80 million years ago
Isolated for 80 million years and contorted by extreme tectonic activity the islands of New Zealand remain one of the best available windows on the world’s deep past. Some of our living species, such as tuatara and kauri, are relatively unchanged since the time of the dinosaurs.
170 MYA: Gondwana
Today’s New Zealand landmass originated from a marginal sliver of the Southern Hemisphere supercontinent Gondwana.
The first rifts in this supercontinent occurred when Africa detached about 170 million years ago. Forty million years later (130 MYA) India with Madagascar broke away and the Atlantic Ocean opened up, separating Africa from South America.
130 MYA: Australis
The remaining eastern part of Gondwana, which contained the future Australia, New Zealand and Antarctica, was tenuously attached to the southern extremity of South America and is referred to as Australis. It straddled the South Pole but remained ice-free and would have supported rich cool-temperate forests for millions of years.
80- 60 MYA: Zealandia
Australis began to fracture about 80 MYA, when the future New Zealand region made its move, splitting away from Australia and Antarctica as the Tasman Sea opened up. Tectonic separation is usually an ‘unzipping’ process and, in terms of today’s maps, the pre-New Zealand rift extended northward from Tasmania through to North Queensland and south along the edge of Antarctica to Marie Byrd Land, splitting off an area about ten times the size of present-day New Zealand; we call this whole continent Zealandia. Full separation took over 20 million years with the Tasman Sea reaching its present width of 2,000 km around 60 MYA.
Today only 10% of the Zealandia continent sits above sea level. It was stretched during its separation, thinning the continental crust to the point where it slowly sank below the waves. Zealandia reached its lowest point about 23 MYA (during the Oligocene period) but has been rising ever since as a result of pressure along the Alpine Fault. Just how far did it sink?
Some bio-geographers suggest New Zealand remained as a few scattered islands throughout the Oligocene period, the largest the size of Canterbury and in total no bigger than present-day New Caledonia. The landscape would have been very low-relief with no hills over 300 metres.
Contrarily, some geologists have suggested the islands were totally drowned. They point to the lack of buoyancy of Zealandia’s crust and the occurrence of limestone over much of the surface of New Zealand (limestone forms on the sea bed from the shells of marine creatures).
If Zealandia did drown entirely, all of our fauna and flora must have arrived from across the ocean less than 25 MYA. However, a continuous fossil pollen record shows no loss of plant diversity through this period and the same dominant species are present before and after this ‘drowning’ period. Notably, molecular techniques have shown the New Zealand kauri, Agathis australis, to be the most ancient of the surviving kauri in Australia, Fiji, Melanesia and Vanuatu, dispersing from New Zealand overseas, not the other way around.
There are also strong arguments for the sustained presence of endemic animal species such as tuatara, moa, kiwi, wētā and many freshwater invertebrate fauna, but unfortunately there is no fossil evidence to verify their deeper history in this country.
For more detailed discussion on the bio-geographic history of New Zealand we strongly recommend George Gibbs’ excellent and popular book Ghosts of Gondwana (Craig Potton Publishing), available at our Store and other good book shops.