Highest point of the Massif Central, Puy de Sancy is a dome-flow. During successive volcanic eruptions, sanidine crystals formed in the magmatic chamber were kept in suspension by convection currents initiated by a permanent arrival of magma in the chamber. The abundance of these centimetre-sized crystals greatly increased the viscosity of the magma before its expulsion. Glacial erosion then shaped the summit, which now takes the shape of a neck of sancyite rock (photo).