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Continents grow by cuddling together

Crash course When a continent collides with another it doesn't just latch on to the landmass, it actually wraps itself around it, a new study shows.

The three dimensional computer simulations, reported in the journal Nature, provide scientists with a detailed view of how continents grow at their margins.

"What we've done is to unify very large scale collision processes, and develop a unifying model that really begins to understand the dynamic balance of what is happening in the crust as these collision processes take place," says lead author, Professor Louis Moresi of the University of Melbourne.

Scientists have known for over half a century that Earth's continents move around on tectonic crustal plates, powered by convection currents in the mantle, which are generated by the intense heat of the planet's core.

The light continental crustal plates are pushed along at between one and three centimetres a year, by the denser oceanic crustal plates on the sea floor, which rise from the mantle along mid-ocean ridges, and dive back down into the mantle at subduction zones near the edges of continental plates to form mountain ranges.

This movement and collision of continental plates at subduction zones is one of the main mechanisms for making continents bigger.

"We've been looking at the process by which these fragments of crust get swept around ... and stick together to make larger continents," says Moresi.

"We didn't really know the dynamics of this process, and didn't understand how the subduction zone recovers from the process of collision."

Evolution of Tasmanides

The authors developed their model by studying the evolution of the Tasmanides, the Australian segment of a 400-million-year-old active tectonic convergence zone known as the Terra Australis orogen, between the Palaeo-Pacific plate and the Jurassic era East Gondwana.

The Tasmanides comprise numerous micro continents which helped to build the eastern third of Australia, and now form a collage of arc shaped contorted geologic belts along the entire length of the country.

Moresi and colleagues found that as a segment of land collided with a continent, it pushed a significant distance in to that continent.

This process displaces some of the continent's original land, which forms mountains, and also spreads out horizontally, forming an arc encompassing the colliding segment of land.

"By building this in computers, we could see exactly how much goes into building the mountains, thickening the crust underneath, and squeezing material around the sides," says Moresi.

The authors also saw the subduction zone beneath the collision site curl backwards in a broad curve to form a new subduction zone at the new continental margin.

"We found that the subduction zone process restarts quite quickly," says Moresi.

"The sheet of cold oceanic crust going down into the Earth has to change its shape quite dramatically bending to match what's happening at the surface."

Moresi and colleagues are now looking at places of active continental collision, such as the Indian subcontinent's ongoing impact into the Eurasian continent.

"The other place that's interesting for us, is to look at Australia and what's happening as we head north," says Moresi.

"There's a collision going on between the Australian continent and the subduction zones north of us. That's a particularly complicated example which will take a vast amount of time to figure out what's going on there.