The Taupo Volcanic Zone is about 50 kilometres wide and about 300 kilometres long and lies over a subduction zone in the earth's crust. Mount Ruapehu marks it's southwestern end, while White Island is considered its northeastern limit. The subduction zone appears to extend further to the northeast, as along the line of the Taupo Volcanic Zone lie several undersea volcanoes known as the Rumbles and also the Kermadec Islands, with another caldera at Raoul Island. There is no volcanic activity to the southeast, although the mountain ranges of the lower North Island as well as the Southern Alps in the South Island indicate that the subduction zone does extend along the length of New Zealand. For this reason the Taupo Volcanic Zone is thought to be the western terminal of the Pacific Ring of Fire, which marks out the subduction zones around the Pacific Ocean.
Recent scientific work indicates that the earth's crust below the Taupo Volcanic Zone may be as little as 5 kilometres thick. Beneath this there appears to be a region of partially melted material that is the source of magma for the volcanoes in the zone. Because of this thin crust, all known volcanoes in the area are considered active, even though they have not erupted recently. Geological records indicate that some of the volcanoes in the area erupt infrequently but have large, violent and destructive eruptions when they do. The last major eruption from Lake Taupo is believed to have first emptied the lake then followed that feat with a pyroclastic flow that covered about 20,000 square kilometres of land with volcanic ash. Over 50 cubic kilometers of material is estimated to have been ejected in one eruption.