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Geological Time

| Geologic Time Scale | Plate Tectonics | Radiometric Dating | Deep Time | Geological History of New Zealand |

The Geological History of New Zealand

Acrobat Icon  NZ Geology Map - PDF file [2.17 MB]

Acrobat Icon  NZ District Map - PDF file [335 KB]

NZ Geology Map

NZ District Map

Modern New Zealand is world renown for being geological active with high mountains, frequent earthquakes, geothermally active areas and volcanoes. This is due to New Zealand's modern position on the boundary of the Australian and the Pacific Plates. The collision of these plates caused the Pacific plate to subduct underneath the Australian plate which carries the North Island. To the south of the South Island, the situation is reversed. The subduction zones in New Zealand are defined by trenches in the north and in the south and by the Alpine Fault which connects the two. This plate boundary has shaped the size of New Zealand and also defines its geology.

The islands forming New Zealand developed as part of a broader continental shield made up of Antarctica and Australia, forming part of Gondwana. Radiometric dating places the oldest rocks in New Zealand being at least 500 million years old.

New Zealand's geological history can be divided into three main periods of sedimentation and three periods of mountain building (orogeny):

1. The early sedimentation depositional phase, Cambrian to Devonian period (about 545 to 370 million years)

This was the period when the earliest major recorded rock formation was taking place, with the oldest rocks being found on the west coast of the South Island. It was the beginning of the area that is now known as New Zealand and was just off the coast of Gondwana. Some volcanic islands built up after they became extinct, were covered with sands and mud, washed down from the land. Sometimes the land uplifted only to be worn down again and be pushed back into the sea.

2. The Tuhua Orogeny, late Devonian to Carboniferous period (about 370 to 330 million years ago)

The long period of sedimentation ended with a period of pressure and uplift. Sea floor sediments were pushed up, folded and melted together to form mountains. Under great heat and pressure, rocks were completely changed and regrouped into new minerals. Sandstones and mudstones, for example, became schist, known for the parallel layering of minerals like mica, giving it a shimmering, silvery surface. Plutonic intrusions formed granite and in some places diorite. The exact mountain building details are not known as materials got deformed, eroded or covered by younger material, but it can be presumed that all these activities happened along the former coast of Gondwana, long before New Zealand existed.

3. The New Zealand Geosyncline, Carboniferous to Jurassic period (about 330 to 142 million years ago)

During this time enormous thicknesses of sediment accumulated, extending northwest from New Zealand to New Caledonia and south far below the South Island. The rocks of this second cycle of deposition have formed much of the foundations of New Zealand. Sediments of the New Zealand Geosyncline form the oldest rocks in the North Island. Two main groups of rocks can be identified from this period: the Torlesse supergroup on the east mostly made up of greywacke with only very few fossils. And the Murihiku supergroup in the west, with a good series of fossils, with sediments rich in volcanic debris. The Torlesse supergroup rocks were deposited further away from a landmass, mostly in deep water, and the Murihiku rocks represent mainly continental shelf and slope deposits.

4. The Rangitata Orogeny, Early Cretaceous period (about 142 to 99 million years ago)

During this orogeny, the previously deposited geosyncline (3) sediments were compressed and folded. Some seafloor was caught in the folding and later exposed when the orogeny had finished and erosional forces had levelled the mountains. These seafloor rocks can be seen today at Dun Mountain, West Dome and Red Hills in the Nelson region of the South Island. The orogeny deformed the sediments deposited during the earlier sedimentation period differently: the western rocks in open simple folds, whereas the eastern block was much more severely deformed, commonly in a stack of folds with complex faulting.

5. The break-up, Cretacous to Oligocene period (99 to 24 million years ago)

This period is characterised by a long period of weathering and erosion of the mountains that were formed during the preceding orogeny, so much so that some places were reduced to so called peneplains, that is areas of low relief. About 85 million years ago a rift valley formed to separate the New Zealand region from the rest of Gondwana, resulting in the formation of a new ocean floor by means of sea floor spreading. This spreading resulted in the gradual formation of the Tasman Sea. The movement of the New Zealand continental block also resulted in a marine transgression and by the Oligocene period (about 35 million years ago) most of the country was submerged. By that time the sea floor spreading had stopped and the land had begun to sink, resulting in characteristic marine deposits: calcareous and fossiliferous, with common limestone. At the end of this period a new rift valley formed between Australia and Antarctica and the sea floor spreading still continues.

6. The Kaikoura Orogeny, Miocene to Quaternary period (24 million years ago to modern)

Due to the new spreading ridge between Antarctica and Australia there was a build up of strain in the southwest Pacific crust that led to vertical and transcurrent fault movements. This resulted in uplift of central Westland and produced the majestic range of the Southern Alps, with its steep, straight western front the Alpine Fault. Between 23 and 10 million years ago the western side of the Alpine Fault was moving northeast relative to the eastern side at a rate of between 1-10 cm per year. This has resulted in about 450 km of displacement along the Alpine Fault. This is why Palaeozoic rock belts in Nelson match reasonably well with those to the south in Fiordland. Widespread tectonic activity continued from 10 million years ago to the modern, and during this time the principal mountain ranges of both islands were uplifted and New Zealand began taking its modern shape. The subduction of the Pacific plate caused much volcanism in the North Island, starting initially in Northland in the early Miocene and moving south over time until it reached its present position along the Taupo Volcanic Zone.

Reference Websites  Reference Websites

Find out more about New Zealand's volcanoes on:

Reference Books  Reference Books

Nelson C. S., Balks M. R. and Chapman R. (2003). Study Guide for ERTH103A Discovering Planet Earth. Hamilton, New Zealand: Department of Earth Sciences, The University of Waikato

Selby M. J., (1985). Earth's changing surface; Clarendon Press, Oxford.

Suggate R. P., Stevens G. R. and Te Punga M. T. (Eds.) (1978). The Geology of New Zealand. Wellington: Government printer

Thornton J., (1997). The Reed Field Guide to New Zealand Geology. Auckland: Reed Books

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