Flow and fracturing of viscoelastic media under diffusion-driven bubble growth: An analogue experiment for eruptive volcanic conduits

doi:10.1016/j.epsl.2006.01.011

Earth and Planetary Science Letters

Volume 243, Issues 3–4, 30 March 2006, Pages 771-785

https://doi.org/10.1016/j.epsl.2006.01.011 Get rights and content

Abstract

To visualize the behavior of erupting magma in volcanic conduits, we performed shock tube experiments on the ductile–brittle response of a viscoelastic medium to diffusion-driven bubble expansion. A sample of shear-thinning magma analogue is saturated by gas Ar under high pressure. On rapid decompression, Ar supersaturation causes bubbles to nucleate, grow, and coalesce in the sample, forcing it to expand, flow, and fracture. Experimental variables include saturation pressure and duration, and shape and lubrication of the flow path. Bubble growth in the experiments controls both flow and fracturing, and is consistent with physical models of magma vesiculation. Two types of fractures are observed: i) sharp fractures along the uppermost rim of the sample, and ii) fractures pervasively diffused throughout the sample. Rim fractures open when shear stress accumulates and strain rate is highest at the margin of the flow (a process already inferred from observations and models to occur in magma). Pervasive fractures originate when wall-friction retards expansion of the sample, causing pressure to build-up in the bubbles. When bubble pressure overcomes wall-friction and the tensile strength of the porous sample, fractures open with a range of morphologies. Both types of fracture open normally to flow direction, and both may heal as the flow proceeds. These experiments also illustrate how the development of pervasive fractures allows exsolving gas to escape from the sample before the generation of a permeable network via other processes, e.g., bubble coalescence. This is an observation that potentially impact the degassing of magma and the transition between explosive and effusive eruptions.

Introduction

During volcanic eruptions, the liberation of volatiles through vesiculation can generate contrasting physico-chemical behaviors of the enclosing magma. Amongst the physical changes, fast volume increase is in most decompressive cases the most relevant, generating a dramatic increase in the rate of deformation of the magma and an acceleration of the processes that often lead to an explosive eruption. Here we investigate the expansion, flow and fragmentation of magma using an analogue material which undergoes rapid vesiculation. In particular, we use a viscoelastic magma analogue to simulate the rate-dependent viscosity of magma, and we investigate the specific case of bubble growth driven by strong supersaturation.

Despite the long-standing acknowledgement of its central role in explosive volcanism, the investigation of nucleation, growth, and coalescence of gas bubbles in magma (magma vesiculation) is still providing new insights into eruptive phenomena. Recent examples come from three aspects of magma dynamics in volcanic conduits: flow, fragmentation, and degassing. Firstly, complex rheology controls how magma flows in volcanic conduits: the abundance, and size and shape distribution of bubbles strongly affect the rheology of magma [1], [2], [3], [4], [5], [6] and its resulting flow profile in volcanic conduits [7]. Secondly, bubble growth in magma forces the liquid phase to deform differentially at small- and large-scales (i.e., around each bubble vs. up the volcanic conduit). At present it is still unclear if, during explosive eruptions, viscoelastic magma fragments in response to the small- or large-scale deformation and stress accumulation [8], [9], [10]. Moreover, total porosity, thickness of bubble walls and bubble pressure likely control magma fragmentation during both steady and unsteady fast decompression events [11], [12], [13], [14], [15]. Thirdly, bubble coalescence can eventually cause magma to become permeable and, via degassing, to erupt effusively instead of explosively [16], [17], [18] or less explosively [19]; coalescence and permeability also affect the final texture and emplacement mode of pyroclasts [e.g., 20].

Published experimental investigations on magma vesiculation used either silicate melts (remelted rock or synthetic) at magmatic temperature or analogues (including water, gum–rosin solutions, and silicon and other polymers) at ambient or lower temperature. The former are best suited for nucleation, growth, and coalescence experiments that involve relatively small strain and strain rates of the sample [e.g., [21], [22], [18]] and the latter are best suited for highly non-equilibrium bubble expansion/fragmentation experiments at higher strain and strain rates [e.g., [23], [24], [25], [26]]. In particular, this last group of experiments does not aim at rigorous scaling of natural processes, but, as noted by [27], represents “a tool to identify and investigate the fundamental processes and interactions operating within the flows”. Within this frame we present a new type of analogue experiment on magma vesiculation and fragmentation that can be used to investigate many of the vesiculation-related processes mentioned above. A novel point of the experiment is the combination of a viscoelastic magma analogue, which has a rate-dependent rheological behavior more similar to magma [28], with the diffusion-driven nucleation, growth, and coalescence of bubbles.

