Geology breaks new ground

October 10, 2003

A team of British scientists has chipped away at one of the bedrock theories of geology to find a new origin for the very ground beneath our feet.

It is widely believed that the continental land masses are made from partially melted rock from the fringes of the oceanic plates.

The process that takes this material from under the oceans to the tops of mountains is called subduction.

But on Wednesday, Nick Petford, reader in geology at Kingston University, presented an alternative at the tenth geological congress in Chile.

"Our work casts a big question mark over modern interpretations of how the earth's continental crust formed," he said.

Along with colleagues at Kingston, Liverpool and Durham universities, Dr Petford proposed that new continental crust comes from the molten rock of the underplate, deep below our feet.

Some of this material could stick to the underside of land masses, partially melting the crust, which could then move up towards the surface.

In effect, the continents extract solid rock directly from the sea of molten rock they rest on.

The traditional model holds that ocean-plate expansion drives solid sheets of basaltic rock under the continents and into the depths of the Earth's mantle until they melt. Some of this molten material then rises to the surface.

Dr Petford said that the chemical signature of ancient rocks, which many have taken as evidence of subduction, could be created as easily by the new mechanism.

He said his fieldwork in the Peruvian Andes suggested that subduction could not account for that part of the South American continent as the ocean floor there was too old and hence too cold to melt and produce new rock.

Tracey Rushmore, a geologist at the University of Vermont in the US, said her fieldwork in New Zealand produced similar results. She felt the new mechanism might work alongside subduction.

Hervé Martin, professor of geology at the Université Blaise Pascal in France, said the Andes work was convincing, but added: "These results cannot easily be generalised."

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