  The transition from static to dynamic friction is often described as a
fracture-like instantaneous slip. However, studies on slow sliding processes
aimed at understanding frictional instabilities and earthquakes report slow
friction transients that are usually explained by empirical rate-and-state
formulations. We perform very slow ($\sim nm/s$) macroscopic-scale sliding
experiments and show that the onset of frictional slip is governed by
continuous non-monotonic dynamics originating from a competition between
contact aging and shear-induced rejuvenation. This allows to describe both our
non-monotonic dynamics and the simpler rate-and-state transients with a single
evolution equation.
