Image: Courtesy of the University of Toronto
Four billion years ago, Earths continents began taking shape. Scientists long held that this process unfolded gradually. It would have taken millions of years for granite, the primary component of the continental crust, to form in the mantle and migrate to the upper crust, they reasoned. But new research suggests that these events may have proceeded at rather a different pace. According to a report published today in the journal Nature, the emergence of granite occurred by way of swift, dynamic and possibly catastrophic events lasting from 1,000 to 100,000 years.
Geologist Alexander Cruden of the University of Toronto and his colleagues turned to experimental studiesmelting rock samples, for exampleto explore how granite magma forms and how fast it can move. Their results proved surprising. "In the past we thought that granite magmawhich cools and crystallizes to form very large granite intrusionsmoved up through kilometers of crust as large, solid blobs at rates of about a meter per year," Cruden says. "But weve 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 through fractures and cracks that are as small as one meter thick. This way, the magma can ascend 20 to 30 kilometers into the upper crust quite rapidly." In fact, Cruden compares the process to subterranean volcanic eruptions. Over time these short, quick bursts of molten granite subsequently cooled and accumulated, building up the continental crust.
According to this model, granite intrusions in Greenland (above) or the Canadian Shield, depending on their size, would have taken only thousands of years to formwhich is extraordinarily fast from a geological point of view, Cruden notes. "This research has important implications for how we understand the basic physics and chemistry of crust formation processes," he remarks, "as well as the formation of economic ore depositsgold and copper, for examplemany of which are associated with granite intrusions."