A speculative model for the nonlinear phase of the collisionless tearing instability is developed for the case of a singel long wavelength tearing mode. Using an energy principle formalism, we find that the nonlinear growth rate is linearly proportional to the mode amplitude. Hence in the nonlinear phase, the tearing mode grows explosively in time, and saturates when the width of the magnetic islands become comparable to the thickness of the current sheet. For typical plasma sheet parameters, the explosive phase lasts 5--10 minutes, and develops cross-tail emf's of several 100 KV.