The dynamics of a random distribution of spontaneously formed 2D GaAs islands are studied using scanning tunneling microscopy. The equilibrium concentration of islands is easily tuned from 0% to 50% coverage by only changing the As-4 overpressure. Images taken during the early stages of island formation reveal the roughening transition primarily occurs through an intermediate pit formation phase. Interestingly, pit formation in the middle of an otherwise pristine terrace is overwhelmingly preferred to atom detachment from the edges of the terraces. (C) 2002 Elsevier Science B.V. All rights reserved.