The oldest known rocks on Earth are 4.02 billion years old.

The Acasta Gneiss is a group of rocks found on an island in the Acasta River in northern Canada. Geologist Samuel Bowring and his team first identified their age in the late 1980s. They used a special machine called a SHRIMP to study the rocks. These are the oldest pieces of Earth's crust ever discovered.Scientists determine the age of these rocks by looking at zircon crystals. Zircons are very tough and do not break down easily over time. When these crystals first form in hot magma, they contain small amounts of uranium. They do not contain any lead when they are new.Over millions of years, the uranium inside the zircon slowly turns into lead. This happens at a steady speed called a half-life. By counting how much lead is inside a crystal compared to the uranium, researchers can calculate its exact age.Most of Earth's original surface has been destroyed or recycled by moving tectonic plates. The Acasta Gneiss is rare because it survived this process for billions of years. It proves that Earth had a solid surface very early in its history. This was a time when the planet was cooling down from a ball of hot liquid.