The permeability of magma in 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
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, which was estimated by Umakoshi et al. (2008) at
The style and timing of activity exhibited during a volcanic eruption are strongly influenced by the presence and mobility of volatiles in magma (Sparks, 1997; Woods and Koyaguchi, 1994) and the surrounding conduit wall rock (Jaupart and Allègre, 1991). During magma ascent, volatiles are exsolved into gas bubbles (Navon et al., 1998; Sparks, 2003) as their solubility decreases with decompression (Liu et al., 2005), crystallisation (Tait et al., 1989), and heat generated by crystallisation (Blundy et al., 2006) and shear (Lavallée et al., 2015). This causes the accumulation of pressurised fluids in vesicles that charges ascending magma, which, if sufficient, may lead to fragmentation (Mueller et al., 2008; Alidibirov and Dingwell, 1996) and an explosive eruption (Sahagian, 1999). The development of a permeable network governs outgassing (Edmonds et al., 2003), pore pressure release (Mueller et al., 2005), and eruptive cyclicity (Michaut et al., 2013), thereby reducing the potential for explosive activity (Klug and Cashman, 1996) and encouraging effusion (Edmonds and Herd, 2007; Eichelberger et al., 1986; Degruyter et al., 2012). Lava dome eruptions – the topic of this study – commonly switch between effusive and explosive modes of activity due to this competition between permeability, pore fluid pressure, and the structural integrity of magma (Melnik and Sparks, 1999; Calder et al., 2015; Cashman and Blundy, 2000; Castro and Gardner, 2008; Edmonds et al., 2003; Lavallée et al., 2013, 2012; Sparks, 1997; Holland et al., 2011; Kendrick et al., 2016; Platz et al., 2012). Considering the water solubility–pressure relationships (Zhang, 1999), permeability–porosity relationships in magma (Westrich and Eichelberger, 1994), and eruptive patterns (Edmonds et al., 2003), it has been suggested that much of the outgassing during lava dome eruptions occurs in the upper few kilometres of the conduit (Westrich and Eichelberger, 1994; Edmonds et al., 2003). This observation is corroborated by rapid shallowing of seismicity leading to explosions (e.g. Rohnacher et al., 2021), and the existence of shallow long-period seismic signals resulting from resonance in fractures and faults (Chouet, 1996; Matoza and Chouet, 2010) as fluids are channelled to the surface (Holland et al., 2011; Kendrick et al., 2016; Gaunt et al., 2014; Nakada et al., 1995; Newhall and Melson, 1983; Pallister et al., 2013b; Sahetapy-Engel and Harris, 2009; Sparks, 1997; Sparks et al., 2000; Edmonds et al., 2003; Varley and Taran, 2003; Stix et al., 2003). Therefore, understanding the evolution of the permeable network during eruptive shearing is central to constraining the evolution of magmatic systems in the shallow crust (Blower, 2001).
Close examination of the architecture of shallow dissected conduits and structures in vent-proximal silicic lavas exposes complex shearing histories that would impact the permeable porous network of erupting magma. These structures reveal porosity contrasts through the lavas, and strain localisation near the conduit margins is commonly identified via the presence of flow bands and variably porous shear zones with a spectrum of configurations (Gaunt et al., 2014; Kendrick et al., 2012; Kennedy and Russell, 2012; Pallister et al., 2013a; Smith et al., 2001; Stasiuk et al., 1996; Tuffen and Dingwell, 2005); these are features that are preserved to differing extents in crystal-poor and crystal-rich magmas (Calder et al., 2015; Lavallée and Kendrick, 2021). For example, crystal-poor obsidian in dissected conduits and dykes commonly exhibits marginal flow bands, showing alternation between glassy, finely crystalline, and microporous bands (Gonnermann and Manga, 2007). Flow bands also occur as variably sintered, cataclastic breccia layers, resulting from fracture and healing cycles (Tuffen and Dingwell, 2005; Tuffen et al., 2003), and as variably sintered tuffisite layers, resulting from fragmentation and entrapment of fragments into narrow fractures (Castro et al., 2012; Heiken et al., 1988; Kendrick et al., 2016; Kolzenburg et al., 2012). Exposed crystal-poor conduits, dykes, and domes are commonly dense, as the porous network may easily collapse (unlike crystal-rich lavas; e.g. Ashwell et al., 2015). The collapse of the porous network occurs as eruptions wane and pore pressure is insufficient to counteract surface tension as well as local magmastatic and lithostatic stresses (Kennedy et al., 2016; Wadsworth et al., 2016), a process which hinders interpretation of the syn-eruptive permeable structure of crystal-poor magma from the study of large-scale relict formations. Studies of erupted crystal-poor pumices (which quench rapidly) help provide constraints on the extent of magma permeability at the point of fragmentation (Wright et al., 2006), but the task of reconstructing the permeable architecture of an entire conduit from these pyroclasts is challenging (Dingwell et al., 2016) and further complicated by post-fragmentation vesiculation (Browning et al., 2020) as well as vesicle relaxation (Rust and Manga, 2002), and it therefore remains to be attempted systematically.
