From the 1995 lecture programme:
Viewed from Space, one of the remarkable features of our planet is that it has continents and oceans. Furthermore, a space traveller flying past every fifty million years, which is not long in the history of the Earth, would see a quite different arrangement of continents every time. The planet looks alive!
The changing pattern of the continents and oceans is the great characteristic of the Earth, and the discovery of how this movement occurs is one of the most exciting scientific stories of modern times. It is a remarkable story of how new instruments, designed to make new or more accurate measurements, led to a range of discoveries that could all be interpreted in a single simple vision known as plate tectonics.
The key to this understanding lay not in the continents at all, but in the oceans, which had remained largely unexplored and unknown. The oceans turned out to have a record of their history as precise as tree rings or a magnetic bar code, and with it came the history of relative motion between the continents. These motions are not fast by our standards — a few centimetres per year — but by geological standards they are very rapid, sufficient to carry a plate ten times round the Earth in the Earth's lifetime! With modern satellite-based technology we can now measure plate motions directly, at even the millimetre-per-year level.
As a description of what is happening in the oceans, plate tectonics is so simple, powerful and accurate that it has dominated our view of the Earth' s behaviour for the last twenty-five years. But there are some problems: it is very difficult to define plate boundaries on the continents, and, although most volcanoes are located near plate boundaries in the oceans, where does the liquid they produce come from?