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

Abstract

Gas-and-ash explosions at the Santiaguito dome complex, Guatemala, commonly occur through arcuate fractures, following a 5-to 6-min period of inflation observed in long-period seismic signals. Observation of active faults across the dome suggests a strong shear component, but as fault propagation generally proceeds through the coalescence of tensile fractures, we surmise that explosive eruptions require tensile rupture. Here, we assess the effects of temperature and strain rate on fracture propagation and the tensile strength of Santiaguito dome lavas. Indirect tensile tests were conducted on samples with a porosity range of 3-30% and over diametral displacement rates of 0.04, 0.004, and 0.0004 mm/ s. At room temperature, the tensile strength of dome rock is rate independent (within the range tested) and inversely proportional to the porosity of the material. At eruptive temperatures we observe an increasingly ductile response at either higher temperature or lower displacement rate, where ductile deformation is manifest by a reduction in loading rate during constant deformation rate tests, resulting in slow tearing, viscous flow, and pervasive damage. We propose a method to conduct indirect tensile tests under volcanic conditions using a modification of the Brazilian disc testing protocol and use brittleness indices to classify deformation modes across the brittle-ductile transition. We show that a degree of ductile damage is inevitable in the lava core during explosions at the Santiaguito dome complex and discuss how strain leading to rupture controls fracture geometry, which would impact gas pressure release or buildup and regulate explosive activity.Plain Language Summary Using instruments we installed at Santiaguito, an active lava dome complex in Guatemala, we detected repeating cycles of inflation and deflation. The inflation took less time leading up to explosions compared to weak gas puffing, which led us to suspect that lava breaking and flowing might be responsible. To investigate this, we made laboratory tests where we put lava samples under tension, which is the most common way they break. We ran tests where we squeezed lavas at faster and slower rates in a press, and we also heated the lavas to their eruption temperatures-about 800°C-for some tests. Since the lavas contain volcanic glass, in some high temperature tests the glass partially flowed. In other tests the lavas were completely brittle, which means they stored up stress and then broke without flowing. The lava's behavior depends on the temperature and how fast they are squeezed. Finally, we considered how fast dome lavas at Santiaguito would have to be deformed to either break or stay intact during inflation of the dome. This study gives us a better idea of how dome lavas deform and how that affects hazardous activity during eruptions.

Physical and mechanical rock properties of a heterogeneous volcano; the case of Mount Unzen, Japan

“…6, Table 4), as has been noted for other volcanic rocks; cold (e.g. Harnett et al, 2019a), hot (Hornby et al, 2019) and fragmented by pore overpressure (Spieler et al, 2004). The average UTS of each group of dry samples revealed that the strongest was UNZ9b (3.39 MPa), the cataclastic sample cored perpendicular to the cataclastic fabric (and fractured in tension parallel to fabric) and weakest was UNZ9a (1.80 MPa), the cataclastic sample cored parallel to the fabric (thus fractured in tension perpendicular) despite having equivalent porosities.…”

“…Volcanic rock strength inversely correlates with porosity, and is frequently defined in terms of uniaxial compressive strength (UCS) at room or high temperature (e.g. Heap et al, 2014b, c;Schaefer et al, 2015;Coats et al, 2018;Bubeck et al, 2017;Pappalardo et al, 2017), direct and indirect tensile strength at room or high temperature (Harnett et al, 2019a;Lamur et al, 2018;Hornby et al, 2019;Lamb et al, 2017;Benson et al, 2012) and triaxial tests at varying pressures, temperatures and saturation conditions (Heap et al, 2016a;Smith et al, 2011;Farquharson et al, 2016;Shimada, 1986;Kennedy et al, 2009;Mordensky et al, 2019). Strength of volcanic rocks also typically positively correlates with strain rate (Schaefer et al, 2015;Coats et al, 2018), which in combination with variability in pore geometry, crystallinity and other textural parameters of volcanic rocks ensures that scatter in volcanic rock strength is high (Lavallée and Kendrick, 2020;Heap et al, 2016b).…”

“…In these tests the disc shaped specimens were loaded diametrically on flat loading platens at an equivalent diametric strain rate of 10 -5 s -1 . Methods and standards utilised for Brazilian disc testing are frequently conflicting in terms of deformation/ loading rate and time to failure (ISRM, 1978;ASTM, 2008;Li and Wong, 2013;Hornby et al, 2019), here we adopt the approach of Lamb et al (2017). All mechanical data were corrected for the compliance of the set-up at the relevant experimental deformation rate.…”

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.…”

UAS-based tracking of the Santiaguito Lava Dome, Guatemala

“…However, changes in lava viscosity resulting from changing extrusion rates, increased cooling or degassing can drastically reduce the free escape of volatiles, resulting in increased explosive activity 8 . Particularly, changes in viscosity will affect the failure mode (brittle-ductile) of the dome magma and control surface strain and, in turn, the development of fractures on the dome surface 46 . Changing viscosity may also impact the mode of extrusion between endogenous and exogenous growth 47 .…”