DESCRIPTION:
Snowpack and Ice Accumulation Hazards

Emplacement or flowage of hot pyroclastic rock debris on or into thick snowpacks on volcanoes can trigger hazardous rapid flows of sediment (including ice grains) and water. Such rapid flows can extend far beyond the flanks of a volcano, as has been observed at volcanoes in many parts of the world (Major and Newhall, 1989). Commonly these sediment-water flows achieve discharges and velocities that produce catastrophic consequences more than 100 kilometers downstream from source areas. The hazardous nature of such flows was most recently demonstrated in 1985 at Nevado del Ruiz volcano in Colombia, where snowmelt-generated sediment-water flows killed about 23,000 people (Lowe and others, 1986; Naranjo and others, 1986; Pierson and others, 1990).

Snow-covered stratovolcanoes like those in the Cascade Range and in Alaska commonly erupt explosively and produce potentially unstable lava domes. On such volcanoes volcanic explosions and mass failures from lava-domes are two primary mechanisms by which a hot rock having a large surface area is brought into contact with snow. Through this contact heat is effectively transferred to the snow, and large volumes of rapidly melted snow can trigger a variety of flows involving sediment (initially the hot volcanic debris) and water. Once triggered, the sediment-water flows are likely to erode and incorporate additional material from older deposits and thus increase in volume. In addition to providing the heat to melt snow, stratovolcanoes commonly provide other conditions favorable to the initiation of sediment-water conditions favorable to the initiation of sediment-water flows:

1. high topographic relief and steep slopes;
2. the presence of poorly consolidated or unconsolidated, erodible pyroclastic rocks on slope surfaces; and
3. weakened structural integrity caused by hydrothermal alteration of rocks forming the edifice.

The details of interactions between hot pyroclastic rock fragments and snowpacks are poorly understood. Hydrologic consequences of eruptive activity are probably fairly common at snow-clad volcanoes but have largely gone unnoticed in the past owing to the remoteness of most such volcanoes, to the small-scale, short-lived nature of most of the phenomena, and to the sometimes enigmatic nature of deposits that once consisted partly of ice particles.

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