Challenging dyke ascent models using novel laboratory experiments: Implications for reinterpreting evidence of magma ascent and volcanism

Janine L Kavanagh,Alec J Burns,Suraya Hilmi Hazim,Elliot P Wood,Simon A Martin,Sam Hignett,David J.C Dennis

doi:10.1016/j.jvolgeores.2018.01.002

Janine L Kavanagh, Alec J Burns + Show 5 more

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https://doi.org/10.1016/j.jvolgeores.2018.01.002

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Abstract

Volcanic eruptions are fed by plumbing systems that transport magma from its source to the surface, mostly fed by dykes. Here we present laboratory experiments that model dyke ascent to eruption using a tank filled with a crust analogue (gelatine, which is transparent and elastic) that is injected from below by a magma analogue (dyed water). This novel experimental setup allows, for the first time, the simultaneous measurement of fluid flow, sub-surface and surface deformation during dyke ascent. During injection, a penny-shaped fluid-filled crack is formed, intrudes, and traverses the gelatine slab vertically to then erupt at the surface. Polarised light shows the internal stress evolution as the dyke ascends, and an overhead laser scanner measures the surface elevation change in the lead-up to dyke eruption. Fluorescent passive-tracer particles that are illuminated by a laser sheet are monitored, and the intruding fluid's flow dynamics and gelatine's sub-surface strain evolution is measured using particle image velocimetry and digital image correlation, respectively. We identify 4 previously undescribed stages of dyke ascent. Stage 1, early dyke growth: the initial dyke grows from the source, and two fluid jets circulate as the penny-shaped crack is formed. Stage 2, pseudo-steady dyke growth: characterised by the development of a rapidly uprising, central, single pseudo-steady fluid jet, as the dyke grows equally in length and width, and the fluid down-wells at the dyke margin. Sub-surface host strain is localised at the head region and the tail of the dyke is largely static. Stage 3, pre-eruption unsteady dyke growth: an instability in the fluid flow appears as the central fluid jet meanders, the dyke tip accelerates towards the surface and the tail thins. Surface deformation is only detected in the immediate lead-up to eruption and is characterised by an overall topographic increase, with axis-symmetric topographic highs developed above the dyke tip. Stage 4 is the onset of eruption, when fluid flow is projected outwards and focused towards the erupting fissure as the dyke closes. A simultaneous and abrupt decrease in sub-surface strain occurs as the fluid pressure is released. Our results provide a comprehensive physical framework upon which to interpret evidence of dyke ascent in nature, and suggest dyke ascent models need to be re-evaluated to account for coupled intrusive and extrusive processes and improve the recognition of monitoring signals that lead to volcanic eruptions in nature.

Highlights

Most volcanic eruptions are fed by magma-filled fractures that propagate through the crust
Here we describe a new series of novel laboratory experiments where, for the first time, coupled magma flow and host rock deformation is studied during dyke initiation, propagation and eruption
Qualitative, polarised light is very useful to demonstrate the evolving stress field during dyke propagation using the photoelastic properties of gelatine

Summary

Introduction

Most volcanic eruptions are fed by magma-filled fractures that propagate through the crust. Crisp, 1984; Gudmundsson, 2002) and diverse geophysical, geochemical and geodetic techniques can be readily employed to monitor magma intrusion in the build up to and during volcanic crises, e.g. retrieving seismic profiles, recording volcanic gas emissions, and detecting surface changes in elevation as the magma-filled fractures grow Kavanagh and Sparks, 2011). Studying dyke propagation in nature is challenging as only remote measurements can be made during an intrusive event. The 2014 dyke intrusion associated with the Bardarbunga volcanic system (e.g. Sigmundsson et al, 2014; Gudmundsson et al, 2014) is arguably the most detailed study of an individual dyking event that led to a fissure eruption. Along with studies of eruptive episodes on Etna, Italy (e.g. González and Palano, 2014, Kahl et al, 2011) and Kilauea, Hawaii (e.g. Cervelli et al, 2002) for example, this has done much to shape and inform our understanding of the geophysical signals of magma intrusion that leads to eruption

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