True lava flows have been a rarity at Fuego. Although lava flows have been observed near the summit, the steep slopes have caused them to be converted to avalanches before descending the cone. These glowing avalanches descend the barrancas surrounding the cone, and deposit as block and ash flows on the fans below the steep barrancas. Deposition of these block and ash flows causes a surge of sedimentation in the downslope drainages.
Twilight photo of Fuego eruptions of 1966, showing incandescent block and ash flow materials descending the barrancas.