Fragmentation of magma during sub-steady phases of volcanic eruptions is related to the achievement of sufficiently large viscosity and rates of strain. This holds for either viscous-to-elastic transition or gas bubble overpressure as a fragmentation criterion. When the viscosity of magma is sufficiently large, a feedback effect is established in the conduit pushing conditions into a region of very large gradients of all flow variables and magma properties and culminating with magma fragmentation. We measured the viscosity of natural silicate liquids from basalt to phonolite to trachyte to rhyolite, and found a correspondence between viscosity and dominant eruptive style. Trachytes have hydrous viscosities nearly as large as those of rhyolites, and comparably dominant explosive style. Phonolites have viscosities intermediate between those of basalt and trachyte. Accordingly, the eruptive style at phonolitic volcanoes like Vesuvius and Teide is either effusive or explosive. In order to investigate conditions leading to fragmentation of phonolitic magma, we performed numerical simulations of magma ascent and fragmentation with the steady, 1D, multiphase non-equilibrium CONDUIT4 code. Conditions pertaining to the AD 1631 eruption of Vesuvius have been selected. Due to relatively low viscosity of liquid magma, fragmentation is only encountered if the crystal content is larger than about 40 vol.%, as for the case of the investigated eruption, or the elastic modulus of magma is significantly less than commonly assumed. Two end-member explanations for fragmentation of phonolitic magma are proposed, one which can explain fragmentation of crystal-rich phonolitic magma, the other which could also explain fragmentation of crystal-poor phonolites but requires additional experimental work to determine the elastic properties of phonolites.
Giordano, D., Polacci, M., Papale, P., Dingwell, D.B. (2004). The problem of fragmentation of low viscosity phonolitic magmas.
The problem of fragmentation of low viscosity phonolitic magmas
GIORDANO, Daniele;
2004-01-01
Abstract
Fragmentation of magma during sub-steady phases of volcanic eruptions is related to the achievement of sufficiently large viscosity and rates of strain. This holds for either viscous-to-elastic transition or gas bubble overpressure as a fragmentation criterion. When the viscosity of magma is sufficiently large, a feedback effect is established in the conduit pushing conditions into a region of very large gradients of all flow variables and magma properties and culminating with magma fragmentation. We measured the viscosity of natural silicate liquids from basalt to phonolite to trachyte to rhyolite, and found a correspondence between viscosity and dominant eruptive style. Trachytes have hydrous viscosities nearly as large as those of rhyolites, and comparably dominant explosive style. Phonolites have viscosities intermediate between those of basalt and trachyte. Accordingly, the eruptive style at phonolitic volcanoes like Vesuvius and Teide is either effusive or explosive. In order to investigate conditions leading to fragmentation of phonolitic magma, we performed numerical simulations of magma ascent and fragmentation with the steady, 1D, multiphase non-equilibrium CONDUIT4 code. Conditions pertaining to the AD 1631 eruption of Vesuvius have been selected. Due to relatively low viscosity of liquid magma, fragmentation is only encountered if the crystal content is larger than about 40 vol.%, as for the case of the investigated eruption, or the elastic modulus of magma is significantly less than commonly assumed. Two end-member explanations for fragmentation of phonolitic magma are proposed, one which can explain fragmentation of crystal-rich phonolitic magma, the other which could also explain fragmentation of crystal-poor phonolites but requires additional experimental work to determine the elastic properties of phonolites.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
