A computed tomography observation of the Unzen lava reveals the frequent existence of vesicles and crystals in proximity

Volcanic outgassing through interconnected bubble networks controls eruption dynamics. Frameworks formed by crystals may facilitate gas escape, but the relative spatial arrangements of bubbles and crystals remain overlooked. We conducted multi-resolution X-ray computed tomography (CT) imaging to examine vesicle-crystal spatial relationships in a dacite bomb, a lava block in a pyroclastic flow deposit, and spine lavas from the 1990-1995 Unzen eruption. We acquired micro-CT, computed laminography (CL), and nano-CT images with progressively higher resolution. Reconstructed 3D images reveal that large vesicles are consistently connected to crystals across all sample types and analysis scales. Size distribution analysis demonstrates preferential connectivity between large vesicles and large crystals. Vesicles in the bread crust bomb that appear isolated actually form interconnected networks, while vesicles in shear-deformed dome samples occupy narrow inter-crystal gaps as sheet-like structures. Compaction timescale calculations and gas flow modelling indicate that interconnected bubble networks enable efficient initial outgassing at depths of 0.5-0.8 km, while crystal-supported pathways subsequently transport ascending gas through shallow conduit regions. These analyses suggest that compaction of the crystal-bearing, interconnected bubbles causes outgassing at depth, thereby developing the crystal frameworks that act as pathways for outgassing during the final ascent to form the dome.