Continental drift

The Earth’s continents have not always been where they are at present. If you look at a map of the world, you might notice what Alfred Wegener noticed – that the continents look as if they could fit together like a big jigsaw puzzle if you were able to move them around. Wegener published his theory in 1915. He tried to explain how the Earth drifted apart, but he was unable to give a scientific explanation. Many years later, though, this theory began to gain popularity, and now we understand more how it is possible that land masses can move.

Continental drift is the concept that the Earth’s continents move relative to each other, with the Earth’s surface being broken into plates.

Nature of science

Although it is reliable and durable, scientific knowledge is neither set in concrete nor perfect. Rather, it is subject to change in the light of new evidence or new interpretation of existing evidence. Because of its tentative nature, we cannot claim ‘absolute truth’ in science. The tentative nature of scientific knowledge also means that laws and theories may change.

What evidence is there that continental drift has occurred?

• The continents can be fitted together rather like a jigsaw.

• Rock records show matching layers, mountain ranges and ancient basement rocks in continents that were once together.

• Glacial striations (scratches) and erratics (rocks moved away by glacial ice from original bedrock) correspond between continents.

• Some distinctive fossils found on the southern continents indicate that they came from one ancient single continent.

• Magnetic records left in the rocks seem to show that the Earth’s poles have changed, but the current thinking is that this is not the case – it is the rocks (plates) that have moved.

How do the plates move?

While the mechanisms are still being discussed, the current thinking is that the upper mantle of the Earth is in a state of convection, with hot material rising under diverging zones (plates moving apart) and cool material sinking in subduction zones (one plate diving and sinking underneath another plate).

Crustal plates

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Movement of crustal plates over semi-molten asthenosphere.

Rights: U.S. Geological Survey (USGS)

At converging plate boundaries, the plates collide and either mountains are formed or one plate is forced down into the mantle (subducted) under another. At transform plate boundaries, one plate slips alongside the other in the opposite direction.

What effect has continental drift had on Antarctica?

• About 300 million years ago (mya ) – There was only one large landmass called Pangaea. At that time, the land that is now Antarctica would have been much closer to the equator.

• 200 mya – Pangaea was separated by the Tethys Ocean. Laurasia was in the north and a southern part of the continent was Gondwana. Laurasia was close to the equator and made up of what is now North America, Europe and Asia. Gondwana consisted of the present day South Africa, South America, India, Australia, New Zealand and Antarctica.

• 180 mya – Gondwana started to break up along the spreading plate boundaries. The climate was warm, and there were animals, trees and no ice sheets, much like New Zealand’s climate today.

• 50 mya – Antarctica moved towards the South Pole, away from Australia and South America and surrounded by the Southern Ocean. Ocean currents and wind could now circle the Earth without being blocked by land masses. These circumpolar winds blocked transfer of heat from the tropics, and heat trapped and carried by ocean currents no longer reached Antarctica. Temperatures cooled, and the East Antarctica ice sheet formed.

• 14 mya – Temperatures were no longer cooling, and the West Antarctica ice sheet formed.

At Antarctica, scientists from GNS are investigating the thickness of Earth’s crust, the way seismic waves move through the crust and are dating rocks to better understand global plate movements and Antarctic’s past environment in relation to global climate change.