A SLICE OF ACASTA GNEISS — ONE OF THE OLDEST ROCKS ON EARTH

This is a slice of one of the oldest rocks in the world. This gneiss (a metamorphic rock subjected to high temperature and pressure) is approximately 4.0 billion years old. Its age can be determined as a result of zircon crystals having trapped a uranium isotope that acts as a timekeeper. Acasta Gneiss originates from an old and stable part the Earth's two topmost layers — the crust and the uppermost mantle — in a geologically inactive portion of Earth’s continental crust. Acasta Gneiss is seen as an outcrop on an island that is middle of Canada’s Northwest Territories and is the oldest rock exposed on Earth’s surface.