Crystal-rich volcanic rocks (the subject of this study) expose a wider range of permeable porous structures (Farquharson et al., 2015; Mueller et al., 2005; Klug and Cashman, 1996; Lamur et al., 2017; Kushnir et al., 2016; Ryan et al., 2020; Kendrick et al., 2021). These rocks frequently share common characteristics and evidence that crystal-rich magmas preferentially shear and accumulate damage near the conduit margins, defined by flow bands and/or cataclastic fault zones, adjacent to brecciated wall rocks (Sparks et al., 2000; Hale and Wadge, 2008; Watts et al., 2002). For instance, dacitic volcanic spines extruded in 2004–2008 at Mount St. Helens (USA) and in 1994–1995 at Unzen volcano (Japan) reveal the presence of a complex “damage halo” near the conduit margin (Calder et al., 2015; Gaunt et al., 2014; Pallister et al., 2013a; Smith et al., 2001; Kendrick et al., 2012; Wallace et al., 2019). Shear zones at Mount St. Helens (Gaunt et al., 2014) and at Chaos Crags, Lassen volcano (Ryan et al., 2020), showed increased porosity and permeability, as well as the development of permeability anisotropy towards the conduit margin, thus describing scenarios in which shearing of dense, crystal-rich magma induced dilation. In the case of Mount St. Helens, in the later Spine 7, the fault zone is defined by the presence of a pseudotachylyte (Kendrick et al., 2012), a feature which can reduce the permeability of shear zones in magmas (Kendrick et al., 2014a). At Unzen volcano, Smith et al. (2001) qualitatively described the character of the shear zone developed in the centre of the lava spine at Mount Unzen, highlighting the presence of a dilational cavity associated with shearing in the core of the magmatic column. However, they did not quantify any porosity–permeability relationships. The cavity (hereafter termed “central shear zone”) was defined by an area in which the groundmass was torn, producing pore spaces in the shadow of phenocrysts. The margin of the Unzen spine also hosts a spectrum of shear textures (Hornby et al., 2015; Wallace et al., 2019), and significant low-frequency seismicity during the eruption indicated flushing of fluids in the marginal fault zone (Lamb et al., 2015). Thus, the study of evolving monitored signals and eruptive products at Unzen depicts a wide range of outgassing pathways, which evolve during the course of magma ascent and lava dome eruptions.
Several studies have explored the permeability evolution of volcanic
materials, but due to the occurrence of many influential structural and
petrological processes in shallow volcanic conduits, no solutions yet
encompass the complete history of magma permeability during volcanic
eruptions, especially its time- and strain-dependent evolution. Following
nucleation and growth, bubbles interact and coalesce beyond a certain
vesicularity, termed the percolation threshold, promoting the onset of fluid
flow through a connected bubble network (Baker et al., 2012; Eichelberger
et al., 1986; Rust and Cashman, 2004; Burgisser et al., 2017). The porosity
of the percolation threshold varies widely (between
In recent decades, laboratory measurements have helped us gain a first-order constraint on the permeability–porosity relationships of volcanic products (Eggertsson et al., 2020; Mueller et al., 2005; Acocella, 2010; Rust and Cashman, 2011; Colombier et al., 2017; Farquharson et al., 2015; Klug and Cashman, 1996). These suggest a nonlinear increase in permeability with porosity; yet, depending on the nature of the porous network as influenced by eruptive history, the permeability of rocks with a given porosity may vary by up to 4–5 orders of magnitude. Controlled laboratory experiments have given us insights on probable permeability trends of magma subjected to different stress, strain, and temperature conditions (Ashwell et al., 2015; Kendrick et al., 2013; Lavallée et al., 2013; Okumura et al., 2012, 2006; Shields et al., 2014), but a complete description of the dynamic permeability of deforming magma requires in operando determination under controlled conditions, which remain scarce (Gaunt et al., 2016; Kushnir et al., 2017b; Wadsworth et al., 2017, 2021); studies have shown that surface tension and/or low-strain-rate conditions under positive effective pressure (i.e. confining pressure greater than pore pressure) promote compaction and reduce permeability. These informative descriptions require further inputs to enable robust relationships with magma rheology as influenced by the presence and configuration of bubbles. Shallow magmas contain bubbles and crystals and exhibit a non-Newtonian rheology (Caricchi et al., 2007; Lavallée et al., 2007; Lejeune et al., 1999; Lejeune and Richet, 1995; Kendrick et al., 2013; Coats et al., 2018) that favours the development of strain localisation, in particular by preferentially deforming pore space (Kendrick et al., 2013; Okumura et al., 2010; Shields et al., 2014; Pistone et al., 2012; Mader et al., 2013). As magma shears, the porous network adopts a new configuration reflecting the stress conditions and magma viscosity (Rust et al., 2003; Wright and Weinberg, 2009), which influences the permeability (Ashwell et al., 2015; Kendrick et al., 2013; Okumura et al., 2010, 2009, 2006, 2008, 2013). Shearing may increase or decrease the permeability depending on the applied stress, strain and porosity of the deforming material, and direction of the permeability measurement due to the development of anisotropy (Ashwell et al., 2015; Kendrick et al., 2013). In cases of extreme shear, magma may rupture, thereby increasing pore connectivity and permeability (Laumonier et al., 2011; Lavallée et al., 2013; Okumura et al., 2013) until the fracture heals via diffusion (Okumura and Sasaki, 2014; Tuffen et al., 2003; Lamur et al., 2019; Yoshimura and Nakamura, 2010), seals via secondary mineralisation (Heap et al., 2019; Ball et al., 2015), or infills with tuffisitic material (Castro et al., 2012; Kendrick et al., 2016; Kolzenburg et al., 2012; Tuffen and Dingwell, 2005), which may densify through time (Kendrick et al., 2016; Vasseur et al., 2013; Wadsworth et al., 2014; Farquharson et al., 2017). The densification of magma under isotropic stresses (due to surface tension) has been reconstructed using high-resolution X-ray computed tomography from synchrotron imaging, providing us with a first complete description of magma permeability evolution as a function of porosity. This indicates that densification intrinsically relates to the evolution of the size distribution and surface area of the connected pore space (Wadsworth et al., 2017, 2021). Nonetheless, a time- and strain-dependent description of the development of the porous network of shearing magma remains incomplete, and information must be sourced from our understanding of permeability evolution in deforming rocks.
