The strength and macroscopic deformation mode (brittle vs.
ductile) of rocks is generally related to the porosity and pressure
conditions, with occasional considerations of strain rate. At high
temperature, molten rocks abide by Maxwell's viscoelasticity and their
deformation mode is generally defined by strain rate or reciprocally by
comparing the relaxation timescale of the material (for a given condition) to
the observation timescale – a dimensionless ratio known as the Deborah
(
In the magmatic state (900
Magma ascends to the Earth's surface and erupts through a wide spectrum of eruptive style (e.g. Siebert et al., 2015), which contributes to the construction of different volcanic edifices (e.g. de Silva and Lindsay, 2015). Amongst this activity, lava domes form when viscous magma accumulates and creates mounds of rocks and lava above the vent (Sparks, 1997; Fink and Anderson, 2000). These dome-building events make up approximately 6 % of volcanic eruptions worldwide (Calder et al., 2015) and their characteristics are governed by the rheology of the erupted magmas (Gonnermann and Manga, 2007; Lavallée et al., 2007). The emplacement of lava domes may be endogenous or exogenous, whether growing through inflation from within or through the piling up of discrete extrusive bodies (Hale and Wadge, 2008). In some extreme cases the latter can manifest as lava spines that extrude in a near-solid state (Heilprin, 1903; Stasiuk and Jaupart, 1997; Young et al., 1998; Tanguy, 2004; Scott et al., 2008; Vallance et al., 2008; Kendrick et al., 2012; Cashman and Sparks, 2013). Dome eruptions can produce a range of primary hazards, from ash fall to large-scale pyroclastic density currents, generated by gravitational collapse (e.g. Sparks and Young, 2002). They also have the potential to generate secondary hazards such as lahars (e.g. Nevado del Ruiz, Colombia; Pierson et al., 1990), edifice failure induced by magma intrusions (Voight and Elsworth, 1997; Reid et al., 2010) and lava dome collapse, as the mass cools or redistributes (e.g. Elsworth and Voight, 1996). In seismically active areas, strong tectonic earthquakes can both initiate activity and promote structural instability (e.g. Mayuyama, Japan; Siebert et al., 1987), even in long-dormant systems (e.g. Merapi, Indonesia; Surono et al., 2012). The eruption, emplacement and stability of lava domes reflects the mechanical properties of their constituent materials; thus, it is essential that the evaluation of monitoring data and development of improved hazard forecasting tools at lava dome volcanoes be based on a description of the mechanical and rheological properties of the materials.
The rheology of silicate melts has been explored extensively (e.g.
Dingwell and Webb, 1989, 1990; Webb and Dingwell, 1990; Webb and Knoche,
1996; Fluegel, 2007; Giordano et al., 2008; Cordonnier et al., 2012b).
Dingwell and Webb (1989) demonstrated that
silicate liquids are viscoelastic bodies, which abide by the glass
transition – a temperature–time space that defines their structural
relaxation according to the theory of viscoelasticity of Maxwell (1867). Maxwell's work established that the structural
relaxation timescale
Viscoelasticity dictates the behaviour of a magma. A rheological description
of viscoelastic materials may be cast via the non-dimensional Deborah
number,
During transport and eruption, magmas crystallise and volatiles are exsolved
(e.g. Cashman, 1992; Martel and Schmidt, 2003), resulting in magmatic
suspensions, undergoing significant rheological changes (e.g. Lejeune and
Richet, 1995; Barmin et al., 2002). In particular, dome-building eruptions
have been observed to produce variably vesicular (generally 0–
Various numerical models have been developed to evaluate the structural stability of lava domes and, with sufficient knowledge of a volcanic edifice and the properties of the materials it holds, collapse events can be modelled effectively (e.g. Elsworth and Voight, 1996). Although elegant and complex, these simulations tend to make non-trivial assumptions regarding vent geometry, dome morphology, and material properties (e.g. Ball et al., 2015). Volcanic domes are composed of materials with a vast spectrum of heterogeneities and degree of coherence (Mueller et al., 2011b; Lavallée et al., 2012, 2018) and although assigning fixed values for the material properties of dome rocks may be computationally beneficial, accounting for the wide range of physical and mechanical properties of dome materials remains a great source of uncertainty. Mechanical testing can be carried out to resolve the behaviour of rocks (see Paterson and Wong, 2005, and references therein) and this has resulted in a recent surge in laboratory testing to advance the understanding of the tensile strength, compressive strength, frictional coefficient and flow behaviour of these heterogeneous dome rocks and magmas as a function of temperature and stresses or strain rates (Smith et al., 2007, 2011; Lavallée et al., 2007; Hess et al., 2008; Kendrick et al., 2012, 2013b, a; Kolzenburg et al., 2012; Heap et al., 2014a; Hornby et al., 2015; Lamb et al., 2017; Lamur et al., 2017, and more.)
