Stress‐induced brittle fragmentation of magmatic melts: Theory and experiments

Abstract

[1] The release of kinetic energy during explosive volcanic eruptions is a key parameter for hazard assessment and civil defense. The explosive production of volcanic ash by intensive fragmentation of magma and host rocks represents a substantial part of this energy. For cases of explosive eruption where predominantly host rock was fragmented (phreatomagmatic eruptions) to form the major part of volcanic ash, rock mechanical parameters could be measured and fragmentation energies assigned. In cases where most of the produced ash is of juvenile origin (magmatic eruptions) a general method for the determination of fragmentation energy is still lacking. In this article we introduce a thermodynamic approach that relates grain size data of the produced ash deposits to shear rates acting during the deformation of magma. With the use of a standardized fragmentation experiment the physical parameters needed to determine the specific fragmentation energy and deformation history were measured. The experiment was calibrated and tested with two case histories of the Campi Flegrei volcanic field (southern Italy). Both eruptions are classified as ''most probable worst-case scenarios'' during the next period of activity, to be expected within the next 10-100 years. Using the experimentally determined specific fragmentation energies, the total mass of produced ash of each eruption, and assuming an energy dissipation as observed in the experiments, the total kinetic energy release of the worst-case Campi Flegrei eruptive events to come were calculated with 25 and 40 kt TNT equivalent.

An Overview of the Monogenetic Volcanic Fields of the Western Pannonian Basin: Their Field Characteristics and Outlook for Future Research from a Global Perspective

“…These volcanic fields can also be classified as typical phreatomagmatic volcanic fields (Németh, 2010a) on the basis of the overwhelming evidence of magma -water interaction driven explosive eruptions, at least in the initial stage of the eruptive history of the majority of the volcanoes of western Hungary. The phreatomagmatic explosive eruption style has been interpreted due to the abundance of preserved pyroclastic rock units in volcanic glass shards with macro-and micro-textural features characteristic of sudden chilling of the rising basaltic melt upon contact with external water, as demonstrated by comparison of experimental volcanology results (Büttner et al, 2002;Büttner et al, 2006) with natural glass shards (Dellino & LaVolpe, 1996;Büttner et al, 1999;Dellino & Liotino, 2002). Textural features, such as the low vesicularity and angular and rugged shape, evident in volcanic glass shards from the western Hungarian volcanic fields are generally accepted to support magma and water explosive interaction in other locations (Heiken & Wohletz, 1986;Büttner et al, 1999;Dellino, 2000;Morrissey et al, 2000;Dellino & Kyriakopoulos, 2003) (Fig.…”

Eos, Transactions, American Geophysical Union Volume 88, Number 24, 12 June 2007

“…In these laboratory experiments we first produce melts with variable degrees of crystallization, and then fragment them by sudden deformation [Büttner et al, 2006]. Our preliminary results show a strong correspondence in shape and crystallinity of the artificial ash with natural eruption products.…”

Investigating Source Conditions and Controlling Parameters of Explosive Eruptions: Some Experimental-Observational- Modelling Case Studies

“…Mechanical energy was provided by expanding overpressured gases in rubber tubes connected to the conduit; when expanding, the gas coupled with the particles in the conduit, generating a multiphase flow. The time scale of mechanical energy release was forced to be similar to that of the mechanical energy release during magma fragmentation experiments [29,30]. Accounting for the link between gas and particle acceleration with magma fragmentation in the conduit in real eruptions [24].…”