A criterion for the fragmentation of bubbly magma based on brittle failure theory

The fragmentation of bubbly magma is a defining point in a volcanic eruption-before fragmentation the magma flows relatively slowly, during fragmentation the bubbles break up to release compressed gas and, afterwards, the eruption becomes a violent gas flow carrying suspended magma particles. Seemingly benign lava flows or domes can suddenly fragment into deadly pyroclastic flows(1-3). Several criteria have been proposed to define the point of magma fragmentation or foam stability(4-7). The criterion of Papale(7) is based on melt relaxation theory and equates magma strain rate with the rate of increase of flow velocity with distance. It ignores, however, the role of bubble pressure in causing fragmentation. Two empirical approaches(4,5) consider the role of high bubble pressure in causing fragmentation but do not address the underlying physics of magma fragmentation. Here I develop a fragmentation criterion for bubbly magma based on brittle failure theory and apply it to the fragmentation of lava domes and flows. On the basis of this theory, a bubbly magma will fragment when the tensile stress at the inner walls of bubbles exceeds the tensile strength of the magma. The fragmentation conditions depend strongly on initial water content, with calculated vesicularity and final water levels coinciding reasonably well with those in observed pumices. This suggests that the proposed criterion captures the essence of the fragmentation process in bubbly magma.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62724/1/402648a0.pd

The Fragility of Volcán de Colima—A Material Constraint

International audienc

Bubble development in explosive silicic eruptions: insights from pyroclast vesicularity textures from Raoul volcano (Kermadec arc)

Critical to understanding explosive eruptions is establishing how accurately representative pyroclasts are of processes during magma vesiculation and fragmentation. Here, we present data on densities, and vesicle size and number characteristics, for representative pyroclasts from six silicic eruptions of contrasting size and style from Raoul volcano (Kermadec arc). We use these data to evaluate histories of bubble nucleation, coalescence, and growth in explosive eruptions and to provide comparisons with pumiceous dome carapace material. Density/vesicularity distributions show a scarcity of pyroclasts with ∼65–75 % vesicularity; however, pyroclasts closest to this vesicularity range have the highest bubble number density (BND) values regardless of eruptive intensity or style. Clasts with vesicularities greater than this 65–75 % “pivotal” vesicularity range have decreasing BNDs with increasing vesicularities, interpreted to reflect continuing bubble growth and coalescence. Clasts with vesicularities less than the pivotal range have BNDs that decrease with decreasing vesicularity and preserve textures indicative of processes such as stalling and open system degassing prior to vesiculation in a microlite-rich magma, or vesiculation during slow ascent of degassing magma. Bubble size distributions (BSDs) and BNDs show variations consistent with 65–75 % representing the vesicularity at which vesiculating magma is most likely to undergo fragmentation, consistent with the closest packing of spheres. We consider that the observed vesicularity range may reflect the development of permeability in the magma through shearing as it flows through the conduit. These processes can act in concert with multiple nucleation events, generating a situation of heterogeneous bubble populations that permit some regions of the magma to expand and bubbles to coalesce with other regions in which permeable networks are formed. Fragmentation preserves the range in vesicularity seen as well as any post-fragmentation/pre-quenching expansion which may have occurred. We demonstrate that differing density pyroclasts from a single eruption interval can have widely varying BND values corresponding to the degree of bubble maturation that has occurred. The modal density clasts (the usual targets for vesicularity studies) have likely undergone some degree of bubble maturation and are therefore may not be representative of the magma at the onset of fragmentation