Section snippets

Rheology of the magma analogue

Our magma analogue is a silicon polymer named “Changeable Silly Putty® ”. It is viscoelastic and solvent of Ar gas depending on pressure. We used a forced oscillation rheometer to measure sample rheology within the linear viscoelastic region of response (stress range from 150.7 to 3037.9 Pa, peak strain of 20%) at 25 °C. The polymer is shear-thinning, its viscosity η decreasing from 5 × 10⁴ to 1 × 10³ Pa s on the strain timescale 10³–10^− 2 s, and its relaxation time τ is 0.2 s (Fig. 1). To evaluate

Overview of experimental phenomena

Upon opening the diaphragms, pressure in the chamber drops to atmospheric value, the sample is suddenly supersaturated with Ar, and bubbles nucleate and grow. Bubble growth forces the sample to expand and flow upward in the chamber (Fig. 3a). As the flow starts, contact between sample and chamber is uniform, likely provided by micron-sized bubble walls touching the chamber. During the flow, fractures open in the sample with variable geometry and timing, and, short after opening, often the

Inferences on the vesiculation and fracturing of magma

Compared with previous analogue experiments on magma flow and fragmentation, the present experiments fall dynamically between those of [25], who observed brittle fracturing but very limited deformation of a similar, viscoelastic analogue during rapid decompression, and those of [24], showing very large and rapid expansion of the sample but involving ductile, surface-tension-controlled fragmentation of their aqueous magma analogue. The most similar experiments are those of [23] that created

Further inferences on eruptive conduit processes

During experimental flow, rim fractures accommodate the local velocity differentials and cause the shear strain rate at the flow margin to drop. At this point the sample relaxes, shifting back to ductile behavior, and the fractures starts to heal. [34] describes how cycles of fracturing and healing in a rhyolitic magma produced tuffisite textures and flow banding, and shows that this mechanism may explain hybrid earthquakes observed during effusive silicic eruptions. They attribute the cracking

Conclusions

Vesiculation experiments with a viscoelastic magma analogue illuminate some of the processes that previous models theorize to occur in magma flowing along an eruptive conduit. In particular, the experiments outline the potential role of friction between magma and conduit walls in controlling the motion, fragmentation, and degassing of the magmatic flow. We observe the following processes relevant for volcanic eruptions: i) brittle fracturing repeatedly interrupting ductile flow; ii) rim

Acknowledgements

This paper benefited from fruitful discussions with Hugh Tuffen, Ulli Küppers, Betty Scheu, Basti Müller, and Jon Castro, and inspiring comments from Ed Llewellin and an anonymous reviewer. Alfonso Fernández Davila (a.k.a. Fon) and Ben Kennedy (a.k.a. Benutzen) provided substantial help in and out of the laboratory. Prof. Nikolai Petersen of the Geophysics Section, LMU, kindly provided the high speed camera, and Prof. Nino Grizzuti of the University of Naples “Federico II” gently provided the

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Flow and fracturing of viscoelastic media under diffusion-driven bubble growth: An analogue experiment for eruptive volcanic conduits

Abstract

Introduction

Section snippets

Rheology of the magma analogue

Overview of experimental phenomena

Inferences on the vesiculation and fracturing of magma

Further inferences on eruptive conduit processes

Conclusions

Acknowledgements

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

Earth Planet. Sci. Lett.

J. Volcanol. Geotherm. Res.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

Earth Planet. Sci. Lett.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

The rheology of a bubbly liquid

Proc. R. Soc. Lond., A

The constitutive equation and flow dynamics of bubbly magmas

Geophys. Res. Lett.

Volcanic dilemma: flow or blow?

Science

Strain-induced magma fragmentation in explosive eruptions

Nature

A criterion for the fragmentation of bubbly magma based on brittle failure theory

Nature