Abstract

Our objective is to show how geology may help to make progress in the understanding of some fundamental concepts and issues dealing with space, time and movement. This leads us to the very theory of relativity in physics. A thought experiment is presented where geology has fully played a role. Deep (geologic) time is a way to speak of very slow movements, involving imperceptible changes of the ordinary space we live in. According to the relative speeds involved, what is space becomes time (when we accelerate the movements of the mountains); conversely, time may become space (at the scale of the femtoseconds where the sand grains in the egg-timer are immobile). This is not just an allegory: we can never stop, thinking we have reached a pure time and a pure space; because we are inside the world and we can only compare what moves more (on which we build time) to what moves less (on which we build space) within a relational thinking. In this context, conventions are necessary, in particular that of the constancy of a standard movement: this is what we do today with light. The uniformity hypothesis in geology plays the same role. The links between space, time and movement is discussed in various fields of geology (sedimentary sequences, metamorphism, metasomatism, magmatism, …) as they are omnipresent in prehistory.