The dynamics of frictional slip have been studied for centuries yet many aspects of these everyday processes are not understood.
One such aspect is the onset of slip. First described by Coulomb and Amontons as the transition from static to dynamic friction, the
onset of frictional slip is central to fields as diverse as physics, tribology, and the mechanics of earthquakes and fracture. We study
the dynamics of how this transition takes place by performing real-time visualization of the true contact area which forms the interface
separating two blocks of like material. The results show that the onset of frictional motion is driven by the interplay of three different
types of coherent crack-like fronts, which propagate along the interface, reducing the contact area as they progress. We show both
how and why these different entities are selected and discuss the relevance of these observations to the dynamics of earthquakes.