PLATE TECTONICS AT 50

50 years ago a small number of people at an even smaller number of laboratories put together a set of ideas that are now know as plate tectonics. I was the youngest of those involved, and, together with Jason Morgan, put the theory into what is probably its final form. But the ideas that were so novel 50 years ago now seem so obvious, even to primary school children, that I am always asked 'So what did you believe before plate tectonics?' This is the question I will try to answer, and to explain why the ideas were not obvious to us at the time. Doing so requires removing the understanding of geodynamics that everyone now has. The underlying issue is technological: we needed accurate global observations, of earthquake locations, mechanisms and of the oceanic magnetic field, none of which we had 50 years ago. I expect technological advances now in progress to have the same impact on the controversial and important ideas that presently concern Earth scientists.