During the summers of 1978 and 1979, Johnston led studies of the ash-flow sheet emplaced in the 1912 eruption of Mount Katmai in the Valley of Ten Thousand Smokes. The gas phase is extremely important in propelling volcanic eruptions. Because of this, Johnston mastered the many techniques required to analyze glass-vapor inclusions in phenocrysts embedded in lavas, which provide information about gases present during past eruptions. His work at Mount Katmai and other volcanoes in the Valley of Ten Thousand Smokes paved the way for his career, and his "agility, nerve, patience, and determination around the jet-like summit fumaroles in the crater of Mt. Mageik" impressed his colleagues.