Exposure to UV light induces structural changes in silica glass. This leads to a change in density, refractive index, optical absorption, and stress. Prolonged exposure causes catastrophic damage in the form of microchannels (extended microscopic voids), not satisfactorily explained so far. We demonstrate how microchannels are created by a plasma spark in the cores of compaction-induced waveguides. The waveguides form as a result of an accumulated self-focusing effect, directly observed by in-situ microscopy. We describe the formation of a complex, permanent 3D refractive index structure in the glass, and explain how the characteristic scales of that structure evolve during the UV exposure. While single pulse dynamic self-focusing has been extensively studied, we report a multi-pulse accumulated effect.