Searching for rare lavas
August 26, 2013
In 1998 and 1999, during an investigation of the caldera on Axial volcano, we used our rock corer to sample some small cones along the east margin of Axial’s north rift zone. These glass samples turned out to be a lava type called “andesite” that are quite rare in ocean ridge and seamount environments. Andesites are usually associated with explosive volcanoes on land—like those in the Andes volcanic chain or Mount St. Helens. They are thought to derive from basalt magmas after they have cooled and partially crystallized. One of the main objectives of our dive today was to see if we could locate these andesitic lavas, to document how they erupted, and to determine how they might be related to the normal mid-ocean ridge basalts that comprise Axial and the surrounding seafloor.
Upon settling on the bottom with the ROV early this morning, we immediately knew the lavas we were seeing were not typical basalts and likely were andesites. How did we surmise this? Because ocean ridge andesites have higher silica contents and contain more gases (mostly CO2 and H2O), the lavas form pillows that are much larger than typical ocean floor basalts, contain abundant frozen gas bubbles called “vesicles,” and have distinctive bread-crust textures or elongate striations on their surfaces.
An inexperienced marine geologist may not initially recognize these differences from normal seafloor basalts, but having seen these before on the southern Juan de Fuca Ridge and the East Pacific Rise, we easily recognized them as andesites. What we were not expecting were the various structures they made and forms they took, as well as the very large area these lavas appeared to cover. In fact, it appears that all of the small cones and mounds are composed of andesite that makes up their broad bases, as well as steep to near-vertical walls covered with elongate andesite pillow tubes.
In some places the ends of these tubes have broken off and cascaded below, but in others they have remained in place along sheer walls. Our hypothesis is that when these lavas erupted they were much more viscous than basalts because of their chemical composition, lower temperature, and high-gas vesicle contents, and that allowed them to build steep-sided cones and elongate mounds. What we don’t yet understand is why this area of the rift zone has erupted such a great volume of these rare ocean rock types and, ultimately, what magmatic process led to their formation.
— Mike Perfit