Subaqueous Cryptodome Eruption on the Andesitic North Su Volcano

Abstract:

Recent subaqueous volcanic dome eruptions offer a rare opportunity to study the associated surface expressions and volcanic deposits that are usually only accessible where ancient seafloor is exposed on land. The North Su volcano, located in the eastern Manus Basin of the Bismarck Sea, offers such opportunity. North Su is a double-peaked active andesite submarine volcano that reaches a depth of 1154 m. Geologic mapping based on ROV observations combined with repeated bathymetric surveys documents the emplacement of a volcanic cryptodome between 2006 and 2011. We use our observations and rock analyses to interpret an eruption scenario where highly viscous, crystal-rich andesitic magma, which failed as a brittle solid upon stress, erupted slowly into the water-saturated, gravel-dominated slope of North Su. An intense fragmentation process produced abundant blocky clasts of a heterogeneous magma (olivine crystals within a rhyolitic groundmass) that only rarely breached through the clastic cover onto the seafloor. Phreatic and phreatomagmatic explosions beneath the seafloor cause mixing of juvenile and pre-existing lithic clasts and produce a volcaniclastic deposit. This central volcaniclastic deposit consists of blocky, non-altered clasts next, variably (1–100%) altered clasts, hydrothermal precipitates and crystal fragments. The parameters usually applied to identify juvenile subaqueous lava fragments, i.e. fluidal shape or chilled margin, were not applicable for distinguishing between pre-existing non-altered clasts and juvenile clasts. This deposit was updomed during further injection of magma and mechanical disruption. Gas-propelled turbulent clast-recycling caused clasts to develop variably rounded shapes.

Such mixed volcaniclastic deposit could easily be misinterpreted as distal redeposited volcanic facies if found in an ancient volcanic outcrop on land. This central heterogeneous volcaniclastic deposit on North Su also has similarities to volcaniclastic deposits found on Snowcap dome on Pual Ridge, 50 km further west in the Manus Basin. Such mixed volcaniclastic deposits with variably rounded clasts may be a common feature in certain subaqueous dome eruptions.