Image
This is an east–west cross-section of the central North Island, passing though Lake Taupō and Taranaki (Mt Egmont). As the Pacific Plate descends, heating causes fluids to rise into the overlying mantle, producing magma. These magmas are mainly basalts. They evolve into andesite that erupts to form cone volcanoes. Heat from the basalt magma causes the continental crust to melt. This leads to huge rhyolite eruptions involving caldera collapse.
Magma erupted from Taranaki comes from a much deeper source than that beneath the Taupō Volcanic Zone. This is reflected in the composition of the andesite magmas.