The connectivity of rocks’ porous structure and the presence of fractures influence the transfer of fluids in the Earth’s crust. Here, we employed laboratory experiments to measure the influence of macro-fractures and effective pressure on the permeability of volcanic rocks with a wide range of initial porosities (1–41 vol. %) comprised of both vesicles and micro-cracks. We used a hand-held permeameter and hydrostatic cell to measure the permeability of intact rock cores at effective pressures up to 30 MPa; we then induced a macro-fracture to each sample using Brazilian tensile tests and measured the permeability of these macro-fractured rocks again. We show that intact rock permeability increases non-linearly with increasing porosity and decreases with increasing effective pressure due to compactional closure of micro-fractures. Imparting a macro-fracture both increases the permeability of rocks and their sensitivity to effective pressure. The magnitude of permeability increase induced by the macro-fracture is more significant for dense rocks. We finally provide a general equation to estimate the permeability of intact and fractured rocks, forming a basis to constrain fluid flow in volcanic and geothermal systems.

Columnar joints form by cracking during cooling-induced contraction of lava, allowing hydrothermal fluid circulation. A lack of direct observations of their formation has led to ambiguity about the temperature window of jointing and its impact on fluid flow. Here we develop a novel thermo-mechanical experiment to disclose the temperature of columnar jointing in lavas. Using basalts from Eyjafjallajökull volcano (Iceland) we show that contraction during cooling induces stress build-up below the solidus temperature (980 °C), resulting in localised macroscopic failure between 890 and 840 °C. This temperature window for incipient columnar jointing is supported by modelling informed by mechanical testing and thermal expansivity measurements. We demonstrate that columnar jointing takes place well within the solid state of volcanic rocks, and is followed by a nonlinear increase in system permeability of <9 orders of magnitude during cooling. Columnar jointing may promote advective cooling in magmatic-hydrothermal environments and fluid loss during geothermal drilling and thermal stimulation.

The rapid discharge of gas and rock fragments during volcanic eruptions generates acoustic infrasound. Here we present results from the inversion of infrasound signals associated with small and moderate gas‐and‐ash explosions at Santiaguito volcano, Guatemala, to retrieve the time history of mass eruption rate at the vent. Acoustic waveform inversion is complemented by analyses of thermal infrared imagery to constrain the volume and rise dynamics of the eruption plume. Finally, we combine results from the two methods in order to assess the bulk density of the erupted mixture, constrain the timing of the transition from a momentum‐driven jet to a buoyant plume, and to evaluate the relative volume fractions of ash and gas during the initial thrust phase. Our results demonstrate that eruptive plumes associated with small‐to‐moderate size explosions at Santiaguito only carry minor fractions of ash, suggesting that these events may not involve extensive magma fragmentation in the conduit.

We combine geophysical and experimental observations to interpret preeruptive unrest at Volcán de Colima in 1998. 17,893 volcanic earthquakes were detected between 1 October and 31 December 1998, including 504 clusters. Using seismic ambient noise interferometry, we observe a drop in velocity prior to the eruption linked to damage accumulation during magma ascent. This is supported by experimental observations where static stress causes a velocity decrease prior to failure. Furthermore, we observe acoustic emission clusters during the experiments, with lower porosity samples producing higher numbers of repeaters. This behavior introduces tensile failure as an additional viable mechanism for clusters during magma ascent. The findings suggest that preeruptive magma ascent may be monitored to variable degrees of accuracy via descriptions of damage accumulation and associated seismic velocity changes.

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.