The uniaxial compressive strength (USC) of
volcanic rocks has been found to inversely correlate with porosity (Al-Harthi
et al., 1999; Kendrick et al., 2013b; Heap et al., 2014a, b, 2016b; Schaefer
et al., 2015) and to positively correlate with strain rate (Schaefer et al.,
2015). In volcanic rocks, porosity is made up of vesicles and microfractures,
which contribute to the mechanical behaviour and strength of the rock (Sammis
and Ashby, 1986; Ashby and Sammis, 1990; Heap et al., 2014a; Bubeck et al.,
2017; Collombet et al., 2017; Griffiths et al., 2017). Two models have gained
traction to explain the strength of rocks. The pore-emanating crack model of
Sammis and Ashby (1986) describes the case of a pore-only system in which
cracks nucleate from the pores and propagate in the direction parallel to the
principal stress, when the applied stress overcomes the fracture toughness of
a rock. As the applied stress increases, the microfractures propagate and
coalesce, leading to macroscopic failure. An analytical estimation of this
model was derived by Zhu et al. (2010) to estimate the uniaxial compressive
stress (
Heap et al. (2014a) experimentally demonstrated that neither model fully satisfied the mechanical data obtained for volcanic rocks and suggested that a microstructural model that combines the two mechanisms must be developed to permit the design of simulations considering the mechanical behaviour of microstructurally complex volcanic materials.
The problem of lava dome stability does not simply require knowledge of hot lavas or cold rocks; it further requires understanding of the effects of temperature (e.g. Harris et al., 2002), chemical alteration (e.g. Lopez and Williams 1993; Ball et al., 2015), pore pressure (Farquharson et al., 2016), thermal stressing (Heap et al., 2009, 2010, 2014a; Kendrick et al., 2013a; Schaefer et al., 2015) and mechanical stressing at different rates such as during seismic shaking (e.g. Cole et al., 1998; Voight, 2000; Calder et al., 2002) or magmatic intrusions (Walter et al., 2005) on the mechanical properties of the materials, many aspects of which have been tested in the context of edifices. The cooling of crystalline lava bodies results in the generation of fractures (Fink and Anderson, 2000; Takarada et al., 2013; Eggertsson et al., 2018; Lamur et al., 2018) – leaving a highly fractured blocky mass, the mechanical impact of which is difficult to quantify (Voight, 2000; Voight and Elsworth, 2000). Furthermore, thermal stressing cycles that could result from proximity of hot magma in a conduit, lava dome or edifice following a new eruptive episode have been found to only weakly modify the strength of commonly microfractured volcanic rocks (Heap et al., 2009; Kendrick et al., 2013a; Schaefer et al., 2015), unless they contain thermally liable minerals. Recent experiments on porous basalt by Eggertsson et al. (2018) have shown that rocks that are essentially void of microcracks (likely due to slow cooling), are however susceptible to fracture damage by thermal stressing (i.e. forming cooling joints); in contrast, microfractured rocks may not necessarily accumulate more damage during cooling; yet upon contraction, pre-existing fracture may widen to give way to the ingression of hydrothermal fluids (e.g. Lamur et al., 2018), further contributing to the stress balance and mechanical response.
The Unzendake volcanic complex is situated on the Shimabara Peninsula in south-western Japan (Fig. 1a). The volcanic complex began to grow at 0.5 Ma and now covers 20 km (E–W) by 25 km (N–S) (Takarada et al., 2013). Unzendake exhibits an intricate eruptive history of lava domes, flows and pyroclastic deposits (Nakada and Fujii, 1993) of predominantly dacitic composition (Nakada and Motomura, 1999).
