Cratons are the stable, ancient cores of continents. They consist of a thick, rigid foundation called the basement rock, which is mostly made of igneous and metamorphic rocks. These rocks were formed billions of years ago, during the early stages of Earth's history.

Within these cratons, we find the shields, which are exposed portions of the basement rock. These shields are the most ancient and stable parts of the Earth's surface.

Some of the oldest known rocks on Earth are found in the Canadian Shield, which is a vast expanse of ancient rocks in North America. Rocks from the Canadian Shield have been dated to over 4 billion years old. Other notable examples include the Australian Shield and the Baltic Shield.

While the ocean floor is constantly being renewed through plate tectonics, the oldest oceanic crust is still much younger than the continental crust, with the oldest oceanic crust being about 200 million years old.

Therefore, if you are looking for the oldest crust on Earth, look for the cratons and their exposed shields.