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To examine the impact of preexisting weaknesses on fracture coalescence during volcanic edifice deformation, we triaxially compressed Mount Etna basalt while acquiring in situ dynamic X-ray microtomograms and calculated the internal strain tensor fields using image correlation. Contraction localization preceded dilation and shear strain localization into the protofault zone. This onset of strain localization preceded macroscopic yielding and coincided with increases in the magnitude and volume of rock experiencing dilation, and spatial clustering of the strain populations. The exploitation of weaknesses by propagating fractures enabled the dominant shear strain to switch senses as propagating fractures lengthened along 30-60 degrees from sigma(1). Scanning electron microscopy images reveal pore-emanated fractures, and fractures linking pores. These experiments provide evidence of internal contraction preceding dilation and shear, consistent with inferences from field and laboratory observations. The transition from contraction to dilation may provide a precursory signal of volcanic flank eruption.

Plain language summary Directly observing how rocks break at seismogenic depths in natural settings is at present impossible. Here we used X-ray imaging techniques to view deforming, and then breaking, basaltic rocks at stress conditions equivalent to the flanks of the upper part of the Mount Etna volcano. We tracked how preexisting weaknesses, including pores and fractures produced during the fast cooling of the lava, controlled the growth and coalescence of new fractures and faults. At 50% of the stress at failure, shear and dilative strains began to concentrate in the volume that eventually developed the largest connected fracture network. The localization of contractive strains preceded this shear and dilation localization. These data sets provide observations of fracture growth within Etna basalt that previous studies could only infer, and thus constraints on how the volcanic edifice deforms under magmatic and tectonic stresses and eventually ruptures in flank eruptions.