The timing of caldera collapse at Mount Katmai in response to magma withdrawal toward Novarupta

Several thin layers of nonjuvenile lithic‐rich tephra from Mount Katmai are intercalated with 1912 plinian pumice‐fall deposits that originated at Novarupta 10 km away, demonstrating that caldera collapse at Katmai and eruption at Novarupta were synchronous. Emplaced by phreatic explosions and avalanche‐generated dustclouds, each Katmai‐derived layer is thickest and coarsest at the caldera rim and pinches out within 5–8 km. In contrast, thicknesses and clast sizes of plinian Layers A‐G decrease radially away from Novarupta. The first Katmai‐derived layer lies between Novarupta‐derived Layers B2 and B3, indicating that collapse began after release of ∼6.9 km³ of magma at Novarupta. The first major earthquake (Ms=6.5) at 2356 h on 6 June, ∼11 hours after the eruption began, may have marked the onset of collapse. The Katmai River landslide and several debris‐flow deposits are also sandwiched within the plinian deposits, indicating that they too were emplaced during the eruption.