Fragmentation of magma during Plinian volcanic eruptions

Abstract

The ratio of the volume of vesicles (gas) to that of glass (liquid) in pumice clasts (V G /V L ) reflects the degassing and dynamic history experienced by a magma during an explosive eruption. V G /V L in pumices from a large number of Plinian eruption deposits is shown here to vary by two orders of magnitude, even between pumices at a given level in a deposit. These variations in V G /V L do not correlate with crystallinity or initial water content of the magmas or their eruptive intensities, despite large ranges in these variables. Gas volume ratios of pumices do, however, vary systematically with magma viscosity estimated at the point of fragmentation, and we infer that pumices do not quench at the level of fragmentation but undergo some post-fragmentary evolution. On the timescale of Plinian eruptions, pumices with viscosities ~10 9 Pa s can expand after fragmentation, as long as their bubbles retain gas, at a rate inversely proportional to their viscosity. Once the bubbles connect to form a permeable network and lose their gas, expansion halts and pumices with viscosities ~10 5 Pa s can collapse under the action of surface tension. Textural evidence from bubble sizes and shapes in pumices indicates that both expansion and collapse have taken place. The magnitudes of expansion and collapse, therefore, depend critically on the timing of bubble connectivity relative to the final moment of quenching. We propose that bubbles in different pumices become connected at different times throughout the time span between fragmentation and quenching. After accounting for these effects, we derive new information on the fragmentation process from two characteristics of pumices. The most important is a relatively constant minimum value of V G /V L of F1.78 (64 vol.% vesicularity) in all samples with viscosities 10 5 Pa s. This value is independent of magma composition and thus reflects a property of the eruptive mechanism. The other characteristic is that highly expanded pumices (`85 vol.% vesicularities) are common, which argues against overpressure in bubbles as a mechanism for fragmenting magma. We suggest that magma fragments when it reaches a vesicularity of F64 vol.%, but only if sheared sufficiently strongly. The intensity of shear varies as a function of velocity in the conduit, which is related to overpressure in the chamber, so that changes in overpressure with time are important in controlling the common progression from explosive to effusive activity at volcanoes.

The size distribution of pyroclasts and the fragmentation sequence in explosive volcanic eruptions

“…Large fragments sample the average mixture, and their densities are expected to be independent of size. This has indeed been verified for fragments larger than about i cm [Houghton and Wilson, 1989;Gardner et al, 1996]. For fragments smaller than this, we assume that the fragment vesicularity e* depends on fragment radius r as follows:…”

Physical volcanology of the 2,050 bp caldera-forming eruption of Okmok volcano, Alaska

“…The answer is probably linked to pumice rafts, which are large accumulations of low-density clasts on the sea surface generated by fallout or pyroclastic currents traveling over water (Whitham and Sparks 1986). Because densities and porosities of pumice and scoria have similar ranges, both pumice and scoria rafts share similar floatation properties (Gardner et al 1996). If the scoria rafts produced by Okmok's eruption became continuous and thick enough, they could act as a skin over the water, bouncing dense clasts as the ground would.…”

Density structure of Io and the migration of magma through its lithosphere

“…Equally, significant gas volumes will be generated by the incorporation into magmas in dikes of even very small amounts of volatile compounds from the dike walls. Whatever the volatile source, if the bubble volume fraction exceeds some critical value, commonly taken to be close to 75-80 vol % [Sparks, 1978] but almost certainly a complex function of bubble size distribution, bubble number density, and magma strain rate [Mader et al, 1994;Alidibirov and Dingwell, 1996;Gardner et al, 1996], the magma is disrupted and an explosive eruption ensues, the velocity of the ejecta being an increasing function of the gas mass fraction in the explosion products [Wilson, 1980]. The adiabatic expansion of the mixture of gas and silicate droplets leads to extreme cooling and eventual freezing of most of the volatile phase so that an assemblage of cold, irregular pyroclasts forms around the vent as a loosely-packed fall deposit [Kieffer, 1982[Kieffer, , 1984.…”