Four billion years ago, Earth's continents begantaking shape. Scientists long held that this processunfolded gradually. It would have taken millions ofyears for granite, the primary component of thecontinental crust, to form in the mantle and migrateto the upper crust, they reasoned. But new research suggests that these events may haveproceeded at rather a different pace. According to a report published in the journal Nature, theemergence of granite occurred by way of swift, dynamic and possibly catastrophic eventslasting from 1,000 to 100,000 years.
Geologist Alexander Cruden of the University of Toronto and his colleagues turned toexperimental studies—melting rock samples, for example—to explore how granite magmaforms and how fast it can move. Their results proved surprising. "In the past we thought thatgranite magma, which cools and crystallizes to form very large granite intrusions, moved upthrough kilometers of crust as large, solid blobs at rates of about a meter per year," Crudensays. "But we've found that magma actually has quite low viscosity and is relatively runny.Because it is runny, it is able to channel its way from the mantle and lower crust throughfractures and cracks that are as small as one meter thick."
According to this model, granite intrusions in Greenland or the Canadian Shield, depending ontheir size, would have taken only thousands of years to form, which is extraordinarily fast froma geological point of view, Cruden notes.
It can reasonably be inferred that one reason scientists thought the continental crusttook millions of years to form was that they had ________
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