On 17 November 1990, after 198 years of quiescence, a phreatic eruption
occurred at Mt. Unzen, which was accompanied by multiple earthquake swarms
(Matsushima and Takagi, 2000). This was followed shortly afterwards by a
phreatomagmatic eruption along with intense edifice swelling, and on 20 May
1991, the extrusion of a lava spine initiated the growth of the
Heisei–Shinzan dome complex (Nakada and Fujii, 1993; Takarada et al., 2013).
This introduced a 45-month-long period of lava dome activity with growth
being primarily exogenous in periods of high extrusion rate and endogenous in
times of low effusion rate (Nakada et al., 1995, 1999). The final stage of
growth was marked by the extrusion of a spine between October 1994 and
February 1995 (which can be seen today; Fig. 1b–c), characterised by
pulsatory ascent and seismicity (Umakoshi et al., 2008; Lamb et al., 2015),
along fault zones defined by compactional shear and mineral reactions,
crystal plasticity and comminution (Wallace et al., 2017). The end of the
eruption was followed by cooling of the lava dome and thermal contraction
that caused multiple joints (Takarada et al., 2013). Fumarole activity has
continued to the present day, with temperatures decreasing from
300
In total, 13 lava lobes were formed, and, at its maximum size, the lava dome
was 1.2 km (E–W) by 0.8 km (N–S) wide. In particular lobe 11, which
dominated the eastern side of the complex (Nakada et al., 1995, 1999), has
long been unstable, which has led to partial collapses that generated several
pyroclastic density currents (PDCs; Nakada et al., 1999; Sakuma et al.,
2008). The flows were estimated to have travelled at 200 km h
Mt. Unzen lava dome is made up of porphyritic, dacite (
In this study, nine samples were selected with different properties. Samples UNZ-1, UNZ-2, UNZ-4, UNZ-5, UNZ-7 and UNZ-8 were collected from easily accessible June 1993 block-and-ash flow deposits in the Minami–Senbongi area, north-east of the spine; UNZ-13 was collected from the May–August 1991 deposits in the restricted area of the Mizunashi River, east of the spine (see Fig. 1b). These rocks were collected as they represent the freshest (unaltered) materials that originate from dome collapse events during eruption, prior to any chemical alteration (e.g. Cordonnier et al., 2009). Sample UNZ-11 was collected on lobe 11 of the dome, selected as it showed signs of hydrothermal alteration (crusted, white and friable). UNZ-12 was collected on the dome, just east of the lava spine, and was chosen specifically for its reddish colour which suggested thermal alteration and oxidation. Each sample block was then cored to make multiple 20 mm diameter cylindrical cores, cut and then ground parallel to 40 mm in length (Fig. S1 in the Supplement) to maintain a 2 : 1 aspect ratio in accordance with the ISRM suggested method (ISRM Turkish National Group, 2015).
The bulk geochemical compositions of selected samples were determined using a Panalytical Axios advanced x-ray fluorescence spectrometer (XRF) at the University of Leicester (using fused glass beads prepared from ignited powders). Sample-to-flux ratio was kept at 1 : 5, 80 % Li metaborate: 20 % Li tetraborate flux. Results are quoted as component oxide weight percent and recalculated to include LOI (loss on ignition).
The geochemical composition of the interstitial glass in sample block UNZ-4
was determined using a Cameca SX-5 field emission electron probe
microanalyser (EPMA) at the University of Oxford. A variety of standards
were used to calibrate the spectrometers, including wollastonite for Ca, and
albite for Al, Na and Si. Secondary reference standards, of which the exact
chemistry was known, were utilised for better precision and accuracy. These
were labradorite and kn18 glass (comendite obsidian, Kenya), used as the
chemistries were similar to those of the Mt. Unzen glass sample. Analyses
used an accelerating voltage of 15 KeV, a beam current of 6 nA and a
defocussed spot size of 10
The porosity and character of the pores (i.e. whether connected or isolated)
was assessed using an AccuPyc 1340 helium pycnometer from Micromeritics.
