  We report spatially resolved, time-dependent, magnetization reversal
measurements of an Fe$_{8}$ single molecular magnet using a microscopic Hall
bar array. We found that under some conditions the molecules reverse their spin
direction at a resonance field in the form of an avalanche. The avalanche front
velocity is of the order of $1$m/sec and is sensitive to field gradients and
sweep rates. We also measured the propagation velocity of a heat pulse and
found that it is much slower than the avalanche velocity. We therefore conclude
that in Fe$_{8}$, the avalanche front propagates without thermal assistance.
