| Geologic Time Scale | Plate
Tectonics | Radiometric
Dating | Deep Time | Geological
History of New Zealand |
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
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
shield made up of Antarctica and Australia,
forming part of
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
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
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
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
spreading. This spreading resulted in the
gradual formation of the Tasman Sea. The movement of the New
Zealand continental block also resulted in a
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.
Find out more about New Zealand's volcanoes on: http://www.gns.cri.nz/what/earthact/volcanoes/nzvolcanoes/index.html
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
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