The History of Zealandia
Present day New Zealand does not appear to be a continent, but simply a group of islands in the South Pacific Ocean.
However, much of New Zealand’s area exists beneath its coastal waters. This is Zealandia - the New Zealand continent, a large fragment of the former super-continent of Gondwanaland. The image shows the present day shape of Zealandia. As well as the New Zealand landmass, it includes the surrounding pale coloured area of continental rock below sea level.
Zealandia's Dynamic Evolution
The geology of New Zealand tells us that for at least 400 million years, up to about 125 million years ago, the Pacific Plate was subducting beneath the eastern edge of the ancient continent of Gondwanaland. Sediments piled up along the margin of the continent and became crumpled and deformed as they were squeezed by the colliding plates.
About 125 million years ago, the tectonic situation changed and instead of compression the eastern margin of Gondwanaland was subject to extension and rifting that lasted for about 100 million years. This process had several significant consequences including formation of the Tasman Sea floor. This began to open 83 million years ago and persisted for about 30 million years until rifting ceased about 53 million years ago. A large fragment of continental Gondwanaland was separated off by the widening gap. This fragment, called Zealandia, was about half the size of Australia. Initially it as all land, but it slowly sank as the Tasman Sea grew, and it continued to be stretched and thinned until about 23 million years ago, when it was mostly under the sea.
Then the tectonic situation changed yet again and instead of extension there was a return to compression, plate collision and subduction. The modern plate boundary developed along the Kermadec Trench and southwards under the North Island, and the Southern Alps began to be uplifted along the Alpine Fault. Modern New Zealand has since been rising slowly out of the sea, with the much larger underwater portion of Zealandia still hidden below sea level.
Over the course of geological time, plate tectonics and oceanic processes have sculpted an underwater landscape that rivals anything onshore.