Igneous rocks of basic character dominate the Central Plateau. A great dolerite sheet of Jurassic caps the Plateau and forms its resistant surface. Later, sporadic basalt lavas of Tertiary age fill old drainage depressions cut in the Plateau. The dolerite is far more voluminous, but less varied in its chemical composition (approx. 1500 cu. km; silica range 52-60%) than the basalts (approx. 15 cu. km; silica range 36-53%). Both these rocks express important events which affected the Southern Hemisphere. The dolerite is the vast molten response to initial fracturing of the southern supercontinent, Gondwanaland, of which Tasmania is a small fragment. The basalts form part of the eastern Australian volcanic province which erupted in response to warping, stretching and increased heat flow along the continental margin as sea-floor spreading opened up the Tasman Sea and Southern Ocean, beginning about 85 million years ago.