Firstly, height (
Thin sections of UNZ-4, UNZ-11, UNZ-12 and UNZ-13 were prepared with a fluorescent dyed
epoxy, selected as they cover a vast range of sample diversity, including
both the lower and upper bounds of porosity, and collection site. Images
were acquired using a DM2500P Leica microscope in plane-polarised light. To
further constrain the microstructures of each sample block, backscattered
electron (BSE) images were taken of each sample using a Philips XL 30
tungsten filament scanning electron microscope (SEM), equipped with an
energy-dispersive X-ray spectrometer (EDS), and a Hitachi TM3000 SEM at
the University of Liverpool. Stubs of the samples were set in epoxy,
polished and carbon coated before being imaged in the Philips XL30 at a
working distance of
To constrain the conditions at which to carry out the high-temperature
uniaxial tests, we evaluated the softening point of the Mt. Unzen dome rock
using a Netzsch 402 F1 Hyperion thermomechanical analysis (TMA) at the
University of Liverpool. Under a 20 mL min
Selected cores of pristine material were thermally stressed in a Carbolite
box furnace to examine the effects of experimentally induced
heating–cooling cycles on the residual strength of rock cores. Cores were
subjected to heating at 4
Schematic of the USC testing set-up in the Experimental Volcanology and Geothermal Research Laboratory at the University of Liverpool. A 100 kN Instron 8862 uniaxial press with a three-zone, split cylinder furnace was used to perform experiments at varying strain rates and temperatures.
Sample properties, measurement data, experimental conditions, mechanical response and resulting properties of each sample. Y: yes; N: no; n/a: not applicable.
Continued.
Continued.
USC tests were carried out using a 50 kN 5969 Instron benchtop press and a
100 kN Instron 8862 uniaxial press with a three-zone, split cylinder furnace
using the parallel plate method in the Experimental Volcanology and
Geothermal Research Laboratory at the University of Liverpool (Fig. 2).
Experiments were carried out at both ambient temperature (
Prepared cores were placed upright in between the pistons of the press; the
furnace was closed around the sample, which was heated at 4
Ambient temperature experiments were carried out on all collected sample
blocks. Prepared cores were placed upright between the pistons, where they
underwent compressive tests at various strain rates until failure. The
thermally stressed samples were tested at a strain rate of 10
The strain at failure for these samples was selected using a semi-automated
MATLAB script which identified the strain value at peak stress. The static
Young's modulus was computed for each experiment that exhibited a brittle
response (e.g.
after Heap et al., 2014a) by calculating the slope of the linear portion of
the stress–strain curve via an automated script written in MATLAB and
available at
For samples that demonstrated a viscous response, the apparent viscosity
(
Normalised chemical composition of bulk rocks obtained by XRF analysis and interstitial glass obtained by EPMA. UNZ-4 was selected as it is representative of fresh lavas tested in this study; in contrast, UNZ-11 and UNZ-12 were deemed to display a certain degree of alteration. Original totals were 99.97, 100.39, 100.09 and 99.95 for UNZ-4, UNZ-12, UNZ-11 and UNZ-4 glass, respectively, before normalisation for direct comparison. The standard deviation of the UNZ-4 glass was taken from two measurements.
Plane-polarised light
Normalised geochemical analysis for bulk and glass geochemistry, obtained by
XRF and EPMA, respectively, are displayed in Table 2. Optical examination of
the samples reveals that they consist of 20–50 vol % phenocrysts and
microphenocrysts of plagioclase (
Average total, connected and isolated porosities for each sample block used. A larger number of cores were measured to calculate the average porosities than those used in strength tests. Note the values are presented to two decimal places but were calculated with four decimal places.
The total porosities of the samples determined by helium pycnometry measurements range from 0.10 to 0.32 (Table 3), a scatter which has previously been studied in an investigation of rock frictional properties (Hornby et al., 2015) and which is consistent with field measurements of Mt. Unzen 1991–1995 eruptive products (Kueppers et al., 2005). The pores of the denser products, notably UNZ-4 and UNZ-12, are fully connected, whereas the higher porosity blocks contain a portion (0.01–0.02) of isolated pores. The small standard deviation for the connected, isolated and thus total porosity of the rocks ensures the comparability of mechanical data obtained on samples with similar porosities during repeats.
Microstructural examination can be used to assess any pre-existing
anisotropy or fabrics in the lavas. Photomicrographs along with SEM images,
of a selected group of samples (UNZ-4, UNZ-11, UNZ-12, UNZ-13), can be seen in Fig. 3.
