starting the dive at the base of the cone and heading up the flanks we immediately encountered a hillside covered with talus—various sized fragments of rock deposited along the steep slopes. The talus consisted of angular blocks of pillow lava covered with significant amounts of sediment that presumably had cascaded downslope from the upper portions of the cone. Talus covered the side of the cone for more than 300 meters before we encountered the solid piles of pillows from which they were derived. Our observations tell us that the cone was not created by explosive eruptions but rather by the erosion and collapse of a very steep and somewhat elongate ridge of pillow lavas. The very slow extrusion and rapid cooling of lava along fissures allows pillows to form very steep constructs that break apart, sending fragments of blocks down their steep sides to form aprons of talus. We took a number of lava samples during the dive and our plan is to analyze them back in our geochemistry labs to determine if they are related to the Axial magma plumbing system or if they have their own separate origin deep within the Earth.
— Mike Perfit