In rock physics, the evolution of the porous network in deforming rocks has been extensively studied. In its simplest description, the modes of deformation differ at low and high effective pressures as rocks adopt brittle or ductile behaviour, respectively. These are defined as a macroscopic behaviour (not a mechanistic description), whereby “brittle” refers to the localisation of deformation leading to rupture, and “ductile” refers to the inability for rocks to localise strain during deformation (e.g. Rutter, 1986); see Lavallée and Kendrick (2020) and Heap and Violay (2021) for reviews of brittle and ductile deformation in volcanic materials. The key distinction between these two deformation modes is that brittle failure generally results in local dilation (i.e. the creation of porosity), whereas ductile deformation results in compaction of the porous network (Heap et al., 2015a). As a result, brittle (dilational) failure generally enhances the permeability of rocks (Heap and Kennedy, 2016; Lamur et al., 2017; Farquharson et al., 2016b), whereas ductile (compactional) deformation generally causes reduction in permeability (Heap et al., 2015a; Loaiza et al., 2012), though there are exceptions. Despite its crucial role in defining deformation mode in rock, the role of effective pressure in dictating the ductile and brittle modes of deformation has not been systematically mapped out for multiphase magmas; instead, we generally consider the effects of temperature and applied stress or strain rate (e.g. Lavallée et al., 2008) over that of stress distribution, as the deformability of magma imparts technical challenges to classic rock mechanics tests and permeability determinations (Kushnir et al., 2017b). We may thus anticipate some similarities between rock and magma deformation modes, whereby at high effective pressure, ductile deformation is favoured via compactant viscous flow or even cataclastic flow (if strain rates are high enough to cause pervasive fracturing of bubble walls), causing porosity and permeability reduction; at low effective pressure, viscous flow may promote compaction at low strain rates, whereas dilation may ensue if strain rate favours localised rupture (Lavallée and Kendrick, 2020). Similarly, embrittlement may take place if a porous magma efficiently compacts, shifting its properties from the ductile to brittle regime (Heap et al., 2015a). Across this viscous–brittle transition, magma rupture may be partial and end abruptly, leaving a blunt fracture tip (Hornby et al., 2019). Most, if not all, of the features observed in experimentally deformed rocks and lavas should be observable in shallow magmatic systems, hinging on a delicate balance between ductile and brittle deformation regimes; these would influence outgassing, prompting temporal and spatial variations in effective pressure. In this study, we examine the well-preserved, dacitic lava spine erupted in 1994–1995 at Unzen volcano to constrain the permeability of dilational and compactional shear zones that developed in the shallow volcanic conduit.
Unzen volcano is a stratovolcano located near the city of Shimabara on the
island of Kyushu, Japan (Fig. 1). The volcano underwent a 5-year period of
protracted dome growth, which threatened the surrounding population with the
occurrence of several thousand rockfalls and many pyroclastic flows, such as
the destructive event on 3 June 1991 that caused 43 fatalities.
Activity initiated in early 1990 with a series of phreatic explosions and
brief extrusion of a spine on 19 May; this was swiftly followed by
continuous growth of a lava dome until early 1995 (Nakada et
al., 1995). Between October 1994 and January 1995, the eruption concluded
with the extrusion of a spine through the dome surface (Fig. 1c). At the
dome surface, gas emissions focused along the spine-marginal faults
(Ohba et al., 2008). The dome products have a dacitic
composition and contain euhedral phenocrysts of plagioclase, amphibole, and biotite in
a groundmass containing microlites of plagioclase, amphibole, pyroxene, and
iron oxides (Nakada et al., 1995; Wallace et al., 2019). Petrological
constraints suggest that degassing initiated at a pressure of approximately
70–100 MPa, i.e. in the upper
Dome growth occurred in stages, forming 13 discrete lobes until
mid-July 1994. Growth was observed to be typically exogenous when effusion
rates were high and endogenous at effusion rates lower than
Location of the lava spine blocks and characteristics of the
marginal shear zone.
The 1994–1995 lava spine was investigated during two field campaigns in
November 2013 and May 2016. Close structural examination at different scales
forms the basis of this study along with porosity and permeability
measurements using field and laboratory equipment. Owing to the inclination
of the spine (extruded towards the east), large blocks ranging from 5 to 20 m wide are dislocated from the front of the in situ western main spine structure
(Fig. 2a, b). Here, we investigated two blocks that reveal a central shear
zone (CSZ) and marginal shear zone (MSZ) that developed in the spine. These
detached, yet fully intact, spine blocks were selected owing to their
contrasting shear textures that would have represented different positions
within the volcanic conduit during magma ascent and extrusion (i.e. central
vs. marginal), thus allowing assessment of syn-eruptive outgassing pathways.
The marginal shear zone (MSZ) block, located
Samples collected from the marginal shear zone were cut and cored parallel to the shear direction and perpendicular to the shear plane in order to constrain the anisotropy developed in shear zones. A total of eight thin sections (fluorescent dyed) were prepared for microtextural analysis (labelled A–H). For the largest samples (A, B, C, E, H; see Fig. 2c–d) a set of two to three cylindrical cores (two parallel and one perpendicular to shear plane) was prepared with a diameter of 26 mm and a length of 30 or 13 mm, depending on the size of the sample. Within the highly sheared sample B (Fig. 2c–d), which is directly adjacent to the fault and gouge zone, multiple sets of cores of 20 mm diameter were prepared, closely spaced, to obtain porosity and permeability determinations at a higher resolution across this defining part of the shear zone.
Two-dimensional (2D) analysis of the microstructures exhibited across the shear zones was carried out using a Leica DM2500P optical microscope in plane-polarised and ultraviolet (UV) light, as well as a Philips XL30 scanning electron microscope (SEM) in backscattered electron (BSE) mode, set at 20 kV and 10 mm working distance. For this purpose, representative features were imaged for each sample across the shear zone (Fig. 3).
