Characteristics and Origin of the Havre 2012 Giant Pumice

Abstract:

Giant pumice (>1 m, silicic, moderately to highly vesicular) has been identified at a handful of locations on the modern seafloor and in ancient subaqueous successions. With one exception, information on the context, distribution and physical characteristics of giant pumice has been limited by difficult access and/or incomplete exposure. The exception is the giant pumice generated by the 2012 Havre submarine silicic eruption, and subsequently mapped and sampled during the MESH research cruise in 2015. The main products of this eruption were an extensive pumice raft and 14 lavas and domes, together amounting to ~0.5 km³ (DRE). Volumetrically subordinate products include a layer of giant pumice (~0.01-0.06 km³ DRE) and a thin (everywhere <50 cm) layer of ash and lapilli.

The Havre 2012 giant pumice clasts are prismatic and bounded by smoothly curved surfaces; close-spaced joints extend from outer surfaces inward. Some clasts are flow-banded. The giant pumice clasts occur in isolation, as a single-clast-thick layer, and locally stacked two- or three-clasts-high. They were deposited by settling from temporary suspension in the water column but they were not part of a raft because there is no evidence of clasts having been abraded. Some giant pumice clasts disintegrated where they landed on the seafloor, forming local clusters of decimetre-size polyhedral pumice clasts.

Five lavas and domes were emplaced before deposition of the giant pumice layer and nine domes post-date it. The composition of the giant pumice falls between the slightly less silicic earlier lavas and domes (~69 wt% SiO₂), and the slightly more silicic later domes (~71 wt% SiO₂).

The characteristics of the Havre giant pumice layer, and its intimate association with contemporaneous domes, support the inference that the eruption style was fundamentally effusive, involving steady discharge of highly vesicular magma at the seafloor, balanced by removal due to propagation of quench fractures and release of the buoyant giant clasts into the water column. There is no evidence that magmatic volatiles or steam drove magma fragmentation; the shape and jointed margins indicate that quenching was the dominant fragmentation process. This combination of an effusive eruption style with detachment and dispersal in suspension is restricted to subaqueous settings.