From the 1995 lecture programme:
We live on the edge of a ball in space, orbiting a star. How do we find out what we are made of and where we came from?
It is a daunting task: the centre of the Earth is more than 6,000 km away and we can't dig a hole that deep! We have to look for clues: and they may be unexpected ones. What can we learn from a large earthquake in Bolivia that was felt in Canada, and made the Earth wobble for months? What can we see in volcanoes, which bring material from deep underground to the surface?
The early navigators knew that the Earth's magnetic field changed with time, but how does that help us to know that the Earth has a core made of liquid iron? What clues are provided by our neighbours in the solar system? We have visited or flown past some of them, and some of them visit us in the form of meteorites. Such clues tell us what the Earth is made of, that it is layered, and that it is very old — it formed about 4,500 million years ago.
But that is not all. As we go deeper into the Earth it gets hotter and this encourages all crystalline materials, including rocks, to flow. The flow happens in the solid state and is called creep: the rocks do not have to melt to do this. The same rocks which deep down are hot and weak can be very strong at the surface, where they are colder. So the Earth has a strong outer lid above a weak, creepy inside. This dual behaviour is the key to how the Earth can be so mobile.