Composite figure of the microtextural characteristics across the marginal shear zone consisting of a photograph of fresh surface textures, plane-polarised light (PPL) photomicrographs, ultraviolet (UV) light photomicrographs, and backscattered electron (BSE) images of the groundmass. Images of the fresh surface were taken following cutting the sample perpendicular to shear. The C fabric (red line) and S fabrics (dashed yellow line) are labelled in gouge, high shear, and moderate shear zones. The C fabric runs consistently parallel to the shear direction, while the S fabric is slightly inclined to variable degrees across the MSZ. Phenocrysts observed include plagioclase (P), amphibole (A), biotite (B), and quartz (Q). Green boxes on PPL photomicrographs show the location of the UV light images, which highlight the pore structures across the MSZ. On UV light images, two white arrows pointing away from each other show the location of fractures within the groundmass (samples G and H), single arrows point to large pores adjacent to large phenocryst (samples G and H), and two arrows pointing towards each other show compaction bands (their spacing represents the width of each band; samples B and C). In the BSE images, a porous diktytaxitic texture is prevalent across the shear zone, although in the high shear zones (samples B and C) these textures are impeded by low-porosity compaction bands that show strong crystal alignment, fractured crystals (FCs), and pulverised crystal bands (PCBs).
To further evaluate the architecture of the porous network in three
dimensions (3D), four samples collected across the shear zone were scanned
using a Phoenix Nanotom®M X-ray computed tomography scanner
to produce high-resolution reconstructions with a voxel size of 11.111
Each core was dried in an oven at 50
The prepared cores were jacketed with a Viton™ tube and inserted in a
hydrostatic cell from Sanchez Technologies to measure permeability and pore
volume as a function of pressure. The jacketed samples were externally
loaded using a Maximator® oil pump to various confining
pressures (
To measure the permeability of rocks in the central shear zone (CSZ; Fig. 1c) that could not be sampled for laboratory testing due to preservation
restrictions, we used a non-destructive, portable air permeameter (TinyPerm II) from New England Research, which estimates permeability by monitoring
pressure recovery rate from a vacuum based on the concept of transient
pulse permeability (Brace et al., 1968). The apparatus is
handheld and needs to be employed carefully to maintain a consistent seal
between the nozzle of the permeameter and rock surface throughout the
measurements (lasting up to a few tens of minutes). It may be used to
determine the permeability of rocks between approximately 10
The 1994–1995 spine structure at Mount Unzen is exposed in several large,
segmented blocks (Figs. 1c–d; 2a–b). A thorough structural description
of the main spine structure and subsidiary block (e.g. CSZ) can be found in
Smith et al. (2001); here we highlight the main features. The
lava spine is split into a few very large primary blocks
Our primary field location for this study was a 4.7 m wide block of the
spine, exposing the northern marginal shear zone consisting of gouge,
sheared lava and the spine core (Fig. 2c–d). The outcrop displayed mild
surface weathering in the form of a thin (micrometre size) veneer of unknown
precipitate on the rock surface (which was inclined at an angle of ca.
40
Tomographic reconstructions of four samples across the shear
zones:
Permeability of the marginal shear zone as a function of effective
pressure and direction to shear: measurements conducted
The spine core, termed low shear herein (
The moderate shear zone is approximately 2 m wide (Fig. 2c, d). In this
zone, we observed an increased fracturing of phenocrysts and changes in the
distribution of porosity. Scrutinising sample E via microscopy, we
observe that the phenocrysts, which rarely exceed 2 mm in size in this zone,
are commonly micro-fractured (Fig. 3). The vesicles are occasionally large (3 mm) and connected (Figs. 3, 4e–f), and while the vesicular texture remains
diktytaxitic (as in the low shear spine core), the vesicles in sample E
appear increasingly aligned and localised around phenocrysts as the
magnitude of shear increases towards the fault; similarly, the microlites
show increasing degrees of alignment (revealed by
undulose extinction angles; see Wallace et al., 2019). Thin bands (
The high shear zone is approximately 1 m wide (Fig. 2c, d) and marks the
beginning of microscopic and mesoscopic shear bands at a scale of the order of
a few millimetres nearly parallel to the direction of shear; these increase
in abundance and scale nearer the fault, especially within the final 0.1–0.2 m (see features denoted in Fig. 2c–d as well as enlarged in the inset). The
bands, which form a pervasive foliation (S), consist of elongate, white
porphyritic plagioclase lenses that are fractured and crenulated. The C–S fabrics
are parallel in this area. These porphyritic bands are flanked by
reddish-brown groundmass as well as thin, elongate biotite phenocrysts (see
sample B “fresh surface” in Fig. 3). The plagioclase and biotite commonly
exhibit a mineral fish texture. Under the microscope, we observe that the
biotite shows undulose extinction from crystal–plastic deformation
(see Wallace et al., 2019, for a detailed crystal
plasticity study). Intense banding (observed as faint lineations of reduced
porosity under UV light in the moderate shear zone; Fig. 3) is observed
adjacent to, and running parallel to, the fault–gouge contact. The bands
are up to
The fault zone hosts up to ca. 0.2 m thick gouge material (Fig. 2c, d). The
contact between the gouge and the high shear zone is generally sharp and
often planar, although we observed small embayments, especially along C–S
fabrics in the neighbouring high shear zone (Fig. 2d). (Note that the extent
of the gouge is not exposed equally across the outcrop as material was
likely lost during separation of this block from the main spine upon
eruption, so the surface does not reflect the contact geometry. This
material loss also led to obliteration of vestiges of a pseudotachylyte,
suggested by local partial melting textures presented by Wallace et al., 2019.) The gouge is typified by well-consolidated, fine-grained
cataclasite with some larger rounded clasts up to
The porosity of the rocks, determined via pycnometry, indicates variations between 8 % and 27 % across the shear zone and in the fault gouge; Fig. 5a displays the average of multiple measurements from the different cores prepared from each sample. The measurements indicate that the high shear zone generally holds slightly lower porosities than surrounding areas. Within the high shear zone (sample B) we measured significant variations in porosity ranging between 8 % and 15 % (at ambient conditions) due to flow bands (e.g. in sample B); yet, the coarseness of samples measured prevents accurate quantification of the highly spatially variable porosities observed in hand specimens.
