Products of neptunian eruptions

Abstract

A common pyroclastic facies in subaqueous volcanic successions comprises massive to graded, very thick (several to tens of meters), laterally extensive (several kilometers) beds of nonwelded pumice lapilli with volumes ranging to tens of cubic kilometers. This facies may be overlain by laminated ash or bimodal ash and giant (>1 m) pumice clasts, and underlain by coarse lithic breccia. The association is inferred to be the typical product of sustained magmatic volatile–driven explosive eruptions from vents at water depths of ~1300–200 m. We propose the term “neptunian” for such eruptions and their products. The eruption column rapidly mixes with the surrounding water, cools, increases in density, and collapses, while remaining under water. Lithic clasts that are too heavy to be entrained in the column are deposited close to the source, forming a neptunian lithic breccia. Pumice lapilli are rapidly waterlogged and form the dominant component in the collapsing column and in eruption-fed, water-supported density currents (neptunian density currents). Hot, buoyant, giant pumice clasts continue to rise and may reach the water surface before being waterlogged and settling, along with temporarily suspended ash, forming neptunian suspension deposits. Eruption magnitude, fragmentation mechanisms, and juvenile pyroclast characteristics, especially vesicularity, are very similar in neptunian and Plinian-style eruptions, but column behavior differs primarily because of the contrasting physical properties of the ambient fluid (water versus air).