GWh pa,. about four percent of the country's total electricity generation. The extra catchment increases the volume of water going into Lake Taupō and down the Waikato River by 20%. The western diversion takes water from six rivers and streams from the Whakapapa River to the Whanganui River, into Lake Rotoaira via Lake Otamangakau. The Tokaanu power station is connected to Lake Rotoaira via a six kilometre tunnel through Mount Tihia. It also draws water from the Tongariro River via the Poutu tunnel and canal. On the eastern side of Mount Ruapehu, water is diverted from the Kaimanawa Ranges and headwaters of the Wahianoa River through a 20 km tunnel into the Rangipo dam. The 120MW Rangipo power station is 230 m below ground. Its turbines are located in a cavern cut from solid rock and lined with concrete. Genesis Energy has developed mitigation measures with stakeholders that aim to lessen the environmental effects of the Tongariro Power Scheme. Some of these measures include lake level management, ecological monitoring programmes, and agreements with local iwi to establish a process for iwi involvement in environmental monitoring and access to information. The electricity generated at the Tongariro Power Scheme goes into the national grid. Then it is distributed to residential, commercial and industrial customers throughout New Zealand. Geology The park's volcanoes are the southern end of a 2500 km long range of volcanoes, below which the Australian Plate meets the Pacific Plate. These volcanoes have resulted from internal tectonic processes. The Pacific Plate subducts under the Australian plate, and subsequently melts due to the high temperatures of the aesthenosphere. This magma being less dense, rises to the surface and goes through the weak parts of the Earth's crust (the faults) resulting in volcanic processes in the area. Volcanic processes have been building the mountains of Tongariro National Park for over two million years. Three volcanoes (Tongariro, Ngauruhoe and Ruapehu) remain active, while the park's two northernmost volcanoes (Pihanga and the Kakaramea-Tihia Massif) last erupted over 20,000 years ago. They have however produced significant historic mudflows. Erosion and deposition by mountain glaciers has also played an important role in shaping Tongariro and Ruapehu volcanoes. Small glaciers are present on the summit of Mt. Ruapehu today, however there is abundant geomorphological evidence for more extensive glaciation in the recent geological past. Glaciers were last present on Tongariro during the Last Glacial Maximum. Biology Flora