Submarine volcanism in the Sumisu Rift, Izu-Bonin arc: submersible and deep-tow camera results

Abstract

Multi-vent volcanic extrusions in the Sumisu back-arc rift have formed individual cones in the outer rift and short segment en echelon ridges in the inner rift. Alternating or coincident phases of constructional volcanism and mass wasting have produced a terrain consisting mainly of large basalt pillow lavas, lesser amounts of fresh-looking lobate, ropy, and sheet flows along the ridge crests, and volcanic debris ubiquitous throughout the rift. A rhyolite volcano and inactive hydrothermal chimneys occur at the step in the largest en echelon ridge. At present, constructional volcanism predominates over tectonism on the features studies. The development of en echelon basaltic ridges is greater in other western Pacific back-arc rifts with more extension. Continued extension in Sumisu Rift may cause a shift in the concentration of volcanism from the cross-rift trend to an along-axis orientation, eventually leading to the coalescing of the inner rift volcanic ridges into a back-arc spreading system.