The Non-Explosive Generation of Pumiceous Material in Subaqueous Felsic Volcanism
Abstract:
First, in felsic dome-building eruptions, modest amounts of vesiculation often occur under non-explosive conditions to generate a pumiceous carapace to the lava. In air this carapace stays in situ, but under water such material may be positively buoyant and can detach and float to the surface. The resulting clasts can be of great size (17 m at Taupo), and have distinctive jointing, density, groundmass and bubble-textural characteristics. We show these from two examples: Taupo (NZ), where degassed rhyolite lava domes were extruded on to the floor of the re-filled Lake Taupo shortly (5-10 years) after the highly explosive 232 AD eruption, and Green Lake (Raoul volcano, Kermadec arc) where, prior to a ~1.4 ka explosive eruption, degassed dacite lava was extruded beneath a pre-existing lake. In both cases, large pumice clasts are now preserved in lacustrine shoreline sediments, often breaking up in-situ to yield abundant fragments that in many cases have a density greater than 1.0.
Second, in subaqueous activity with intermediate eruption rates, the behaviour of magma can follow a fundamentally different path. Vesiculation of magma reduces its density, and the possibility then arises that magma becomes buoyant with respect to the water and will float. Detachment of non-degassed vesicular material is thus divorced from both explosive fragmentation and lava effusion, and yields clasts with textural characteristics that are distinct from those of either degassed dome-derived pumice or explosively fragmented material. We have used these features to define an intermediate eruptive style (Tangaroan) that is unique to the subaqueous realm. We document the resulting products from dredge hauls at Macauley, but the processes involved have been captured in the 2011-2012 eruptions at El Hierro (Canary Islands) and may have played an important role in the dispersal of material from the 2012 Havre eruption.