Leading Edge / Spring 2001
Rocks of Ages

Discovering the path of granite magma

A new study says that granite intrusions, one of the principal building blocks of Earth’s continental crust, form in rapid volcanic-like bursts of geological activity, and not by slow and steady movements of rocks over millions of years. Many scientists have long believed that molten granite magma moves up through the crust in large semi-solid blobs at a rate of about one metre per year. Instead, Professor Alexander Cruden and an international research team used theoretical experiments and fieldwork to show that granite magma is, in fact, quite fluid and channels its way up through small fractures in the crust at a much faster rate. Large parts of the Earth’s continental land masses, such as the Canadian Shield, formed between one billion and four billion years ago. The land masses were created by tens of thousands of quick injections of molten granite that ascended from the lower crust and then intruded into the upper crust as huge horizontal sheets. These intrusions took between 1,000 to 100,000 years to form, depending on their size (a typical intrusion covers an area about the size of Toronto). “These sheets then cooled and crystallized to form the large bodies of granite that we see exposed at the surface of all continents today,” says Cruden, associate professor of geology.

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