To learn how plate tectonics operated billions of years ago, look to the continents. Why? The oldest ocean floor is only 200 million years old. But continents contain vast areas of rock, called cratons, that have remained intact for billions of years. Cratons preserve information about ancient tectonic events and how the continents came to be. North America's craton is quiet today ... but its present tranquillity belies an eventful plate-tectonic history.

• United Plates of America
• Rocks From a Vanished Landscape
• Clues to a Very Different Earth