Puu Mahana Near South Point in Hawaii Is a Primary Surtseyan Ash Ring, Not a Sandhills-type Littoral Cone

Abstract

Puu Mahana has previously been interpreted to be a littoral cone, formed at a secondary rootless vent where lava flowed from land into the ocean, but a number of lines of evidence point to it being a remnant of a Surtseyan tuff ring built on a primary vent. The differences between it and littoral cones are highlighted by a comparison of Puu Mahana with the undoubted littoral cone of the Sandhills that was observed to form in the 1840 flank eruption of Kilauea Volcano. Puu Mahana contains abundant lithic debris and accretionary lapilli, absent in the Sandhills deposit. Compared with the Sandhills, the Puu Mahana pyroclastic deposit is finer grained and more poorly sorted, and its juvenile component is less dense and more highly vesiculated. Puu Mahana lies 3 to 4 km offMauna Loa's southwest rift zone. Identification of it as a primary vent implies that the lower rift zones of Hawaiian volcanoes can be much wider and more diffuse or more mobile than is currently acknowledged. The olivine grains that compose the well-known green-sand beach at Puu Mahana are likely derived from the ash, strongly concentrated and somewhat abraded by wave action.