These samples are shown due to their contrasting nature, covering the span
of textures studied here: UNZ-11 and UNZ-12 are visually altered samples,
UNZ-13 has a different pore anisotropy than UNZ-11 and UNZ-4 is a typical
product of the block-and-ash flow and is representative of the remaining
samples tested. The images in Fig. 3 show the original materials,
orientated so that the direction of principal stress,
It is evident from Fig. 3 that the pores in the Mt. Unzen dome rock samples
are preferentially elongate. In some cases, the elongation has a
preferred orientation (e.g.
UNZ-11, UNZ-13), while in others it is unsystematic (e.g. UNZ-4, UNZ-12). In
UNZ-11 vesicles, and microlites, appear to bottleneck around phenocrysts in a
horizontal direction (i.e. perpendicular to
The groundmass of UNZ-12 contains a scaly textured silica polymorph that
appears to have filled vesicles. Common silica polymorphs seen at Mt. Unzen,
and other domes across the world (e.g. Mt. St. Helens; Voight et al., 1981),
are cristobalite precipitates, formed from hydrothermal activity (Nakada and
Motomura, 1999; Voight et al., 1981, 1999). This silica deposit has filled a considerable number (
In UNZ-12 the phenocrysts are visually more abundant (
The skeletal volume, mass and dimensions of each core were measured before and after thermal stressing in order to assess changes in porosity that may accompany microstructural adjustment in the process. Results showed that over the 12 cores subjected to thermal stressing, the change in connected porosity was less than 0.001, which is within the resolution of the method. Thus, it may be said that thermal stressing did not markedly create pores or connect isolated vesicles. It did however cause a slight decrease in the values of Young's modulus.
USC tests were conducted on 66 cores at ambient
temperature. For those samples which had a brittle response to uniaxial
compression, the failure process can be segregated into four stages (e.g. Hoek and
Bieniawski, 1965; Brace et al., 1966; Scholz,
1968; Heap et al., 2014a). An initial build-up of stress has been attributed
to the closure of microcracks perpendicular to
Examples of compressive stress–strain curves for
The strength of the rocks was observed to decrease with porosity (Fig. 5a). The range of strength of dense rocks is higher than porous rocks. We observe that rock strength increases with applied strain rates at all porosities, although this effect is more pronounced for dense rocks. The data suggest that the rocks deemed altered (UNZ-11, UNZ-12) are not weaker but indeed stronger than pristine rocks with equivalent porosities (see circled data points in Fig. 5a).
The overlap among the datasets obtained for thermally stressed and as-collected samples suggests that thermal stressing did not impart significant damage or mineralogical changes (if any) to modify the strength of these rocks (Fig. 5). Yet, a closer look at the mechanical data suggests that the initial convex increase in stress with strain is more pronounced for the thermally stressed samples than for their pristine equivalent (Fig. 4c), indicating that the thermally stressed samples have more cracks to close than their untreated equivalents. It is therefore likely that thermal stressing has caused the creation or opening of microfractures, dislocating the rocks slightly in the process but not enough to cause a notable increase in porosity or decrease in strength
Backscattered electron images of polished stubs for samples after
strain
The mechanical data of lavas show a wider range of behaviour than those
obtained on rocks at ambient temperature (Fig. 4a, b). At slower strain rates
of 10
The evolution of apparent viscosity is strain rate dependent as shown by
the stepped strain rate experiment (Fig. 7). An increase in the strain rate
resulted in an order of magnitude decrease in viscosity– a thixotropy of
similar magnitude to that described for highly crystalline magmas in
Lavallée et al. (2007). In this
experiment, deformation at low strain rates of 10
High-temperature uniaxial experiment results, including
stress–strain curves for samples tested at strain rates of
At strain rates of 10
When a strain rate of 10
The apparent viscosities calculated from the responses at 10
These results indicate that the transition in deformation mode from
macroscopically ductile to brittle behaviour is straddled by our experiments
in the range from 10
Apparent viscosities of porous lavas at 900
The experimental findings presented here suggest that the mechanical
response of lavas and rocks is similar, but important differences remain.
