We present evidence for explosive vaporization in fused silica, initiated by picosecond pulses from a tunable free-electron laser (FEL) operating in the wavelength region from 2 to 10 mum. The unusual pulse structure of the FEL, which produces 1 pr micropulses at intervals of 350 pr in a macropulse lasting up to 4 mus, makes it possible to test separately the effects of intensity and fluence. We show that thermal descriptions of the ablation process fail in the regime when there is high vibrational excitation density in the solid due to resonant absorption of mid-infrared laser light.