When loading the samples (cored parallel to the spine extrusion direction) in the hydrostatic pressure vessel, we observed a nonlinear decrease in porosity of up to 4 % by increasing the effective pressure to 100 MPa (Fig. 5b). We observe a similar dependence of porosity on effective pressure for the coherent samples from the low, moderate, and (densest part of) high shear areas, with a slightly larger reduction in porosity with effective pressure in the initially most porous, high shear bands and the granular gouge sample (Fig. 5b).
The permeability of the rocks collected across the spine segment reveals a
There are considerable differences in the permeability parallel and perpendicular to the plane of shear (Fig. 3c, d) across the shear zone (Fig. 6a, b). In the high shear zone permeability was found to be higher in the plane of shear (i.e. parallel to the extrusion direction) than perpendicular to it, whereas in the moderate and low shear zones, as well as in the gouge, permeability was essentially isotropic. Anisotropy is cast here as a ratio between the permeability parallel and perpendicular to the shear plane (Fig. 6c). The anisotropy is most pronounced in the high shear zones, where, in one instance, the permeability ratio increases dramatically from 3 to over 7 times larger parallel than perpendicular to the shear plane with increasing confining pressure in a hydrostatic pressure vessel (Fig. 6c). In other samples, the anisotropy increase with pressure is less or even negligible, indicating the heterogenous nature of the high shear zone. This sensitivity to confinement is due to the presence of the distinct dense and porous bands in the sheared lava (Figs. 5b, 6); in the cores parallel to the shear plane, fluid can flow through porous bands from the top to bottom of the sample, whereas perpendicular to shear, fluids must pass through both and dense and porous bands to traverse the sample. Fluid flow in the denser areas will be dominated by channelling through narrow fractures (sub-horizontal in BSE images in samples B and C in Fig. 3), which are more susceptible to closure by increasing effective pressure than equant pores (e.g. Kendrick et al., 2021). Although this process occurs during confinement in both orientations, it only impacts permeability perpendicular to the shear direction and therefore contributes to enhanced anisotropy of permeability in banded shear fabrics under confinement (Kendrick et al., 2021).
The second feature of interest is the cavity exposed in the central shear zone block (Figs. 1c and 2a). This section of the spine has been described in detail by Smith et al. (2001); here, we review key aspects observed in the field as no samples were collected to conserve the exposure of this world-class feature. We only examined the rocks forming this structure and performed non-destructive, in situ testing.
The central shear zone (CSZ) is located near the centre of the spine core (Fig. 1c). Its primary feature is the presence of a porous cavity, which curves and pinches out (upward) from the end of a dominant, 9 cm wide fracture extending approximately 3 m in length (determined from the visible extent of the exposure). Unlike the aforementioned marginal shear zone, which displays an increased degree of shear towards the spine margin, the central shear zone exhibits an increase in shear towards the centre of the spine. From left to right (i.e. northward) in Fig. 7, we note an increase in aligned, bent, and broken phenocrysts as well as aligned shear bands (ostensibly parallel to the dominant fracture), fractures, and surface roughness, which terminates upon intersecting the end cavity; beyond this point, the rocks show no clear evidence of shear, including shear bands, elongate pores, or aligned crystals. This is evident in the field photograph (Fig. 7) as steeply inclined porous bands which end against the southern (i.e. left) side of the cavity; on the southern side the sheared lava exhibits a higher porosity than the surrounding undeformed rocks (although this could not be quantified in the field). Approximately 1 m above the pinched-out tip of the main cavity, we observe the presence of a secondary porous cavity (Fig. 1c inset) that is approximately 60 cm long and elongated parallel to the fracture that connects to the main cavity.
The permeability of the rocks in the central shear zone was measured along
three transects in two field campaigns (in November 2013 and May 2016) to
negate potential influence from variable degrees of water saturation of the
rocks at different times of year. Our field measurements are consistent with
one another. The permeability varies very little in the undeformed areas of
the outcrop (i.e. on the right-hand side of the fracture in Fig. 7) for all
transects, with an abrupt increase in permeability up to 3 orders of
magnitude in the 9 cm wide central cavity and elevated permeability in the
The contrasting permeability, porosity, and (micro)structural changes observed across the marginal and central shear zones reveal the impact of shear and distinct modes of magma deformation during shallow conduit ascent. Here we interpret each of these key features for the development of volcanism at lava domes.
