Interior boundaries

A CHUNK of the earth's crust off Canada's west coast is beginning to disappear, says a report by North American geologists. The fragment, known as the 'Explorer plate', is fusing with its neighbouring plates to form a new plate boundary and will cease to exist independently. This is the first time that geologists have been able to assess a boundary formation in the act (New Scientist, Vol 153, No 2067).

The tectonic plates making up the earth's rigid shell are in constant motion, grinding past their neighbours, to transform boundaries or diving beneath one another in regions called subduction zones. A rarer and shorter-lived event is where a new boundary is born and another dies. A similar event took place in California where the San Andrea's Fault was formed 20 million years ago when a plate cracked as it was being gobbled up beneath the continent of North America. Kristen Rohr of the Geological Survey of Canada and Kevin Furlong of Pennsylvania State University, US, found that something similar might be happening in the case of the Explorer plate because a large number of earthquakes had been mapped recently with it. "This is Canada's most seismically-active region, but it shouldn't be," says Rohr. In a subduction zone, he explains, the earthquakes should occur under the continental edge, rather than offshore within the oceanic plate.

So, in a study, Rohr and Furlong used seismic reflection profiles, made by sending pulses of sound into the ocean floor, to try and see where the plate was deforming as a result of the earthquakes. They found a large field of faults, not at its border but near its middle. The researchers estimate that the plate has transformed about 90 per cent into a new plate boundary. The whole transition takes about million years, which is a mere moment on the geological timescale.

Dave Sanderson, a geologist at the University of Southampton, UK, Says that if the Explorer plate is still splitting, it provides the only known contemporary scene of this important event. "This would be a real chance to study these fundamental processes," he says.

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