Experiments carried out on rocks at ambient temperature (all strain rates),
and on some lavas at strain rates of 10
Strength and Young's
moduli of Unzen rocks and lavas at different conditions. Shades of blue
represent tests carried out at ambient temperatures, shades of red indicate
those performed at 900
From the results of the uniaxial compressive experiments it is evident that porosity is a major control on the strength of dome materials. Previous studies on volcanic rocks (Al-Harthi et al., 1999; Heap et al., 2014a, b, 2016b; Schaefer et al., 2015) have found a similar correlation in which, to a first order, strength is inversely proportional to the porosity of the rock.
Here, the strength of samples with higher porosities displays less scatter
than that of samples with lower porosities (Fig. 10a). Microstructural
examination of the samples (Fig. 3) reveals the porosity of the porous
specimens to be dominated by vesicles, whereas the porosity of the denser
samples is dominated by microfractures, which may define a change in the
microstructural control on the strength and failure of low- and high-porosity
samples. In these lower-porosity specimens, the non-systematic orientation of
microfractures could be responsible for the large scatter in strength. The
USC was calculated for the samples for both the
pore-emanating crack model of Sammis and Ashby (1986) (Eq. 3) and the sliding
wing crack model of Ashby and Sammis (1990) (Eq. 4). For the former, the
USC was calculated with varying values of
Plot of uniaxial compressive stress against porosity showing the
ambient temperature mechanical data (black dots) alongside contours of
various values of
This transition in the preference of fracture nucleation site from pore to
crack is likely to be gradual and dependent on the pore network architecture
of a suite of samples; in these Mt. Unzen samples it is found at a porosity
of
Samples UNZ-11 (porosity: 0.30) and UNZ-13 (porosity: 0.32) both have
elongated vesicles. The cores were cut so that the vesicles were either
perpendicular or parallel to the applied principal stress,
At ambient temperatures, the static Young's modulus decreases from
Lavas deformed at 900
In addition, thermally stressed samples have slightly lower (
Remarkably, when in the brittle regime at high temperature, samples exhibited
strengths
A similar increase in strength with temperature was also noted in basaltic rocks from Pacaya volcano (Schaefer et al., 2015). There, the authors attributed the increase in strength of the glass-poor rock to the closure of microcracks (likely formed upon cooling after their eruption) due to thermal expansion, a process that equally occurs in Mt. Unzen dome rocks. Rocks may also become weaker from thermal stressing; this can be due to crack initiation (Heap et al., 2016a) or alteration, via processes such as decarbonation and dehydroxylation (Heap et al., 2012, 2013a, b). A recent study by Eggertsson et al. (2018) found that samples that hosted microfractures (like Mt. Unzen dome rock) were not affected by thermal stressing, while those that showed a trivial fraction of pre-existing microfractures were more readily fractured through thermal stressing and as a result became more permeable.
The style of an eruption – effusive vs. explosive – depends on the rheological response of magma (Dingwell, 1996). The urge to understand the alarmingly variable nature of volcanoes and recent advances in experimental capabilities and computational modelling have encouraged the community to focus efforts on the development of two- and three-phase models of magma rheology (e.g. Lejeune and Richet, 1995; Caricchi et al., 2007; Lavallée et al., 2007; Costa et al., 2009; Mueller et al., 2011b; Truby et al., 2015). Truby et al. (2015) combined two two-phase flow models (considering melt and crystals, and melt and gas bubbles) to elaborate a three-phase model of magmatic suspensions, further tested against a set of controlled analogue laboratory data. Their model shows that while the addition of crystals increases the viscosity of a suspension, leading to a shear thinning rheology, the addition of gas bubbles (which can deform during shear) has variable consequences. Depending upon the initial crystal volume and maximum packing fraction of those crystals, the addition of gas bubbles may result in a further increase in viscosity or, in other cases, a levelling or a decrease in the apparent viscosity of the suspension. Their model suggests that the addition of bubbles to lavas, above their glass transition, with high normalised crystal fractions, like those seen in volcanic domes, would likely decrease the viscosity of the suspension. However, here, the data show that the presence of vesicles (between 0.09 and 0.33) in dome lavas may not necessarily influence the apparent viscosity (at least not systematically). We advance that this could be due to the high connectivity of the pores present in dome lavas, which allows efficient outgassing; thus the gas cannot act as an isolated phase that can pressurise during shear. Thus, it may be that lavas hosting permeable porous networks may have mostly porosity-independent apparent viscosities (at least across the range examined here), as suggested by Lavallée et al. (2007). Current models relating porosity to viscosity simply account for the presence of isolated gas bubbles via a capillary number to calculate the apparent viscosity of a multiphase suspension (e.g. Rust and Manga, 2002; Llewellin and Manga, 2005; Truby et al., 2015). However, this result highlights important shortcomings to the modelling of shallow magmas, in which porous networks tend to develop connectivity, especially in sheared crystal-bearing lavas (e.g. Laumonier et al., 2011; Kushnir et al., 2017). This connectivity controls outgassing, and thus pressure build-up or release, which is responsible for rheological variations in magma and therefore eruption style (effusive vs. explosive). Our findings suggest that we need to revise three-phase models to account for gas flow through evolving deformable bubbles, which may also be connected, in order to constrain the apparent viscosity of magmas in lava domes and other open-system settings.