The marginal shear zone is characterised by a 3 m wide zone in which strain caused changes in the porous structure via crushing of the pore walls as well as distortion and failure of the crystalline phase; these promoted an increased reduction in pore volume and permeability towards the fault, especially in the high shear zone. Smith et al. (2001) invoked the effects of gravitational forces during post-emplacement flow of the lobes as a mechanism for the development of “ragged” pores and porous and/or dense flow banding in dome lavas at Unzen volcano. Yet, such diktytaxitic structures have been observed in small surficial dome blocks at Santiaguito volcano (Guatemala), which have not suffered from gravitational effects associated with flow along the flanks (Rhodes et al., 2018); they have also been observed at Merapi volcano, where they were attributed to late-stage gas filter pressing of a silica-rich melt phase (Kushnir et al., 2016). The commonality between these observations is that they occur in crystal-rich magmas, where crystals hamper the presence and distribution of exsolved fluids and interstitial melt, leading to ragged pore boundaries with protruding crystals. At Unzen, the character and distribution of the porous network rather evidence the importance of deformation, which was pervasive and commonly compactant in the marginal high shear zone. Experiments have shown that in the ductile field, material may deform by sustaining substantial compaction without the propensity for developing localised strain (Rutter, 1986) – a regime that generally results in a permeability reduction through shear (Ashwell et al., 2015; Kushnir et al., 2017b; Heap et al., 2015a, b). In this regime, magma deformation may result in crystal–plastic distortion and failure (Kendrick et al., 2016), as witnessed at Unzen (Wallace et al., 2019). Thus, we interpret the bulk of the marginal shear zone as the result of ductile deformation, which resulted in distributed, pervasive shear over a width of 3 m. Within this part of the conduit, the high shear zone displayed the highest degree of shear-enhanced compaction.
However, ductility alone is insufficient to describe the marginal shear
zone. For instance, the high shear area exhibits a foliation (S plane) and
fractures (C plane) parallel to the shear plane, which is then crosscut
(parallel but undulating) by a marginal fault hosting gouge formed by
comminution and cataclasis containing conjugate fractures. The composite
C–S fabric in the high shear zone is increasingly penetrative towards the
fault core (at the gouge contact), and its parallel C and S planes indicate
that the shear zone accommodated significant strain. This is supported by
observation that curvilinear Riedel fractures have developed and overprinted
the C–S fabric at an angle of 57
The central shear zone detailed in this study has a very different
character. Macroscopic observations of numerous cracks suggest that it is
dominantly dilational, as supported by the drastic increase in permeability
towards the fault and cavity. Despite having opened by
The shear zones studied here indicate that the dominant deformation regime of magma may evolve spatially and temporally during ascent in volcanic conduits, which would modify the magma's permeability and its ability to localise and channel outgassing during the effusion of lava domes.
The power of volcanic eruption models relies on an understanding of the coupling between magma and volatiles in volcanic conduits (Sparks, 1997), yet a description of dynamic permeability of deforming magma eludes us. The studies of eruptive products have provided first-order constraints on the relationship between permeability and porosity (Fig. 8; Klug and Cashman, 1996; Mueller et al., 2005; Farquharson et al., 2015) for various types of volcanic rocks (e.g. explosive clasts vs. effusive lavas), including the presence of heterogeneous structures (Farquharson et al., 2016c; Kolzenburg et al., 2012; Lamur et al., 2017; Kendrick et al., 2021), and these constraints have been invoked in diverse models to assess how magma permeability may evolve leading to eruption (Burgisser et al., 2019; Edmonds et al., 2003). However, the deformability of magma imposes constant changes to the porous permeable network, and to date, only a few studies have measured or assessed the transience of permeability and porosity during magma deformation (Okumura et al., 2010, 2012; Kendrick et al., 2013; Ashwell et al., 2015; Kennedy et al., 2016), especially in operando (Wadsworth et al., 2017, 2021; Kushnir et al., 2017a; Heap et al., 2017b). Considering the range of pressure conditions (e.g. pore pressure gradient, local deviatoric stress) and magma properties, none of these studies have yet succeeded in fully reconstructing the evolution of porosity and permeability of magma shearing during ascent in volcanic conduits.
Permeability–porosity relationship for Unzen dome lavas and other volcanic products. Blue and red circles represent data from this study, made parallel and perpendicular to the plane of shear, respectively. Grey circles show porosity data for Unzen from Mueller et al. (2005) and Kendrick et al. (2021), and open circles show permeability measurement on USDP drill cores from Watanabe et al. (2008). Other symbols indicate porosity–permeability measurements for volcanic materials from Soufriere Hills volcano (Harnett et al., 2019), Volcán de Colima (Farquharson et al., 2015), Mount Meagre (Heap et al., 2014), and Merapi volcano (Mueller et al., 2005).
The rocks sampled across the shear zone and in the fault gouge at Mount
Unzen vary in porosity between 8 % and 27 %; this range is slightly
narrower than the porosity range (4 %–48 %) covered by blocks shed by
pyroclastic density currents originating from the domes during the 5-year
eruption (see Fig. 8; Kueppers et al., 2005; Coats et al., 2018; Kendrick
et al., 2021; Scheu et al., 2007; Mueller et al., 2005). The narrower range
exhibited by the spine shear zones may reflect the occurrence of fewer
