Oldest intact rock (Acasta Gneiss)

The Acasta Gneiss, an outcrop in Canada's Northwest Territories about 300 kilometers north of Yellowknife, is the oldest dated intact rock on Earth. In 1999 Samuel Bowring and Ian Williams used uranium-lead dating on its zircon crystals and found that the original rock crystallized about 4.03 billion years ago. Gneiss is a banded rock formed when an older igneous rock is baked and squeezed deep in the crust. Here the original rock, a granite-like body, later underwent metamorphism, leaving a record of more than one event in its crystals.

The Acasta rocks matter because their chemistry resembles that of continental crust, which is rich in silica and lighter than ocean floor. That implies that continent-like crust existed within the first 500 million years of Earth's history, soon after the Moon-forming impact. Their age displaced the earlier record holder in Greenland, about 3.8 billion years old, by more than 200 million years. Older minerals exist, such as the Jack Hills zircons, but they are loose grains, not an intact rock. Other claims of older rocks, including some in northern Quebec, remain disputed.