Geologically the volcanoes of the Tongariro National Park are of recent origin, dating back to about two million years, reaching their greatest heights during the last Ice Age when glaciers extended down the slopes of Ruapehu to about 1220m. The Tongariro National Park lies at the south-western extremity of a volcanic chain which extends through White Island in the Bay of Plenty and the Kermadec Island to the islands of Tonga 2000km to the north-east.
The volcanoes in Tongariro National Park are part of the long line of volcanic and earthquake activity that extends around the Pacific Ocean as the Ring of Fire. This activity is caused by the Pacific and Australian tectonic plates pushing against each other, with the Pacific Plate being forced under the Australian Plate in the North Island.
The land mass of Tongariro National Park was formed by many different eruptions from at least six cones. They all share the same alignment with the oldest lava dated at about 275,000 years ago, near what is now the Tama Lakes on the southern flanks of Mt Ngauruhoe. The eruptions continued for the next 200,0000 years, as the ice retreated it carved our valleys clearly visiable in the lower Mangatepopo and Oturere Valley. Clearly you can see this on your hike on the Tongariro Crossing or the Tongariro Northern Circuit.
Mt Ruapheu, Ngauruhoe and Tongariro are Stratovolcanos (Composite cone Volcanos) The last eruptive activity on Mt Tongariro was a gas and steam driven eruption on Monday 6 August 2012. The eruption lasted only a couple of minutes and occurred partly from existing vents at the Upper Te Maari Crater. It also involved the formation of a new crater and eruption fissure.
Mt Ngauruhoe last erupted in 1977 and true to her name (meaning; throwing hot stones) ejected red hot blocks of lava and ash thrown 3km away.
All the volcanoes in the Tongariro National Park are monitored and alerts are emailed and posted on the Geonet Website.