Magma Fragmentation

Abstract

Magma fragmentation is the breakup of a continuous volume of molten rock into discrete pieces, called pyroclasts. Because magma contains bubbles of compressible magmatic volatiles, decompression of low-viscosity magma leads to rapid expansion. The magma is torn into fragments, as it is stretched into hydrodynamically unstable sheets and filaments. If the magma is highly viscous, resistance to bubble growth will instead lead to excess gas pressure and the magma will deform viscoelastically by fracturing like a glassy solid, resulting in the formation of a violently expanding gas-pyroclast mixture. In either case, fragmentation represents the conversion of potential energy into the surface energy of the newly created fragments and the kinetic energy of the expanding gas-pyroclast mixture. If magma comes into contact with external water, the conversion of thermal energy will vaporize water and quench magma at the melt-water interface, thus creating dynamic stresses that cause fragmentation and the release of kinetic energy. Lastly, shear deformation of highly viscous magma may cause brittle fractures and release seismic energy.14.1

Blowing Off Steam: Tuffisite Formation As a Regulator for Lava Dome Eruptions

“…It is by behaving as a solid for short timescales that the melt is able to fail or fragment, and the once-coherent liquid can exist as fragments that are able to be transported through fractures. From the point at which failure relieves the bulk of the stress, the fractured material may relax any remaining stress viscously (e.g., Gonnermann, 2015), provided that temperature remains above the glass transition temperature (T g ). In the case of tuffisites, pyroclastic fragments that are trapped in fractures in the lava dome, in the magma column or in the country rock relax, reverting to a viscous mechanism and slowly sintering together, eventually healing the system (Tuffen et al, 2003).…”

“…Since the conditions which lead to it, and the processes by which magma fragmentation occurs are relatively well understood (e.g., Alidibirov and Dingwell, 1996;Papale, 1999;Sahagian, 1999;Mueller et al, 2005;Fowler et al, 2010;Arciniega−Ceballos et al, 2015;Cashman and Scheu, 2015;Gonnermann, 2015) our reconstruction of tuffisite formation focuses on the postdeposition sintering of fragmental materials. By recreating the physical properties of tuffisites and applying established sintering models (see Supporting Information for a discussion of competing models) to describe the process we aim to constrain the conditions which form them.…”

Unravelling textural heterogeneity in obsidian: Shear-induced outgassing in the Rocche Rosse flow

“…Magma viscosities on the order of 10 8 to 10 12 Pas (e.g., Farquharson et al, 2015) inhibit bubble growth and contribute to volatile oversaturation in the melt as rhyolitic magma ascends to lower pressures and depths in the conduit (e.g., Sparks, 1978). Solubility calculations show that if magma contained dissolved water in concentrations typical of obsidian lava (0.1-0.4 wt.%) decompression during ascent would result in vesicularities far beyond those inferred for fragmentation (Gonnermann, 2015). Therefore to avoid explosive fragmentation of ascending magma there must be significant volatile removal from the melt i.e., open-system degassing (outgassing) in order to reduce gas overpressure (e.g., Taylor et al, 1983;Eichelberger et al, 1986;Gonnerman & Manga, 2003) and result in effusion of degassed obsidian lava.…”

Magma decompression rates during explosive eruptions of Kīlauea volcano, Hawaii, recorded by melt embayments

“…The style of basaltic explosive eruptions is thought to be predominantly dependent on the magma ascent velocity, which asserts a strong control on the movement and spatial distribution of the magmatic vapor (e.g. Wilson and Head, 1981;Parfitt et al, 1995;Parfitt, 2004;Goepfert and Gardner, 2010), and in turn affects the fluid dynamics and fragmentation of the erupting magma (Vergniolle and Jaupart, 1986;Parfitt, 2004;Houghton and Gonnermann, 2008;Namiki and Manga, 2006;Gonnermann, 2015). The presence of pre-existing exsolved volatiles (i.e.…”

“…Swanson et al, 2012Swanson et al, , 2014May et al, 2015) and the conditions that accompany such intensely explosive eruptions remain largely unresolved. Since a number of important processes are directly dependent on the decompression rate, such as bubble nucleation, bubble growth, and fragmentation itself (Parfitt and Wilson, 1995;Mangan and Cashman, 1996;Mangan et al, 2014;Gonnermann, 2015), robust constraints on this parameter will enable a fuller understanding of the processes that determine the intensity of explosive eruptions driven by magmatic volatiles. The average decompression rate between magma storage reservoir and surface should also provide information about the potential presence of abundant pre-existing exsolved volatiles.…”