Mafic volcanic activity is dominated by effusive to mildly explosive eruptions. Plinian and ignimbrite-forming mafic eruptions, while rare, are also possible; however, the conditions that promote such explosivity are still being explored. Eruption style is determined by the ability of gas to escape as magma ascends, which tends to be easier in low-viscosity, mafic magmas. If magma permeability is sufficiently high to reduce bubble overpressure during ascent, volatiles may escape from the magma, inhibiting violent explosive activity. In contrast, if the permeability is sufficiently low to retain the gas phase within the magma during ascent, bubble overpressure may drive magma fragmentation. Rapid ascent may induce disequilibrium crystallization, increasing viscosity and affecting the bubble network with consequences for permeability, and hence, explosivity. To explore the conditions that promote strongly explosive mafic volcanism, we combine microlite textural analyses with synchrotron x-ray computed microtomography of 10 pyroclasts from the 12.6 ka mafic Curacautín Ignimbrite (Llaima Volcano, Chile). We quantify microlite crystal size distributions (CSD), microlite number densities, porosity, bubble interconnectivity, bubble number density, and geometrical properties of the porous media to investigate the role of magma degassing processes at mafic explosive eruptions. We use an analytical technique to estimate permeability and tortuosity by combing the Kozeny-Carman relationship, tortuosity factor, and pyroclast vesicle textures. The groundmass of our samples is composed of up to 44% plagioclase microlites, > 85% of which are < 10 µm in length. In addition, we identify two populations of vesicles in our samples: (1) a convoluted interconnected vesicle network produced by extensive coalescence of smaller vesicles (> 99% of pore volume), and (2) a population of very small and completely isolated vesicles (< 1% of porosity). Computed permeability ranges from 3.0 × 10−13to 6.3 × 10−12m2, which are lower than the similarly explosive mafic eruptions of Tarawera (1886; New Zealand) and Etna (112 BC; Italy). The combination of our CSDs, microlite number densities, and 3D vesicle textures evidence rapid ascent that induced high disequilibrium conditions, promoting rapid syn-eruptive crystallization of microlites within the shallow conduit. We interpret that microlite crystallization increased viscosity while simultaneously forcing bubbles to deform as they grew together, resulting in the permeable by highly tortuous network of vesicles. Using the bubble number densities for the isolated vesicles (0.1-3−3 × 104 bubbles per mm3), we obtain a minimum average decompression rate of 1.4 MPa/s. Despite the textural evidence that the Curacautín magma reached the percolation threshold, we propose that rapid ascent suppressed outgassing and increased bubble overpressures, leading to explosive fragmentation. Further, using the porosity and permeability of our samples, we estimated that a bubble overpressure > 5 MPa could have been sufficient to fragment the Curacautín magma. Other mafic explosive eruptions report similar disequilibrium conditions induced by rapid ascent rate, implying that syn-eruptive disequilibrium conditions may control the explosivity of mafic eruptions more generally.
Conduit models of volcanic eruptions simulate magma evolution through phase transitions and material changes during ascent. We present a time‐dependent one‐dimensional model of a chamber‐conduit system to examine the temporal evolution of dome‐forming eruptions. As magma ascends, volatiles exsolve and may escape vertically through the column or laterally through the conduit walls. Magma solidifies which increases viscosity, leading to a natural transition from viscous flow at depth to frictional sliding along the conduit walls near the surface, resulting in the extrusion of a semisolid plug. The model evaluates time‐ and depth‐dependent pressure, velocity, porosity, and relative amounts of exsolved water to carbon dioxide. Transient effects arise when magma outflux from the chamber appreciably decreases pressure over the magma ascent timescale. For low magma permeability, transient effects increase porosity and velocity relative to steady‐state solutions. For high magma permeability, efficient vertical and lateral gas escape depresses porosity and velocity at later times. We use the model to predict three time series data sets from the 2004–2008 eruption of Mount St. Helens: extruded volume, ground deformation, and carbon dioxide emissions. We quantify sensitivity of model predictions to input parameters using the distance‐based generalized sensitivity analysis. Chamber volatile content, volume, and excess pressure influence the amplitude of observables, while conduit radius, frictional rate dependence and magma permeability influence temporal evolution. High magma permeability can cause marked departures from exponentially decaying flux and may explain the unique temporal evolution of deformation observed at the only nearby continuous GPS station in operation at the eruption onset.
more » « less- NSF-PAR ID:
- 10455146
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 124
- Issue:
- 11
- ISSN:
- 2169-9313
- Page Range / eLocation ID:
- p. 11251-11273
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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