Over time sand and silt eroded down the Gondwana land mass. This compacted and formed a sedimentary rock called graywacke.
Formation process of Tongariro Volcano Centre
The Tongariro complex of volcanic cones and craters was formed by eruptions from at least 12 vents over more than 275,000 years. Erosion during the last Ice Age has worn away what was once a substantial mountain into the world famous hiking destination that it is today.
The Pacific plate subducted under the Australian plate and volcanism began. This process resulted in magma forming 150km below the Tongariro Volcanic Centre.
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