porosity-modifying mechanisms (e.g. post-fragmentation vesiculation) in the
highly viscous spine lava compared to those which occurred throughout the
entire course of the eruption; these are represented by the blocks at the
foot of the volcano. We see the largest contrast when we compare the
permeability range of the lavas which erupted through the spine at the end
of the eruption (
Previous investigations of permeability in shallow volcanic conduits have highlighted the existence of dilational shear zones, whereby the conduit margin is bound by a permeable “damage halo”; this has been proposed through both field (Saubin et al., 2019; Pallister et al., 2013a; Gaunt et al., 2014; Wallace et al., 2019; Ryan et al., 2020; Sparks et al., 2000; Watts et al., 2002; Holland et al., 2011) and laboratory (Lavallée et al., 2013; Laumonier et al., 2011) studies. These constraints indicate a dilation zone, with permeability higher by up to 1.5 orders of magnitude and variable degrees of anisotropic shear fabrics, causing preferential channelling of fluids in the direction of extrusion (Wright et al., 2006; Gaunt et al., 2014; Wallace et al., 2019; Ryan et al., 2020). Pore space connectivity is enhanced by fracturing (Lamur et al., 2017; Tiab and Donaldson, 2016), which would contribute to the development of anisotropy and would preferentially channel fluids along the conduit margin, promoting concentric or ring-like gas emissions, for instance as exemplified at Santiaguito, Guatemala (Lavallée et al., 2013; Holland et al., 2011). Connectivity may, however, be lost at the expense of fracture healing (Lamur et al., 2019) or sintering (Ryan et al., 2020; Wadsworth et al., 2016). Here, at the conduit centre at Unzen, we observed a localised dilational shear zone up to 3 orders of magnitude more permeable than the surrounding magma; thus, the scale of dilation exceeds that observed in marginal shear zones at Mt. St. Helens (Gaunt et al., 2014) and at Chaos Crags (Ryan et al., 2020). This zone spans a relatively narrow section of the conduit and appears to be a late, immature feature that is possibly related to shear during the final stages of ascent of the magma plug and/or structural readjustment during failure and calving of portions of the spine to the ENE. Instead, the primary (and volumetrically most significant) marginal shear zone studied at Unzen is mostly compactional and exhibits a lower permeability than the surrounding magma, particularly in the plane perpendicular to shear direction. Compaction may have been favoured in the marginal shear zone at Unzen compared to dilation at Mt. St. Helens and Chaos Crags due to the relatively higher porosity of the ascending magma (20 % at Unzen vs. 10 % and 12 %–15 % at Mt. St. Helens and Chaos Crags, respectively; Gaunt et al., 2014; Ryan et al., 2020); it may also reflect lower viscosities and/or deformation at greater effective mean stress in the system or at relatively lower strain rates (Fig. 9). Indeed, the marginal shear zone is overprinted by faulting, which suggests that compaction took place at greater depth and/or during inter-seismic periods of slower ascent. Seismic analysis indicated that seismogenic faulting was episodic and shallow, likely originating in the upper 500 m of the conduit (Umakoshi et al., 2008; Lamb et al., 2015); so, whilst below this depth shear may have prompted compaction, above this depth pulsatory magma shearing may have resulted in switches between compactional and dilatant shear, causing locally higher-permeability fractures through the sheared magma and a permeable marginal fault gouge by cataclasis (Fig. 9). Such intermittent seismic stressing may also serve to weaken surrounding country rocks and modify permeable pathways (Schaefer et al., 2020).
The presence and overprinting of compactional and dilational shearing modes in close proximity within a given magmatic extrusion demand appraisal. The ductile–brittle transition of materials has long been studied and is generally better understood for rocks than magmas as more low-temperature tests have been carried out (Paterson and Wong, 2005; Rutter, 1986; Heap et al., 2015a). Reconstruction of yield caps (or curves), based on the shear stress required for rupture or flow of materials at different effective mean stress, has shown that porous rocks undergo a transition from macroscopically brittle to ductile deformation mode with increasing effective pressure (Fig. 9b); this transition sets in at lower effective pressure (i.e. either at shallower depths or with higher pore pressures) if the material is more porous (Heap et al., 2015a). However, magma is viscoelastic; thus, depending on the timescale of observations, magma may behave as a solid: in essence, as a rock. Magmas abide by the glass transition so that at long observation timescales or under slow deformation, they flow; but at short timescales or if strain rate is high, they may rupture (Dingwell, 1996). The strain rate to meet this transition decreases if melt viscosity increases due to cooling, crystallisation, degassing, and/or vesiculation (Wadsworth et al., 2018; Dingwell and Webb, 1989, 1990; Cordonnier et al., 2012, 2009; Coats et al., 2018; Lavallée et al., 2013, 2008). The glass transition of silicate melts, which controls the deformation mechanisms of magmas (viscous or brittle), thus impacts their deformation mode, brittle or ductile (be it viscous flow or cataclastic flow); applicability of the concept of yield caps to volcanic rocks and magmas, as shown in Fig. 9b, has been reviewed by Lavallée and Kendrick (2020). In a scenario in which magma ascends, deforms, and outgasses during an eruption, such as during spine extrusion at Unzen, magma may undergo a transition from a macroscopically ductile to brittle deformation mode due to a reduction in effective pressure (from ascent or due to pore pressure increase; Heap et al., 2017b), densification (Heap et al., 2015a; Coats et al., 2018), viscosity increase (see Dingwell and Webb, 1990), or if the strain rate locally increases (Coats et al., 2018; Lavallée et al., 2013, 2008).
