Preprints
https://doi.org/10.5194/se-2021-127
https://doi.org/10.5194/se-2021-127

  20 Oct 2021

20 Oct 2021

Review status: this preprint is currently under review for the journal SE.

Transient conduit permeability controlled by a shift between compactant shear and dilatant rupture at Unzen volcano (Japan)

Yan Lavallée1, Takahiro Miwa2, James D. Ashworth1, Paul A. Wallace1,3, Jackie E. Kendrick1,4, Rebecca Coats1, Anthony Lamur1, Adrian Hornby5, Kai-Uwe Hess6, Takeshi Matsushima7, Setsuya Nakada8, Hiroshi Shimizu8, Bernhard Ruthensteiner9, and Hugh Tuffen10 Yan Lavallée et al.
  • 1Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool, United Kingdom
  • 2Earthquake Research Department, National Research Institute for Earth Science and Disaster Resilience (NIED), Tsukuba, Japan
  • 3Department of Geosciences, Environment and Society, Université Libre de Bruxelles, Brussels, Belgium
  • 4Geosciences, University of Edinburgh, Edinburgh, United Kingdom
  • 5Earth and Atmospheric Sciences, Cornell University, United States of America
  • 6Earth and Environmental Sciences, Ludwig-Maximilians University of Munich, Germany
  • 7Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara, Nagasaki, Japan
  • 8National Research Institute for Earth Science and Disaster Resilience, Tennodai, Tsukuba, 305-0006, Japan
  • 9Staatliche Naturwissenschaftliche Sammlungen Bayerns (SNSB), Zoologische Staatssammlung München, München, Germany
  • 10Earth Sciences, University of Lancaster, United Kingdom

Abstract. The permeability of magma in shallow volcanic conduits controls the fluid flow and pore pressure development that regulates gas emissions and the style of volcanic eruptions. The architecture of the permeable porous structure is subject to changes as magma deforms and outgasses during ascent. Here, we present a high-resolution study of the permeability distribution across two conduit shear zones (marginal and central) developed in the dacitic spine that extruded towards the closing stages of the 1991–1995 eruption at Unzen volcano, Japan. The marginal shear zone is approximately 3.2 m wide and exhibits a 2-m wide, moderate shear zone with porosity and permeability similar to the conduit core, transitioning into a ~1-m wide, highly-sheared region with relatively low porosity and permeability, and an outer 20-cm wide cataclastic fault zone. The low porosity, highly-sheared rock further exhibits an anisotropic permeability network with slightly higher permeability along the shear plane (parallel to the conduit margin) and is locally overprinted by oblique dilational Riedel fractures. The central shear zone is defined by a 3-m long by ~9-cm wide fracture ending bluntly and bordered by a 15–40 cm wide damage zone with an increased permeability of ~3 orders of magnitude; directional permeability and resultant anisotropy could not be measured from this exposure.

We interpret the permeability and porosity of the marginal shear zone to reflect the evolution of compactional (i.e., ductile) shear during ascent up to the point of rupture, estimated by Umakoshi et al. (2008), at ~500 m depth. At this point the compactional shear zone would have been locally overprinted by brittle rupture, promoting the development of a shear fault and dilational Riedel fractures during repeating phases of increased magma ascent rate, enhancing anisotropic permeability that channels fluid flow into, and along, the conduit margin. In contrast, we interpret the central shear zone as a shallow, late-stage dilational structure, which partially tore the spine core with slight displacement. We explore constraints from monitored seismicity and stick-slip behaviour to evaluate the rheological controls, which accompanied the upward shift from compactional toward dilational shear as magma approached the surface, and discuss their importance in controlling the permeability development of magma evolving from overall ductile to increasingly brittle behaviour during ascent and eruption.

Yan Lavallée et al.

Status: open (until 10 Dec 2021)

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  • RC1: 'Comment on se-2021-127', Michael Heap, 20 Oct 2021 reply

Yan Lavallée et al.

Yan Lavallée et al.

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Short summary
Volcanic eruptions are controlled by the presence of gas bubbles in magma, which, in excess, can cause explosions. Eruption models lack an understanding of how gas percolates in magma flowing in a conduit. Here we study gas percolation in magma associated with the 1994–95 eruption at Mt. Unzen, Japan. The results show that the pathways for gas escape depend on the depth and ascent rate of magma. Pathways closed at depth but opened along fractures when magma ascended rapidly near the surface.