Experiments have been carried out on initially gas-aerated and gas-fluidized granular flows propagating into a horizontal channel. After lateral acceleration following release of the originally fluidized bed, two contrasting flow behaviors were observed, which reflected the degree of initial fluidization and the grain size. Initial fluidization disrupts the interparticle contact network, which controls internal friction of the static bed. The flow regime then depends on the timescale needed to reestablish a strong contact network, and this time increases as the grain size decreases. Initially aerated and fluidized flows of coarse particles (>≈100 ¿m) and initially aerated flows of fine particles (<≈100 ¿m) behave as their nonfluidized counterparts and they propagate as a wedge, with decelerating velocities so that the front position increases as the ~0.8 power of time. In contrast, initially fluidized flows of fine particles propagate for most of their duration at constant thickness and frontal velocity in a similar fashion to the slumping regime of buoyancy-driven gravity currents of Newtonian fluids. We have determined a Froude number Fr for such flows of ≈2.6 consistent with published data from experimental and theoretical investigations on inviscid fluids. This implies that internal particle friction can be neglected in describing the dynamics of initially fluidized, concentrated fine granular flows. However, all flows are characterized by a short, final stopping phase whose timescale gives an estimate of the kinetics required to reestablish a strong contact network and form a static deposit. These results suggest that fines-rich pyroclastic flows may propagate as inviscid fluids for most of their emplacement.