During magma ascent, the strain rate, which is proportional to effusion rate
(e.g. Goto, 1999), plays a key role in determining whether the response of
magmas and extruding lavas is that of a solid or liquid (Webb and Dingwell,
1990). Here, the macroscopic deformation mode (viscous, viscous-dominated
transitional, brittle-dominated transitional or brittle) of lavas was
characterised based on their resulting stress–strain curve (Sect. 3.2.2;
Fig. 12a); these are further supported by microstructural observations (see
Figs. 6, 12a). Note that sample UNZ-4–28 was not given a classification as
its response to deformation was likely an experimental artefact due to a
chipping of the sample edge. The distinction among these rheological regimes
can be made using the Deborah number (Eq. 2). In a recent study on the
failure of single-phase silicate melts, Wadsworth et al. (2017) suggest that
fractures can propagate above
For the Mt. Unzen material
Thus, both the addition of crystals (as seen by the fact that
The findings observed here help constrain the impact of rheological evolution on lava domes as they erupt and cool following emplacement. The rheology of magma has a fundamental influence on the style of a volcanic eruption, be it explosive or effusive (Dingwell, 1996; Gonnermann and Manga, 2007). Understanding how magmas respond to changes in petrology, stress and eruptive shearing conditions that occur during ascent in a volcanic conduit may help to enhance models that aim to predict volcanic activity. The work undertaken here constrains the material behaviour of erupting dome lavas and the relics that remain once the lava cools.
As magma crystallises, its apparent viscosity (generally) increases as the
melt evolves, and an increasing fraction of the suspension becomes solid
(with slower diffusivity and a lower rate of plasticity than the viscous
liquid melt); thus the suspension becomes increasingly solid-like. For
crystalline magmas, we would expect
Upon extrusion, lava cools, contracts and fractures (Lamur et al., 2018). Here we show that the strength of a dome is reduced upon cooling due to contraction and microfracturing, leaving a weaker relic structure. This situation may favour the progressive creep of cooling dome structures, as observed in lobe 11 at Mt. Unzen (Kohashi et al., 2012).
Post-emplacement, through time and prolonged exposure to corrosive fluids, dome material may alter (Ball et al., 2015). In this study, the altered rocks tested showed a higher strength than pristine rocks with equivalent porosities. However, previous studies have found that altered volcanic rocks can also be weaker (e.g. Pola et al., 2012). From this distinction we surmise that the structure of the rocks as well as the type of alteration (developing under different conditions in cooling volcanic rocks) may have a contrasting effect on the strength of cooled dome lavas. Thus, the data shown here beg for an increased focus on the impact of alteration on volcanic rock strength for improved lava dome structural stability models.
The rate of deformation imposed on dome materials is also an important
variable to be considered. In this study, and in others (e.g. Schaefer et al., 2015;
Lavallée et al., 2018), volcanic rocks have been shown to withstand
higher stresses when deformed at higher strain rates. Previous studies have
suggested earthquakes with high ground acceleration have provoked lava dome
collapse (Voight, 2000); therefore, it is
essential to understand the effect of strain rate on the strength of
materials. This is of particular importance for Mt. Unzen as it is located
in a very seismically active area. Slow, continuous strain (or recurring
stressing cycles) can induce fatigue in a material and promote brittle creep (e.g.
