A numerical simulation of impulsive growth and sudden disruption of a thin current sheet due to inward boundary flows in a collisionless plasma, governed by the equations of Hall magnetohydrodynamics, is presented. After a period of slow growth followed by a much shorter period of impulsive enhancement, the thin current sheet becomes unstable to secondary tearing instabilities which produce multiple islands. These secondary islands coalesce to form a large plasmoid. Much of the thin current sheet disrupts rapidly to produce magnetic turbulence in less than a characteristic Alfv¿n time. Possible implications for magnetotail substorms are discussed. ¿ 1999 American Geophysical Union