  We investigate quench dynamics of spin ice after removal of a strong magnetic
field along the [100] crystal direction, using Monte Carlo simulations and
theoretical arguments. We show how the early-time relaxation of the
magnetization can be understood in terms of nucleation and growth of strings of
flipped spins, in agreement with an effective stochastic model that we
introduce and solve analytically. We demonstrate a crossover at longer times to
a regime dominated by approximately isotropic clusters, which we characterize
in terms of their morphology, and present evidence for a percolation transition
as a function of magnetization.
