How did Earth's continents first form? A University of Alberta geochemistry PhD student studied a 4-billion-year-old rock and concluded: Look at modern-day Iceland.

The student, Jesse Reimink, collected ancient rock samples from the Acasta Gneiss complex in Canada's Northwest Territories, which contains some of the planet's oldest rocks.

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Because they're so old and have undergone so many changes, many of the Acasta Gneiss rocks don't lend themselves to precise geochemical analysis. But some of the ones Reimink collected were preserved well enough to reveal what Reimink called, in a University of Alberta (U of A) article, "crust-forming processes that are very similar to those occurring in present-day Iceland."

Today it's accepted that plate tectonics -- in which pieces of the Earth's crust shift beneath each other into the Earth's mantle and cause magma to rise to the surface -- are responsible for the buildup of continents.

But, Reimink told the U of A, it's not as clear whether plate tectonics existed a couple of billion years ago. One theory holds that the land masses formed in the ocean as liquid magma rose from Earth's mantle and then, over much time, cooled and became a solid crust.

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Iceland's crust built up when magma from the mantle rose to shallow waters and incorporated volcanic rocks already in place. Reimink says this makes today's Iceland a good proxy for how continental crust formed in the early life of Earth.

Reimink's 4-billion-year-old rock seems to back up that notion. "This provides the first physical evidence that a setting similar to modern Iceland was present on the early Earth," he told the U of A.

Reimink, is lead author of a new study in Nature Geoscience that details his findings.