Tongariro National Park covers an area of 79,600 ha and is characterized by its three volcanic peaks arising in the heart of the North Island. From north to south, these are Mount Tongariro (1967 m), and his second peak Mount Ngauruhoe (2287 m), as well as the highest in North Island: Mount Ruapehu (2800 m).
Lake Rotoaira marks the park's most northern border. From here the Alpine Crossing leads through the Okahukura Bush, past the Sulfur Lagoon, Kehetahi Springs and the Blue Lake at 1725 m above sea level leading to Tongariro's summit. The Whanganui River, the third longest river in the country, arises here and flows to the park's northwestern border. In the very west lies the plateau of Mount Hauhungatahi iat an altitude of 1521 m. The entire south surrounding Mount Ruapehu is rather flat and covered in forest. Unlike the south, the Rangipo Desert suffers from fast and dry western winds that have been drained by mountaintops before reaching the park's eastern side.
Tongariro National Park lies within the Taupo Volcanic Zone and is part of the Pacific Ring of Fire. It is created by the Hikurangi Subduction Zone, a result of tectonic movement, as the Pacific plate slowly shifts underneath the Indian-Australian plate. The volcanoes of the Taupo Volcanic Zone are believed to be less than 500,000 years old.
In general, the park contains mostly sediments and volcanic rocks. The mountains are made up of andesites, an igneous rock type that is composed of slightly more silicon dioxide (SiO2) than basalt.
A typical volcanic eruption is, as we imagine it, a crater that continuously catapults lava and debris in all directions. This phenomenon is caused when gases are released from rather thick deposits of lava. The less the viscosity of such deposit, the more easily gases are set free. The lava produced in the Taupo Volcanic Zone is intermediate in terms of composition. Thus, the lava may reach the surface as a result of explosions, or as a passive outflow. The North Island's rocks were created from extrusion, meaning that the fine-grained rocks once were magma that had solidified on the earth's surface.
Mount Ngauruhoe is a stratovolcano composed of several layers (strata) that form a steep volcanic cone. In comparison, others may have a collapsed crater, called 'caldera'. In fact, Lake Taupo covers such crater, the massive Taupo Volcano, that is believed to have caused the two biggest supereruptions ever in New Zealand: The Oruanui Eruption (nearly 27,000 years ago) and the Hatepe Eruption that dates back to about 180 A.D. Some historians believe that they have traced the Hatepe Eruption back to parts of China and Europe where it caused weather phenomena such as ashy colours in the sky.
Tongariro National Park was clearly shaped by its local volcanoes, which are influencing the landscape today - whether it be the Kehetahi Springs (fumaroles), the crater lakes or the massive lahars that bring lava debris down the slopes to the surrounding areas. A fumarole is a steam emitting crack in the earth's crust that may release volcano-typical chemical compositions such as carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen chloride (HCl), and hydrogen sulphide (H2S).