Bubble growth in visco-elastic magma: implications to magma fragmentation and bubble nucleation |
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| Authors: | Kurzon Ittai Lyakhovsky Vladimir Navon Oded |
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| Institution: | (1) The Fredy and Nadine Herrmann Institute of Earth Sciences, The Hebrew University, Jerusalem, 91904, Israel;(2) The Geological Survey of Israel, 30 Malkhe Israel St., Jerusalem, 95501, Israel |
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| Abstract: | We present a visco-elastic bubble growth model, accounting for viscous and elastic deformations and for volatile mass transfer
between bubbles and melt. We define the borders between previous bubble growth models accounting for incompressible viscous
melt, and our new model accounting also for elastic deformation; this is done by a set of end-member analytical solutions
and numerical simulations. Elastic deformation is most prominent for magma of small vesicularity, where the growth regime
depends on the shear modulus. For high shear modulus, bubble growth is slow and follows an exponential law in a viscous growth
regime, while for low shear modulus bubbles quickly follow a square-root diffusive solution. Our model provides all the elastic
components (stresses, strains and strain rates) required for defining criteria for failure and magma fragmentation. We suggest
two failure criteria, a stress related one based on the internal friction and the Mohr-Coulomb failure theory, and a strain
related one based on fibre elongation experiments. We argue that both criteria are equivalent if we consider their shear modulus
dependency and its effect on magma rheology. Last, we apply our model to the process of bubble nucleation. In the incompressible
case, following nucleation, growth is slow and leads to long incubation times during which bubbles may be dissolved back into
the melt. The elastic response in magmas with low shear modulus results in a short incubation time, increasing the probability
of survival. The above effects emphasize the significance of visco-elasticity for the dynamic processes occurring in magmas
during volcanic activity. |
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