The dynamics of surface roughening and healing induced by Cl on Si(100)-(2×1) were studied with variable-temperature scanning tunneling microscopy. Clean samples were exposed to Cl2 at room temperature, heated to as high as 700 K, and imaged for periods that exceeded 20 h. Chlorine caused surface roughening via a Cl recycling pathway that created pits and regrowth islands with minimal desorption of SiCl2. This reaction pathway is accessible at ∼675K if the surface is not saturated with Cl. Images showed dimer vacancy creation and pit growth, together with vacancy capture by pits. They also showed dimer vacancy escape from pits and pit annihilation, which had not been reported previously. When the terrace coverage was 0.93 ML, the diffusivity of dimer vacancies along the dimer rows was ∼0.7Å2/sec, about three orders-of-magnitude lower than that on clean Si(100). Single Si adatoms were created on terraces and bonded to Si dimers that were Cl-free. They could form new regrowth dimers or they could be accommodated at the ends of existing islands. Adatoms could also be released from regrowth islands. Pairs of atom vacancy lines separated by one or two Si dimers were observed and the transition between those lines and dimer vacancy lines was recorded. Local areas with (3×2) symmetry were created from the latter. These dynamic changes all require Cl-free Si dimers, and the rate of change of the surface morphology reflects the surface concentration. The rate of change increases with time as the density of bare dimers increases due to the creation of new sites for Cl on an increasingly rough surface and limited desorption.