Glowing avalanches from the 1974 eruption of the volcano Fuego, Guatemala

Glowing avalanches from the 1974 eruption of the volcano Fuego, Guatemala, deposited between 0.005 and 0.01 km³ of nonimbricated, nongraded, nonwelded debris on the southern flanks of the volcano. Avalanches emerged from notches in the crater rim, and traveled some 7 km from the vent at an average speed of 60 km/hr. Each glowing avalanche consisted of two parts, (1) the main mass of moving debris, or underflow, and (2) a superjacent gas and dust cloud. The postdepositional thickness of individual underflows is generally less than 2 m, but successions of glowing avalanches produced deposits as much as 15 m thick in some ravines. The underflows and their superjacent gas and dust clouds followed topographic lows. Where underflows were confined laterally by valley walls, the surface of the underflow was featureless, after deposition. Where underflows were able to spread laterally, they developed a surface morphology of channels, levees, and overbank areas. The coarsest fragments were deposited in the channels.

The underflows probably flowed in laminar fashion as high concentration dispersions, similar to normal debris flows. No unequivocal evidence of large-scale gas expansion of the underflows was noted. Gas emission is considered to have occurred during transportation, but it probably played a minor role in fluidization of the underflow. The ability of these glowing avalanches to transport particles as much as 5 m in diameter some 7 km from the crater is considered to be a reflection of the high yield strength of the underflow.

Deposits from the 1974 eruption had a mean grain size of −3.00ϕ (8 mm), were extremely poorly sorted (σ₁ = 4.6) strongly fine skewed (SK_I = 0.63), and platykurtic (K_G = 0.71). All grain size distributions were bimodal with a dominant intermodal low at −1.0ϕ (2 mm). The small percentage of particles in the −1.0ϕ size interval is considered to reflect the inherent mechanical instability of particles of this size.

Turbulently flowing gas and dust clouds, as much as 1,000 m high, accompanied each avalanche, and cloud height increased with increasing distance of travel. The fine-grained dust of the superjacent gas and dust clouds was not deposited on top of the underflow. Prevailing winds blew the fine-grained component to the west-southwest.