Examination of Volcanic Activity: Auv and Submersible Observations of Fine-Scale Lava Llow Distributions Along the Southern Mariana Trough Spreading Axis

Abstract

A high-resolution acoustic investigation using the AUV Urashima has revealed detailed volcanic and tectonic features along the neo-volcanic zone of the intermediate-rate spreading Southern Mariana Trough, where the high magma flux forms fast-spreading type axial high morphology. Side-scan sonar imagery suggests that the survey area mainly consists of two types of terrain: high-backscattering lumpy terrain occupies the majority of the neovolcanic zone, and low-backscattering terrain is scattered over the entire area to form various bathymetric features. Visual observations by the submersible Shinkai 6500 show that the former corresponds to bulbous pillow lava and the latter to jumbled or wrinkled sheet lavas. The estimated proportion of sheet lava with respect to study area is approximately 10 %. Pillow lavas are flatly distributed and do not form the pillow mounds that are common in the slow-spreading Mid-Atlantic Ridge. Furthermore, we did not observe any pillars, collapse features, or axial summit troughs, all of which are frequently reported in the fast-spreading East Pacific Rise.