Puy de la Vache is a Strombolian-type volcano, meaning that it produced mixed eruptions of projections of ashes, slags, volcanic bombs, and lava flows. Its cone is a volcanic edifice with flanks formed of slags and lava. The red color of some slags means that they have been exposed for a long time to the heat, favoring their oxidation. The surface of the volcano's cone also consists of phenocrysts, large crystals visible to the naked eye in the igneous rock.