Molten granite speed shifts

It was once assumed that molten granite took thousands or even millions of years to travel from its source deep within the earth to just below the surface. But this movement could take place over a few years or even months, according to new research from Kingston University.

If this research is correct, it will change the way geologists think about how continents grow. Continents are about 35km thick, with thin strata of rocks on the surface underlain by kilometres of crystalline material. It is the movement of molten granite from deep within the earth towards the surface that determines the evolution of these crystalline regions.

If the granite melt is moving far faster, as John Clemens, professor of geosciences at Kingston suggests, one result could be that the chemicals required to form valuable deposits of minerals could be brought to the earth's surface more frequently than was previously thought.

Professor Clemens presented his research at the third international symposium on the origin of granitic rocks, which was held last month at the University of Maryland, attracting more than 180 delegates from countries.

Initiated in 1987 to commemorate the bicentenary of the publication of James Huttons Theory of the Earth, the meetings, held on a four-yearly basis, are intended to bring together experts on granitic rocks to discuss recent developments and identify future research directions.

Professor Clemens's research was opposed at the conference by scientists arguing that molten granite moves towards the surface in a huge, bulbous mass and does indeed take thousands, or even millions of years. Scott Paterson of the University of Southern California and Roberta Weinberg of the Australian National University both argued for the longer time frame.

The meeting also discussed questions about how the molten granite, once formed, segregates into fractures and veins, and how the pre-existing rocks make room for the rising molten rock. New techniques, such as rhenium-osmium isotopes, which can tell scientists where molten granite first originated within the crust, were also discussed.