Wandering continents

The Earth's continents have wandered all over the surface of the globe in past eras, producing major climatic changes that may have killed off many species.

What are now Arrow Rocks in subtropical Northland were laid down 250 million years ago when New Zealand lay south of Australia and East Antarctica, almost astride the South Pole. Two hundred million years before that, New Zealand and Australia sweltered just north of the equator.

These extraordinary meanderings are driven by slow-moving currents deep inside the Earth which pull out hot rock at ridges, usually in the middle of the oceans, and move the rock across the ocean floors before diving back down at the edges of giant "plates".

Over the past 750 million years, the continents first coalesced as a supercontinent, Rodinia, centred in the middle of what is now the Pacific Ocean. About 600 million years ago bits broke off this assemblage near the South Pole and moved north into a vast ancestral sea which then covered most of the planet, eventually forming a new supercontinent, Pangaea.

As the bulk of the land moved away from the South Pole, the average temperature on Earth rose, making the end-Permian era, 250 million years ago, almost 10C warmer than today.

Since then, Pangaea has broken up again, first into the northerly Laurasia and the southerly Gondwana, and then into the relatively isolated continents we see today. As the southern continents pulled away from Antarctica, the Southern Ocean opened up, creating cold circumpolar winds, freezing Antarctica and lowering the whole planet's average temperatures.

New Zealand split away from Australia and Antarctica about 85 million years ago, carrying a cargo of Gondwanan plants and animals.

But Te Papa geologist Hamish Campbell says the country sank completely under the sea about 25 million years ago, with the possible exception of a few small islands. Any Gondwanan mammals that had come along for the ride may have died out.

New Zealand was lifted up again when the clash between the huge Pacific and Australian plates became much more active about 20 million years ago, pushing up the Southern Alps and the main North Island ranges.

Campbell says that means the kiwi, moa and other "native" New Zealand animals and plants must have come here in the past 20 million years or so.

"There are semi-flightless and flightless birds on many islands in the oceans, so you have to ask, how did they get around? I suspect it's quite simple. They have enough capability to glide in a decent wind."