During accretion of the Earth, a layer of liquid iron may have formed above a cold undifferentiated nucleus. Taylor instability of this configuration leads to a deformation with spherical-harmonic order l=1, a lower symmetry than assumed heretofore in fission analyses. When this deformation exceeds a few tens of kilometers, strength of the materials becomes insignificant, and the deformation may grow catastrophically, the cold nucleus being displaced laterally at a velocity limited only by inertia. The asymmetric distribution of angular velocity leads to large dynamic pressure in regions of velocity convergence, so that the excess angular momentum of the iron layer may be transferred to a jet of mantle material. Sufficient energy and angular momentum might have been released by core formation to eject material to form the Moon.