Pagan Cross, of Balclutha Primary School, asks :-
When did New Zealand get made and how?
Hamish Campbell, a geologist with GNS Science and the co-author (with Gerard Hutching) of the book 'In search of ancient New Zealand', responded.
The New Zealand landmass is 'made' largely of sedimentary rocks that formed in the Panthalassa Ocean (a name for the paleo-Pacific Ocean) along the eastern margin of the Gondwanaland super-continent between 510 and about 125 million years ago (from Cambrian to Cretaceous time).
For at least 385 million years, the eastern margin of Gondwanaland was an active plate boundary involving collision and hence subduction of the Panthalassa Ocean floor beneath Gondwanaland. As it was subducted westwards, the mobile Panthalassa Ocean floor behaved like a bulldozer and rocks were scraped off the ocean floor and plastered back on to the margin of Gondwanaland. As a result, the margin of Gondwanaland slowly grew (by accretion) in an easterly direction.
The rocks that were scraped off and accreted were formed from: 1) sediments that accumulated on the ocean floor as a result of erosion of the Gondwanaland landmass by large rivers and 2) from eruptions of subduction-related (Pacific Ring of Fire) volcanoes, rather like Ruapehu today.
Things changed dramatically 125 million years ago and instead of collision, eastern Gondwanaland was subject to the opposite which is stretching.
This resulted in a great elongate fragment of Gondwanaland being rifted away to form the continent of Zealandia. This was a large area of land about half the size of Australia. The early phase of this stretching produced a great deal of granite formation, between 125 and 105 million years ago (best seen in Nelson, Fiordland and Stewart Island).
With ongoing stretching, the Tasman Sea floor formed (it took 35 million years, between 85 and 50 million years ago) and Zealandia became increasingly isolated and was slowly thinned and hence slowly sank (literally), reaching a low point with very little land about 25 million years ago (Oligocene time). It has been suggested that there may have been no land at all about 23 million years ago but this is unproven.
Another dramatic change happened about 25 million years ago and instead of stretching, there was a resumption of plate collision and subduction. As a result, New Zealand was pushed up out of the water so to speak. In fact, New Zealand may be regarded as the emergent part of the otherwise sunken 7th continent of Zealandia.
As Zealandia sank, sedimentary formations accumulated on the sea floor. Then New Zealand was pushed up and much of those marine formations (sandstones, siltstones, mudstones, limestones) have either been eroded away or buried by glacial outwash and river gravels, wind-blown sands and silts (loess), vocanic ash (tephra) and soil.
With subduction come volcanism, and the addition of significant rock, especially in the Taupo Volcanic Zone, central North Island. In a few places, there has been intraplate (basalt) volcanism such as the Chatham Islands, Campbell Island, Auckland Island, Otago Peninsula, Banks Peninsula, Auckland, Oamaru, Timaru.
So New Zealand is the product of 510 million years of normal crustal processes.