Melt stripping and agglutination of pyroclasts during the explosive eruption of low viscosity magmas

Abstract

Volcanism on Earth and on other planets and satellites is dominated by the eruption of low viscosity magmas. During explosive eruption, high melt temperatures and the inherent low viscosity of the fluidal pyroclasts allow for substantial post-fragmentation modification during transport obscuring the record of primary, magmatic fragmentation processes. Here, we show these syn-eruption modifications, in the form of melt stripping and agglutination, to be advantageous for providing fundamental insights into lava fountain and jet dynamics, including eruption velocities, grain size distributions and melt physical properties. We show how enigmatic, complex pyroclasts termed pelletal lapilli form by a two-stage process operating above the magmatic fragmentation surface. Melt stripping from pyroclast surfaces creates a spray of fine melt droplets whilst sustained transport in the fountain allows for agglutination and droplet scavenging, thereby coarsening the grain size distribution. We conclude with a set of universal regime diagrams, applicable for all fluidal fountain products, that link fundamental physical processes to eruption conditions and melt physical properties.

Magma fragmentation: a perspective on emerging topics and future directions

“…pure melt analogues define a critical deformation timescale that promotes fragmentation via fluid dynamic instabilities (Jones et al 2019). Applications include fragmentation within lava fountains, splashing of spatter clasts (Sumner et al 2005) and melt stripping from pyroclast surfaces (Moss and Russell 2011;Jones et al 2022). Importantly, these results must now be extended to multicomponent mixtures; most promising is combining both numerical models and experimentally derived fragmentation criteria (La Spina et al 2021).…”

“…Secondary fragmentation processes during fountaining of low viscosity, crystal-poor mafic magmas are less well studied. Here, primary magma fragmentation is predominantly ductile and further ductile fragmentation can occur in-flight (Jones et al 2019(Jones et al , 2022Edwards et al 2020;Thivet et al 2020). Recent field observations and numerical models have shown, however, that under conditions of high gas flux, rapid adiabatic cooling of the exsolved gas phase can quench the surface of molten pyroclasts fast enough to prevent development of permeability (Namiki et al 2021).…”

Inflated pyroclasts in proximal fallout deposits reveal abrupt transitions in eruption behaviour

“…Furthermore, the physical processes governing explosive eruption of low viscosity magmas are fundamentally different to those operating in the now relatively well-studied silicic systems 24 – 27 . Most commonly their fragmentation processes are fluid dynamic in nature such that breakage does not occur simply by decompression upon crossing the glass transition 7 , 27 – 29 . Instead, their low melt viscosities allow surface tension-driven reshaping and bubble nucleation, growth, and coalescence to operate on syn- and post-eruptive timescales.…”

“…A common example includes achneliths such as Pele’s tears where in-flight surface tension-driven relaxation 30 transforms irregular pyroclast morphologies into more spherical shapes. The consequence is that pyroclast properties are susceptible to modification after the initial fragmentation 12 , 29 – 33 . As a result, the eruptive deposits derived from low viscosity magmas are challenging to interpret as: (1) vent structures evolve and migrate during an eruption; 34 – 36 (2) early products are frequently buried or rafted by subsequent eruption episodes 37 – 39 and (3) rheomorphic processes and in-flight secondary fragmentation processes destroy primary fragmentation features 1 , 11 , 17 , 29 , 33 , 40 , 41 .…”

“…The consequence is that pyroclast properties are susceptible to modification after the initial fragmentation 12 , 29 – 33 . As a result, the eruptive deposits derived from low viscosity magmas are challenging to interpret as: (1) vent structures evolve and migrate during an eruption; 34 – 36 (2) early products are frequently buried or rafted by subsequent eruption episodes 37 – 39 and (3) rheomorphic processes and in-flight secondary fragmentation processes destroy primary fragmentation features 1 , 11 , 17 , 29 , 33 , 40 , 41 . These multiple factors hinder our understanding of mafic eruption dynamics solely by viewing their associated products.…”