The volcanic ash problem

Explosive volcanic eruptions are the result of intensive magma and rock fragmentation, and they produce volcanic ash, which consists of fragments < 2 min in average diameter. The problem with volcanic ash is that its formation is poorly understood from the standpoint of eruption energetics. Because the source of explosive eruption energy should be the thermal energy of magma, and because an explosion requires rapid conversion of energy into a mechanical form, and because of the physical properties of magma thermal energy is dominantly released by conduction, the energy release on a short time scale (explosion) in volcanic processes has to be the result of a special mechanism, probably a positive feedback mechanism of fragmentation and heat exchange. In fact, the most explosive volcanic explosions are characterized by the most intensive fragmentation. In any fragmentation mechanism the generated particle sizes reflect the kinetic energy available (i.e. the fragmentation energy density). Consequently, fine ash (less than or equal to 64 mum) provides information on fragmentation mechanisms that are the most energetic and related to the highest explosive energy release. In this letter we discuss mechanisms of formation of fine volcanic ash, using experimental results, theoretical considerations, and field observations. We focus on the potency of these mechanisms to explain fine ash produced by explosive volcanism. We conclude that quantitative analysis of fine ash particles is necessary to estimate the mechanical energy of volcanic explosions. (C) 2002 Elsevier Science B.V. All rights reserved.