Abstract

Volcaniclastic sedimentation during and subsequent to the 1971-74 eruption cluster of the Volcano Fuego in Guatemala has occurred in four distinct phases which are part of a 15 to 25 year cycle of sedimentation. In phase 1, the eruption cluster generated 6 × 108 cu m of tephra, one-third in the form of glowing avalanches, the remainder as an elongate airfall ash blanket west-southwest of the cone. Glowing avalanches with a volume of 5 × 107 cu m formed two fans, each 1 to 3 m thick, east and west of the crater. Further avalanches flowed down seven narrow canyons radiating to the south of the crater forming 40-m-thick deposits totaling 1.3 × 108 cu m. During phase 2, debris flows and flash floods removed about one-third of the phase 1 canyon deposits in the first 2 years following eruption. Fan deposits remained intact. Three digitate, 1 to 2.5-m-thick, debris-flow deposits (2.2 × 107 cu m) and two 1-m-thick flood fans (1.8 × 107 cu m) formed south of the crater. In phase 3, terraced, meandering, suspended-load streams were metamorphosed to braided, aggrading, bed-load systems annually eroding 6 million tons of phase 1 and 2 debris, primarily from the canyon deposits. Transport of about two-thirds of this debris to the sea has produced rapid coastal progradation. During phase 4, 15 to 25 years of phase 1 and 2 activity will remove canyon avalanche deposits, redistributing the material in stable fans on the lower volcanic slopes. Phase 1 and 2 processes become inactive while stream incision produces discontinuous terracing. Fluvial systems return to meandering, suspended-load streams.

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