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The oldest rocks exposed on Earth are nearly 4.0 billion years old. These metamorphic rocks — the Acasta gneisses — are found in Canada. It is probably no coincidence that the oldest rocks found are those that formed as the rate of asteroid bombardment in our solar system slowed. In Australia, outcrops of sedimentary rocks contain zircon grains that formed more than 4.2 billion years ago. The mineral zircon occurs mostly in rocks of the continents, which suggests that there were land masses at this time. |