Origin and early evolution of lahar channel at Mabinit, Mayon Volcano, Philippines

Abstract

Mabinit Channel, 5 km long, 15 to 70 m wide, and 2 to 22 m deep, was formed by lahars on the southeastern slope of Mayon Volcano during its 1984 eruption. In 1985, the channel was drastically modified by a lahar triggered by a typhoon. Sediment-budget calculations from surveys conducted in 1985 and 1986 corroborate inferences from other volcanoes that lahars can grow significantly in volume by eroding their channels. The 1984 eruption produced a deep summit ravine and, at its base, a fan of avalanche and pyroclastic-flow deposits 2 km2 in area and 40 x 106 m3 in volume. Lahars initiated in the ravine by heavy rains from eruption updrafts carved out 5-km-long Mabinit Channel along the west margin of the pyroclastic-avalanche field, a position that could not have been predicted from pre-eruption topography. By eroding the channel, the lahars generated more than half of their own solid contents. Below 240 m, where the volcano slope decreases from 5° to 3°, the first eruption debris flows were unconfined and spread out over a 512,000-m2 area, depositing a layer locally more than 4 m thick with a volume of 1.25 x 106 m3. The thin margins of these unconfined flows were non-erosional; however, as they stopped, their thick central portions continued flowing, extending the new channel downslope through the depositional field. Subsequent eruption lahars modified the channel and overflowed at its bends. A moderately intense typhoon in October 1985 triggered a single 9-hr lahar event that widened the channel by an average of 25 m and caused as much as 66 m of lateral bank erosion. Debris flows overtopping the channel at four sites coalesced to cover a 2-km-long, 200,000-m2 area with bouldery sediment. In wider channel stretches, debris-flow margins left prominent paired levees, while their central portions continued moving down-channel as rigid plugs. The lahar plugged the lower 0.5-km stretch of the channel and replaced it by eroding a new channel of comparable size. This demonstrated how easily lahars can change the courses of channels on the upwardly convex debris apron of a stratovolcano, with serious consequences for farmland and communities.