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

Abstract

Abstract. 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 (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 the macroscopic failure of variably vesicular (0.09–0.35), crystal-rich (∼ 75 vol %), 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 the precipitation of secondary mineral phases in the original pore space 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 conditions of low experimental strain rate (≤ 10−4 s−1), ductile deformation dominated (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 essentially equivalent, independent of vesicularity, likely due to the lack of pore pressurisation and efficient pore collapse during shear. At high experimental 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) of these lavas to that of pure melt (Demelt = 10−3–10−2; Webb and Dingwell, 1990). We found that the presence of crystals decreases Dec to between 6.6×10−4 and 1×10−4. The vesicularity (φ), which dictates the strength of lavas, further controls Dec following a linear trend. We discuss the implications of these findings for the case of magma ascent and lava dome structural stability.

Thermal Liability of Hyaloclastite in the Krafla Geothermal Reservoir, Iceland: The Impact of Phyllosilicates on Permeability and Rock Strength

“…The resultant strengths measured in all test types display a porosity control, irrespective of the stress field experienced (Figure 7(a)), which follows the common porosity-strength trend (Figures 7(b) and 7(c)) for a range of igneous rock types, regardless of TT [7,11,63,87,88]. The treatment temperature has a strong impact on porosity (Figure 4(c)), thereby further influencing mechanical compaction; however, comparison with strength and Young's modulus data collected from subsurface samples [63] shows that temperature alone cannot explain the mechanical changes occurring within the reservoir, instilling the roles of compaction and alteration on strength changes.…”

“…The strength is dependent upon the fracture toughness (or critical stress intensity factor), K IC , the porosity (ϕ), and pore size (r). This model was demonstrated to successfully approximate the UCS of limestone [91] and porous glass sintered in the laboratory [92] but show arguable efficiency to approximate the strength of heterogeneous, coherent volcanic rocks, owing to their common abundance of microfractures [7,61]. Here, the UCS values decrease as porosity increases with TT (Figure 8), suggesting that K IC / ffiffiffiffiffi πr p would decrease from~5 MPa down to~3 MPa.…”

“…These fluctuations range from~250°C of cooling during thermal stimulation practices [3] to heating of up to 400°C during flow testing [4] and up to~1200°C during basaltic magma intrusions [5]. Importantly, for the sustainability of hydrothermal systems, temperature variations cause volumetric changes that may impart damage [6][7][8][9][10][11][12] and have the potential to trigger mineral reactions, prompting precipitation or breakdown [13] in altered reservoir rocks [14]. These reactions can affect key reservoir rock properties, such as porosity, permeability, and strength [15], which may influence the capacity for fluid circulation [16,17] and thus dictate energy production potential [18].…”

Brittle‐Ductile Deformation and Tensile Rupture of Dome Lava During Inflation at Santiaguito, Guatemala

“…Brittle behavior can be met at high strain rates (Figure a) preventing stress relaxation and strain, thus favoring the development of a localized throughgoing fracture. As temperature drops the lava also becomes increasingly brittle as the degree of strain localization increases (Coats et al, ; Lavallée et al, ). In many cases where the applied strain rate is lower, a certain degree of stress relaxation (see Figure ) and pervasive strain (see Figures and ) results in slow tearing and the development of blunt crack tips (see Figure c).…”

“…In such cases, the onset of deformation will lead to transient stress accumulation as the material stiffens, before partially dissipating the stress through viscous relaxation and flow, resulting in loading rate decrease from an initial maximum (Figure ). This degree of viscous relaxation is determined by the ratio of the deformation timescale to the relaxation timescale (Dingwell & Webb, ; Maxwell, ) and equivalent relationships when considering multiphase suspensions (Coats et al, ; Wadsworth et al, ). With increasing ductility, primary fracture propagation becomes increasingly time dependent and incremental.…”