Nakada and Motomura (1999) proposed that faulting of this spine formed due to a lower effusion rate that resulted in more complete degassing and crystallisation that increased the magma viscosity. We advance the idea that fluctuations in pore pressure (Farquharson et al., 2016a) and local strain rates (Coats et al., 2018; Lavallée et al., 2013; Wadsworth et al., 2019) may be especially important in triggering embrittlement of otherwise ductile magma. In the ductile regime, strain is accommodated over a prolonged duration without necessarily leading to any substantial stress drop (Coats et al., 2018). Thus, under such conditions, we do not expect to detect any, or much, seismicity that would characterise magma rupture near the conduit margin (e.g. Neuberg et al., 2006; Thomas and Neuberg, 2012; Kendrick et al., 2014b). As a result, we anticipate that magma shearing below the point of rupture (ca. 0.5 km at Unzen; Umakoshi et al., 2008) would have compacted and partially shut the permeability of the conduit margin, with the shear zone creating an impermeable barrier preventing gas from escaping to the surrounding country rock and promoting outgassing through the more permeable conduit core, at least up to the point of rupture (see Collinson and Neuberg, 2012). Upon further ascent, changes in the stress fields and physical properties of the magmas during pulsatory ascent would have favoured transition to a macroscopically brittle response to shear (Lavallée and Kendrick, 2020), triggering seismic rupture (Umakoshi et al., 2008; Lamb et al., 2015) and initiation of predominantly fault-controlled, stick-slip dynamics in the final stint of magma ascent and spine extrusion (Hornby et al., 2015). In brief periods of high discharge rate, shear may have localised along the primary seismogenic fault, simultaneously creating a Riedel fracture, but in periods with lower discharge rates, shear would have been distributed over a wide area and the fault would become inactive (stick phase), shifting the Riedel fractures to shallower depth; upon renewed discharge rate increase, shear would narrow again, faulting would generate another Riedel fracture, and so on (Fig. 9a). Indeed, using seismic events as a proxy for the ductile–brittle transition it was possible to identify its migration through time as the inclined spine loaded and compacted its lower shear zone as it grew, dilating the upper fault zone (Lamb et al., 2015). This is further indicated by the localisation of fumaroles along the upper spine margin (also observed during our latest field campaign in 2016), showing that the fault zone around the inclined spine controlled fluid circulation in the upper conduit (Lamb et al., 2015; Yamasato, 1998). Finally, a late lateral shift in dilational shearing, from the conduit margin to the conduit core, suggests that the location of shear may migrate during magma ascent in conduits as a result of changes in local stresses (e.g. upon extrusion and/or block calving), likely resulting from a combination of pore pressure fluctuations, strain rate reduction, and progressive inward cooling, which would have favoured deformation in the core of the spine. Thus, the rheology of magma and the dominant shearing mode may evolve during ascent, which in turn dynamically modifies the permeability distribution across the conduit through time (Fig. 9a).
The above rheological description is primarily based on the unavoidable
decompression of erupting magma (which degasses, crystallises, and viscously
stiffens), yet previous observations at Unzen suggest that the conditions
for magmatic flow may have fluctuated (Umakoshi et al., 2008; Lamb et
al., 2015), thus contributing to rheological shifts. Here, we invoke
findings from the literature to assess the conditions leading to rupture.
The discharge rates associated with spine extrusion in 1994–1995 varied,
although Yamashina et al. (1999) constrained a relatively constant
spine protrusion rate of 0.8 m d
Coats et al. (2018) studied the rheology of Unzen's
porous lavas to define a failure criterion. Considering the estimated
eruptive temperature of ca. 870–900
We can then turn our attention to geometrical constraints from our
structural analysis to frame magma ascent conditions that satisfy the above
failure criterion. The Riedel fractures that are observed at regular
intervals of
In concert the physical and structural descriptions bolstered by the rheological analysis argue for changes in magma rheology during decompression and pulsatory ascent. We propose that throughout its journey to the Earth's surface, magma may undergo several cycles of expansion (from vesiculation and dilation) and collapse (from outgassing and compaction) due to variable permeability and pore pressure, which may promote switches in shearing regimes that trigger further changes in the permeability structure of shallow conduits. For instance, the vesicles of low-permeability magma may accumulate fluid, thus reducing the effective pressure and promoting brittle, dilatant rupture; rupture would in turn allow magma outgassing and a reduction in effective pressure, promoting compaction and lowering of permeability, and the cycle may recur. The picture portrayed here highlights the need to understand the coupling between magma, fluid flow dynamics, and, importantly, pressure fluctuations (Michaut et al., 2013) in volcanic conduits with increased spatial and temporal complexities in order to resolve the transient state of magma and reconcile gas emission data and volcanic eruption style (Edmonds and Herd, 2007).
The present detailed study of the Mount Unzen spine reveals the competing occurrence of compactional and dilational shear regimes during magma ascent in volcanic conduits. At depth, in areas subjected to high effective pressure, shearing may induce pore compaction, thereby lowering the permeability of the system and inhibiting lateral outgassing to the country rock. At shallower depth, where the effective pressure may be low, shearing may favour localised dilation that enhances permeability. Both shear regimes result in the development of permeability anisotropy, with permeability generally being highest parallel or sub-parallel to the direction of extrusion and lowest perpendicular to the shear plane. The observation of shearing mode overprints suggests that fluctuations in effective pressure and strain rates, during stick-slip cycles, may result in magma switching between compactant and dilational shearing regimes, thus dynamically reshaping fluid circulation at a range of scales and in turn controlling outgassing efficiency during magma ascent and eruption.
All original data collected as part of this study are presented within the paper. A copy of these data or any further information sought may be obtained upon request from the corresponding author. Queries regarding the samples should also be directed to the corresponding author.
YL designed the research programme, conceptualised the study, and wrote the paper. As local scientists, TMi, TMa, SN, and HS supported the design of the research programme, coordinated field logistics, and provided scientific detail and context of the 1991–1995 eruption at Unzen. YL, TMi, JDA, PAW, JEK, RC, AH, and HT undertook the field survey, measurements, and sampling. JDA, AL, JEK, and AH performed the permeability and porosity measurements. PAW and YL performed the textural and microstructural analysis. RC, KUH, and BR performed the computer tomographic scans and reconstructions. All authors contributed to the preparation of the article.
The contact author has declared that neither they nor their co-authors have any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We are grateful to Guðjón Eggertsson for help with the maintenance
of the permeameter. This project was financially supported by a European
Research Council (ERC) Starting Grant on Strain Localisation in Magma (SLiM,
no. 306488) and an award from the DAIWA Anglo-Japanese Foundation (grant no.
11000/11740). Yan Lavallée and Jackie E. Kendrick acknowledge support from the Leverhulme Trust
(ECF-2016-325 and RF-2019-526
This research has been supported by the European Research Council, FP7 Ideas: Strain Localisation in Magma (grant no. 306488), the Daiwa Anglo-Japanese Foundation (grant no. 11000/11740), the Leverhulme Trust (grant nos. ECF-2016-325 and RF-2019-526
This paper was edited by Antonella Longo and reviewed by Michael Heap and Ulrich Kueppers.