Size distribution data obtained from detailed field study of bubbles and amethyst-geodes in the basaltic lava flows of the Serra Geral Formation (Ametista do Sul, Rio Grande do Sul, Brazil) are used in cooling and vesiculation models to infer the origin and the formation processes of the geodic cavities. Coupled field observations and modelling results emphasize that (1) the formation of geodes in the studied lava flow can be explained, qualitatively and quantitatively, by the exsolution of magmatic gas from the supersaturated melt with no need for external surface water supply; (2) the vertically elongated habits of the geodes result from higher cooling rate of the magma in contact with the accumulating bubbles; and (3) the abnormal metre-sized geodes with their branching habits result not only from the diffusive/decompressive bubble growth but also from the coalescence of inwards-progressing tubular cavities.