Explosive eruptions usually involve thick, highly viscous, silicic or felsic magma, high in volatiles like water vapor and carbon dioxide. Pyroclastic materials are the primary product, typically in the form of tuff. Eruptions the size of that at Lake Toba 74,000 years ago, at least 2,800 cubic kilometres (670 cu mi), or the Yellowstone eruption 620,000 years ago, around 1,000 cubic kilometres (240 cu mi), occur worldwide every 50,000 to 100,000 years.[1][n 1]