Failure criteria for porous dome rocks and lavas: a study of Mt. Unzen, Japan

Abstract

Abstract. The strength and 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 to Maxwell’s viscoelasticity and their deformation mode (brittle vs ductile) 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 (De) number. Volcanic materials are extremely heterogeneous, with variable concentrations of crystals, glass/ melt and vesicles (of different sizes), and a complete description of the conditions leading to flow or rupture as a function of temperature, stress and strain rate (or timescale of observation) eludes us. Here, we examined the conditions which lead to failure for variably vesicular (9–35 %), crystal-rich (~ 75 %), pristine and altered, dome rocks (at ambient temperature) and lavas (at 900 °C) from Mt. Unzen Volcano, Japan. We found that the strength of the dome rocks decreases with porosity and is commonly independent of strain rate; when comparing pristine and altered rocks, we found that alteration caused minor strengthening. The strength of the lavas (at 900 °C) also decreases with porosity. Importantly, the results demonstrate that these dome rocks are weaker at ambient temperatures than when heated and deformed at 900 °C (for a given strain rate resulting in brittle behaviour). Thermal stressing (by heating and cooling a rock up to 900 °C at a rate of 4 °C min−1, before testing its strength at ambient temperature) was found not to affect the strength of rocks. In the magmatic state (900 °C), the rheology of the dome lavas is strongly strain rate dependent. Under low strain rate conditions (≤ 10−4 s−1) the lavas behaved ductilly (i.e., the material sustained substantial, pervasive deformation) and displayed a non–Newtonian, shear thinning behaviour. In this regime, the apparent viscosities of the dome lavas were found to be independent of vesicularity, likely due to efficient pore collapse during shear. At high strain rates (≥ 10−4 s−1) the lavas displayed an increasingly brittle response (i.e., deformation resulted in failure along localised faults); we observed an increase in strength and a decrease in strain–to–failure as a function of strain rate. To constrain the conditions leading to failure of the lavas, we analysed and compared the critical Deborah number at failure (Dec, the ratio between the relaxation time and the experimental observation time) of these lavas to that of pure melt (Demelt=10−3–10−2; Webb & Dingwell, 1990). We found that the presence of crystals decreases Dec to 2.11×10−4. The vesicularity (φ), which dictates the strength of lavas, further controls Dec following −5.1×10−4φ+2.11×10−4. We discuss the implications of these findings for the case of magma ascent and lava dome structural stability.

Increasing the Permeability of Hydrothermally Altered Andesite by Transitory Heating

“…This distinction is important because fresh, primary volcanic phenocrysts and groundmass are often microcracked (e.g., Heap, Lavallée, et al, ) as a result of their cooling histories (e.g., David et al, ; Vinciguerra et al, ). Hydrothermal alteration infills and/or seals these microfractures (e.g., Pola et al, , ; Wyering et al, , and references therein), which can strengthen a rock mass (e.g., Pola et al, , ; Wyering et al, , and references therein; Coats et al, ). In addition, hydrothermal alteration will change the mineral assemblage, changing the types of reactions possible during heating and cooling.…”

Evolution of Mechanical Properties of Lava Dome Rocks Across the 1995–2010 Eruption of Soufrière Hills Volcano, Montserrat

“…Long-term petrology across the eruption was explored by Christopher et al (2014) and although they found systematic changes in Fe-content across time, they concluded that there was no progressive change of bulk composition, with SiO 2 content consistently between 56 and 62% throughout the eruption. However, previous studies have documented that geomechanical rock properties of chemically indistinguishable lavas can vary broadly as a result of distinct pore structures (Kendrick et al, 2013;Schaefer et al, 2015;Heap et al, 2016a), local heterogeneities (Farquharson et al, 2016), anisotropy (Bubeck et al, 2017), and post-emplacement alteration (Pola et al, 2014;Siratovich et al, 2014;Coats et al, 2018). We therefore aim to explore how the petrographic textures of the Soufrière Hills products and the temporal variation in these textures affect both rock strength and volcanic behavior, even where there is a narrow range in bulk rock compositions.…”

“…This is an important consideration when modeling structural dome instability, as using a constant UCS/UTS ratio in numerical models could result in overestimation of a dome's tensile strength, and therefore underestimation of the failure likelihood of the unconfined portion of lava domes. The current SHV dome at Montserrat is likely to have cooled to an extent where viscous flow no longer dominates eruptive behavior (Ball et al, 2015); as such, tests of rock properties at ambient temperatures are relevant to the modeling of ongoing stability of the volcano, but moreover, a number of studies have demonstrated that the strength of volcanic rock at elevated temperature is either comparable (Heap et al, 2014a(Heap et al, , 2018a or higher (Schaefer et al, 2015;Coats et al, 2018) than at room temperature, suggesting that domes are at their weakest following cooling.…”

“…Volcanic products are typically very heterogeneous, with varied eruptive conditions leading to large ranges in pore architecture (i.e., connected vs. isolated vesicles vs. fractures) and permeability (Mueller et al, 2005;Heap et al, 2014aHeap et al, , 2018cFarquharson et al, 2015;Colombier et al, 2017). Experimental investigations into volcanic rock properties have increased in recent years, including compressive and tensile strength, elastic properties, and resultant physical changes induced during deformation (e.g., Lavallée et al, 2007Lavallée et al, , 2008Lavallée et al, , 2013Schaefer et al, 2015;Heap et al, 2016aHeap et al, , 2018aLamur et al, 2017;Marmoni et al, 2017;Coats et al, 2018), as well as research into the relationship between activity at dome-building volcanoes and their respective rock properties (e.g., Smith et al, 2009Smith et al, , 2011Kendrick et al, 2013Kendrick et al, , 2016Heap et al, 2015Heap et al, , 2016aKushnir et al, 2016;Lavallée et al, 2019). This increase in research has started to show the importance of understanding how mechanical properties of rock influence the eruptive style at a volcano, for example at Mt.…”

“…Unzen a temporal change in chemistry due to phenocryst abundance was shown to correlate with temporal changes in effusion rate (Nakada and Motomura, 1999), and such evolution in eruptive style will also alter dome stability. Similarly, the occurrence of secondary mineralization may modify the porous structure and coherence of rocks, affecting the structural stability (Horwell et al, 2013;Coats et al, 2018) especially when water is present in the pore space (Heap et al, 2018b).…”