Intra-crater clast recycling and flow confinement during the moderate 1963–1965 eruption at Irazú volcano (Costa Rica)

Abstract

Despite extensive contemporary documentation of the 1963–1965 Irazú eruption, a detailed analysis of its proximal tephra deposits has been lacking. Here, we characterize the stratigraphy and sedimentology of these deposits, interpret their depositional processes including tephra recycling, and constrain magma recharge timing through geochemical data. The eruption was predominantly phreatomagmatic, with subordinate Strombolian and rare phreatic events, producing ~6.2 × 10⁷ m³ of tephra (DRE: 5.68 × 10⁷ m³; VEI 3). Four eruptive stages were identified by correlating deposits with contemporary observations. The irregular pre-1963 crater morphology controlled the development of an asymmetrical tuff cone and a maar-like crater. Intra-crater deposits (~28 m) and rim deposits (~5.7 m) include fallout beds, ballistic curtain deposits, dilute pyroclastic density currents, and massive ash beds. The steep, funnel-shaped crater promoted significant tephra fallback and recycling, and confinement of pyroclastic flow deposits. The deposits record compositional enrichment in incompatible elements from June 1964 indicating magma recharge, while mineral zoning patterns show evidence of magma mixing. These results demonstrate that moderate-intensity summit eruptions can exhibit complexities typically associated with maar-diatreme systems.