High-resolution transmission electron microscopy (HRTEM) observations of unaltered volcanic air-fall deposits from the ongoing lava dome explosive eruption at Chaitén Volcano, Chilean Patagonia, revealed the presence of highly crystalline silica nanofibers in the respirable fraction of the volcanic ash (<4 μm). The nanofibers are identified as the high-temperature (>240 °C), beta form of cristobalite, with average lengths of hundreds of nanometers and widths on the order of tens of nanometers. We propose that the beta-cristobalite nanofibers are formed during explosive eruptions by the reduction of amorphous silica by carbon monoxide to its reactive suboxide SiO, which is later oxidized to form one-dimensional crystalline silica nanostructures. Nucleation and growth of the nanofibers are enhanced by the high surface area of the micrometer- to nanometer-sized fragments of silica glass in the volcanic column. The formation of nanocrystalline cristobalite fibers during explosive lava dome eruptions poses new challenges for the assessment of the short- and long-term health hazards associated with the respirable nanofibrous components of volcanic ash.