  A catastrophe model for the onset of fast magnetic reconnection is presented
that suggests why plasma systems with magnetic free energy remain apparently
stable for long times and then suddenly release their energy. For a given set
of plasma parameters there are generally two stable reconnection solutions: a
slow (Sweet-Parker) solution and a fast (Alfv\'enic) Hall reconnection
solution. Below a critical resistivity the slow solution disappears and fast
reconnection dominates. Scaling arguments predicting the two solutions and the
critical resistivity are confirmed with two-fluid simulations.
