Abstract <p>The hydrothermal transformation of hazardous volcanic ash into functional zeolites was investigated to elucidate the role of silicon-to-aluminum ratio (SAR) and temperature in the crystallization pathways. The alumina-rich system yielded pure SOD phase consistent with a thermodynamically controlled reaction pathway via Ostwald ripening. The silica-rich system followed a kinetically driven pathway consistent with the Ostwald’s rule of stages progressing through a dissolution–recrystallization mechanism: FAU → SOD → ANA (particle size: 3.2 → 0.9 → 11.2&#xa0;µm). The final ANA phase exhibited a surface area of 138.4 m²&#xa0;g⁻¹ with significant intercrystalline mesoporosity. These findings provide a roadmap for the sustainable valorization of volcanic debris into high-performance materials for potential industrial and catalytic applications.</p> Graphical abstract <p></p>