Geologists recognize lavas and ash deposits from about 60 past eruptions in the area around Mammoth Mountain and Devils Postpile, California. This raises the unanswerable question, “When will it erupt again?” An alternative, answerable, and informative question is, “How often has it erupted?”
In the Mammoth Lakes Sierra, geologists have mapped in great detail all the lavas and ash deposits produced by those 60 eruptions. They have dated almost all of them by laboratory methods, showing that eruptions have been repetitive and persistent, though not quite regular, over the last quarter-million years. For few volcanoes in the world is the long-term eruptive frequency so well calibrated as in the Mammoth Lakes Sierra.