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Microseismicity from a dike intruding the mid-crust of\ud Iceland occurs episodically on fractures orientated parallel\ud to the dike plane. We interpret it as caused by fragments of\ud chilled magma being broken and pushed along the conduit\ud by melt flow. Laboratory experiments on basalt samples\ud under shear strains of 104 show that the shear strength\ud of a sample cooled through the glass transition initially increases to a maximum at 0.7 homologous temperature,\ud but subsequently decreases until failure at 0.58 homologous\ud temperature. We interpret the failure as due to the connection\ud of microcracks. Episodicity in microseismicity on\ud timescales of hours to days can arise from a cycle in which\ud magma in 0.1–0.5 meter thick dikes first cools and becomes\ud stronger, but then weakens along the dike margins with continued\ud cooling against the country rock. Continued pressure\ud of magma from below may then cause failure along dikeparallel\ud fractures.