We discuss results from 3+1-D numerical simulations of SU(2) Yang--Mills equations for an unstable Glasma expanding into the vacuum after a high energy heavy ion collision. We expand on our earlier work on a non-Abelian Weibel instability in such a system and study the behavior of the instability in greater detail on significantly larger lattices than previously. We establish the time scale for the onset of the instability and demonstrate that the growth rate is robust as one approaches the continuum limit. For large violations of boost invariance, non-Abelian effects cause the growth of soft modes to saturate. At late times, we observe significant creation of longitudinal pressure and a systematic trend towards isotropy. These time scales however are significantly larger than those required for early thermalization in heavy ion collisions. We discuss additional effects in the produced Glasma that may speed up thermalization.

19 pages, 6 figures, uses REVTEX4; v2: added discussion, matches published version