By reading the rocks, geologists and other scientists are answering the great questions about the evolution of our planet. How old is the Earth? Where did its materials come from? How did they form an iron core and a stony mantle? Ancient rocks tell the history of continents that float like blobs of scum among the ocean floor rocks from which they were made. Moonrocks help to fill in a missing 800 million years of our own world's history. And a meteorite tells of an astonishing coincidence at the birth of our solar system which gave us all the rocks and, eventually, the living forms of Earth.

Although we know the Earth is a hot, radioactively powered machine, as yet there is no satisfactory explanation of how that machine works. This programme outlines some of the clues.

On the Rocks: a series on the geology of Britain, next Tuesday on BBC2