The massif is much older than the nearby chaîne des Puys, dating to the end of the Tertiary era. It is a deeply eroded stratovolcano, similar to the Cantal massif to the south, but more reduced in area and volume. Its history began about 3 million years ago with a Plinian eruption that created a large caldera, the contours of which are no longer clearly defined. This eruption resulted in the emission of ignimbrite rhyolitic pumice that covered over of land. Afterwards, phonolite domes were created on the exterior of the caldera. A period of calm lasting between 1 and 1.5 million years ensued, succeeded by a second major eruptive period that formed the modern Puy de Sancy, the highest peak in the Massif Central, followed by the establishment of smaller domes, dikes, and maars.