Their findings appeared in the December 6 issue of Nature. Because the continental crust is largely made up of these intrusions, the prevailing view was that the continents grew slowly and steadily over millions of years. Because it is runny, it's 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. Because of this, an intrusion of granite 50 km wide in Greenland or the Canadian Shield may have taken just 50,000 years to form, whereas researchers previously would have estimated it to have taken millions of years.
Smaller intrusions form even faster, some in as little as one thousand years.
From a geological perspective, that's extremely fast, says Cruden. The scientists studied melting rock samples in their experiments to try to figure out how granite magma forms and how quickly it can move in the upper mantle and lower crust. They applied the results to theoretical models that were based on a combination of studies and fieldwork in the Canadian Shield, Sweden, the Sierra Nevada of California, Greenland and the Andes of South America. The research is unique in that the three phases of granite formation - generation, ascent, and emplacement, are studied together as one system. In the past, the three processes have been studied separately.