Heap and Faulkner, 2008; Heap et al., 2009; Brantut et al., 2013; Kendrick
et al., 2013a; Schaefer et al., 2015); thus weakening the rocks which undergo
failure at lower stresses. Thus, over long periods (years) of deformation,
such as for lobe 11 at Mt. Unzen, the actual strength of the dome rocks may
be lower than those reported here at the lowest strain rate of 10
Volcanic structures are made of heterogeneous rocks and lavas, with intricate mineralogical assemblages, textures and fabrics, with variable degrees of coherence; thus, their mechanical responses may vary widely. Although here we have only tested material from the 1991–1995 eruption of Mt. Unzen, this study has the potential to be applied to other dome-forming volcanoes of similar composition, crystallinity and porosity. Additionally, the work can also be applied to parts of larger volcanic edifices dominantly constructed by the accumulation of lavas, which may be prone to collapse (Ball et al., 2015). The work presented here can help constrain the behaviour of lavas and rocks involved in lava dome eruptions. We anticipate that the results will form the basis for more advanced numerical simulations of dome eruption and related hazards.
Uniaxial experiments carried out at ambient and high temperature (900 In the brittle regime, strength decreases with increasing pore volume at both
ambient and high temperatures. Magmas deformed in the brittle regime at high temperature are stronger than
rocks of equivalent porosity deformed at ambient temperature. Thermal stressing did not affect the strength of dome rocks within the
conditions tested ( The presence of alteration may have variable effects, sometimes
strengthening volcanic rocks. The strength of rocks and lavas (in the brittle field at high temperature)
increases with strain rate. The viscosity of dome lavas decreased with strain rate (shear thinning) and
did not vary for the range of material crystallinity and porosity studied. Lavas deformed at high temperature and strain rates of The critical Deborah number,
These results reveal that current stability models of cooling lava domes,
like that of lobe 11 at Mt. Unzen, require an integration of the complex
nature of the materials. The outcome of this study suggests that, as a
primary control on rock strength, porosity heterogeneities must be included
when modelling failure mechanisms. As secondary controls, it would also be
beneficial to include deformation conditions such as temperature and strain
rate. Conclusions drawn from high-temperature experiments suggest that
current three-phase models may not be fully applicable to dome lavas and
other crystal-rich lavas. We suggest a new formulation of the Deborah
number that applies to porous crystal-rich lavas and propose that it may
help refine the accuracy of models attempting to describe rheological
evolution to explain geophysical data monitored during lava dome eruptions.
Supplementary data are available in the Supplement
Figs. S1 to S6. The script for the Young's modulus calculation is freely
available on GitHub (Coats, 2018). Further information can be obtained upon
request to the corresponding author. The AST14DEM used in Fig. 1 was
retrieved from the online data pool, courtesy of Land Processes Distributed
Active Archive Center (LP DAAC) and the NASA the Japan Ministry of Economy,
Trade and Industry (METI),
(
The Supplement related to this article is available online at:
YL and JEK designed the experiments. RC, AJH, TMi, JEK, PAW and JDA carried out the mechanical experiments and physical measurements. PAW collected microprobe data and conducted softening point determination. RC wrote the processing codes, analysed the data and prepared the paper with contributions from all co-authors. RC, JEK, PAW, TMi, AJH, JDA, TMa and YL contributed to the collection and selection of samples and preparation of the paper.
The authors declare that they have no conflict of interest.
Rebecca Coats, Jackie E. Kendrick, Paul A. Wallace, Adrian J. Hornby, Takahiro Miwa, James D. Ashworth and Yan Lavallée acknowledge funding from the European Research Council (ERC) Starting Grant on Strain Localisation in Magma (SLiM; no. 306488). Jackie E. Kendrick was supported by an Early Career Fellowship of the Leverhulme Trust. Paul A. Wallace acknowledges the NERC ATSC training for providing time to conduct EPMA measurements. Fieldwork was funded by the DAIWA Anglo-Japanese Foundation (grant number 11000/11740). The authors would like to thank Hiroshi Shimizu for his guidance throughout the study. Edited by: Joachim Gottsmann Reviewed by: Steve Quane and Jamie Farquharson