Quick Dips

Geological Jigsaws

A team of geologists from New Zealand, the United States and Britain, are trying to sort out how Antarctica and New Zealand once fitted together in the great Gondwana landmass millions of years ago.

"Data from the ocean floor and satellite gravity shows very clearly where and how New Zealand and the Marie Byrd Land West Antarctica region used to fit together. One of the odd things is that despite very good evidence for the fit, there are geologically speaking some misfits," says team scientist Associate Professor John Bradshaw.

"When you re-assemble it, for instance, Thurston Island (off the Antarctic coast, near South America) fits alongside the Chatham Islands, but the geology of both islands is totally different. And so we have a situation where the pieces of the jigsaw fit together, but the picture doesn't match."

He says this shows that where the break took place was along a major geological boundary that existed before the split occurred. This in turn is specially relevant to New Zealand.

"New Zealand is peculiar because it is made up of a lot of rock units which are not closely related to each other. We are coming to the conclusion that New Zealand is composed of a whole lot of scraps, which are wedged together to form a new piece of continental crust.

"We are now learning, I think, the limits of the basket that came to contain New Zealand."

Pieces of New Zealand that match Marie Byrd Land include rock units from the West Coast of the South Island. However a lot of the rocks that form the eastern part of the North Island and the Southern Alps have no counterpart in Marie Byrd Land.

"I think the split occurred along what could be called the original natural limit of what was to become New Zealand, and so it tore apart along what was already a major boundary," Dr Bradshaw says.