The South American continent as we know it formed during the break-up of West Gondwana between 150 and 110 million years
ago, when the South Atlantic Rift system evolved into the South Atlantic ocean. Using state-of-the-art global tectonic reconstructions
in conjunction with numerical and analytical modelling, we investigate the geodynamics of rift systems as they evolve into
an ocean basin. We find that rifts initially stretch very slowly along the future splitting zone, but then move apart very quickly before
the onset of rupture. In case of the split between South America and Africa, the divergence rate increased from initially 5 to 7
millimetres per year to over 40 millimetres per year within few million years. Intriguingly, abrupt rift acceleration did not only occur
during the splitting of West Gondwana, but also during the separation of Australia and Antarctica, North America and Greenland,
Africa and South America, in the North Atlantic or the South China Sea. We elucidate the underlying process by reproducing
the rapid transition from slow to fast extension using analytical and numerical modelling with constant force boundary conditions.
The mechanical models suggest that the two-phase velocity behaviour is caused by a rift-intrinsic strength–velocity feedback
similar to a rope that snaps when pulled apart. This mechanism provides an explanation for several previously unexplained
rapid absolute plate motion changes, offering new insights into the balance of plate driving forces through time.