Geophysics

Modeling Magma Explosions

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Science  11 May 2001:
Vol. 292, Issue 5519, pp. 1023
DOI: 10.1126/science.292.5519.1023b

During a volcanic eruption, magma rises through a conduit toward the surface, decompressing as it rises, and the volatile components start to form bubbles. The magma will fragment and erupt explosively if the pressure from the growing bubbles exceeds the strength of the magma. Lensky et al. estimated when a rising magma could become explosive by modeling the radial viscosity profile in a spherical shell of melt that develops around a bubble. In their model, the magma contains a close-packed lattice of bubbles, and each bubble has a shell of melt that is oversaturated in water. As the magma rises, the melt becomes dehydrated as water vapor is transferred to the growing bubble. This process creates a radial gradient of the viscosity in the melt layer such that the inner edge of the shell has a higher viscosity. The higher viscosity allows the bubble to build up additional pressure that leads to magma fragmentation and explosive degassing. Although the bulk viscosity of the magma is not sufficient to create an explosive eruption, the higher effective viscosity of the melt around each bubble is. — LR

Earth Planet. Sci. Lett.186, 1 (2001).

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