Following about two months of intense outward movement and strength deterioration associated with magmatic intrusion, seismicity and gravitational creep, an earthquake on 18 May 1980 marked the collapse of the hot, fluid-rich, north sector of Mount St Helens. Pressure release associated with slide movements resulted in hydrothermal and magmaiic explosions. These explosions produced a lateral blast that partly overran the first slide pulse and devastated a landscape of over 550 km2. Disruption of the sliding masses resulted in the formation of an enormous avalanche of debris that travelled for about 10 min, as far as 23 km. Average velocity was about 35 m/s, and peak velocity about 70 m/s. An area of 60 km2 was buried with 2·8 km3 of hummocky, poorly sorted debris to an average depth of 45 m, and levees to 30 m high were plastered against valley walls and impounded tributaries. One avalanche lobe entered a lake and caused wave run-up to 260 m. Limiting equilibrium analyses and laboratory testing of slide debris suggest that initial failure occurred in a material with c <6 bar, &hi&i;40°, with significant pore fluid pressures and transient shear stresses from a trigger earthquake. Early motion can be characterized by block sliding with an apparent basal friction coefficient of about 0·1. Disintegration of the slide blocks then led to fully developed avalanche flow, to a first approximation involving a Bingham material with strength of about 0·1–1 bar and viscosity 105–106P. The high mobility of the Mount St Helens avalanche, typical for volcanic avalanches, was induced by hot fluids of the depressurized magmatic-hydrothermal system. À la suite d'environ deux mois de poussée intense vers l'extérieur et d'une réduction de résistance due à des intrusions magmatiques accompagnant des événements séismiques et au fluage par gravité, l'affaissement du secteur nord du Mont St Helens, chaud et riche en fluides, fut marqué par un tremblement de terre qui eut lieu le 18 mai 1980. Une chute de pression associée à des mouvements glissants causa des explosions hydrothermales et magmatiques. Ces explosions créèrent un souffle latéral qui dépassa en partie la première pulsation de glissement et dévasta un paysage de plus de 550km2. La rupture des masses glissantes créa une coulée énorme de débris qui se déplaça pendant environ 10 min jusqu'à une distance de 23 km à une vitesse moyenne d'environ 35 m/s, la vitesse maximale étant d'environ 70 m/s. Un terrain couvrant 60 km2 fur enseveli sur une profondeur moyenne de 45 m sous 2·8 km3 de débris accidentés et mal assortis, et des levées jusqu'à une hauteur de 30m furent plaquées contre les côtés des vallées et bloquèrent les affluents. Une branche de l'avalanche pénétra dans un lac, causant des vagues jusqu'à une hauteur de 260 m. Des analyses de l'équilibre limite et des essais de laboratoire des débris de glissement donne l'impression que la rupture initiale a eu lieu dans une matière de c′<6 bar, ø&i;40°, avec des pressions interstitielles considérables et des contraintes transitoires de cisaillement à partir d'un tremblement de terre causateur. Les premiers mouvements peuvent être qualifiés de glissements de blocs avec un coefficient de frottement évident à a la base d'environ 0-1. La désintégration des blocs glissants entraîna alors à une véritable coulée d'une matière du type Bingham ayant en première approximation une résistance de l'ordre de 0·1 — 1 bar et une viscosité de 105—106P. La grande mobilité du Mont St Helens typique des coulées volcaniques fut provoquée par des fluides chauds du système magmatique-hydrothermal décomprimé.

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The Chillos Valley Lahar (CVL), the largest Holocene debris flow in area and volume as yet recognized in the northern Andes, formed on Cotopaxi volcano's north and northeast slopes and descended river systems that took it 326 km north-northwest to the Pacific Ocean and 130c km east into the Amazon basin. In the Chillos Valley, 40 km downstream from the volcano, depths of 80-160 m and valley cross sections up to 337 000 m 2 are observed, implying peak flow discharges of 2.6-6.0 million m 3 /s. The overall volume of the CVL is estimated to be ;3.8 km 3 . The CVL was generated approximately 4500 years BP by a rhyolitic ash flow that followed a small sector collapse on the north and northeast sides of Cotopaxi, which melted part of the volcano's icecap and transformed rapidly into the debris flow. The ash flow and resulting CVL have identical components, except for foreign fragments picked up along the flow path. Juvenile materials, including vitric ash, crystals, and pumice, comprise 80-90% of the lahar's deposit, whereas rhyolitic, dacitic, and andesitic lithics make up the remainder. The sand-size fraction and the 2-to 10-mm fraction together dominate the deposit, constituting ;63 and ;15 wt.% of the matrix, respectively, whereas the silt-size fraction averages less than ;10 wt.% and the clay-size fraction less than 0.5 wt.%. Along the 326-km runout, these particle-size fractions vary little, as does the sorting coefficient (averagep2.6). There is no tendency toward grading or improved sorting. Limited bulking is recognized. The CVL was an enormous non-cohesive debris flow, notable for its ash-flow origin and immense volume and peak discharge which gave it characteristics and a behavior akin to large cohesive mudflows. Significantly, then, ash-flow-generated debris flows can also achieve large volumes and cover great areas; thus, they can conceivably affect large populated regions far from their source. Especially dangerous, therefore, are snowclad volcanoes with recent silicic ash-flow histories such as those found in the Andes and Alaska.

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An explosive eruption of Mount St. Helens on 19 March 1982 had substantial impact beyond the vent because hot eruption products interacted with a thick snowpack. A blast of hot pumice, dome rocks, and gas dislodged crater-wall snow that avalanched through the crater and down the north flank. Snow in the crater swiftly melted to form a transient lake, from which a destructive flood and lahar swept down the north flank and the North Fork Toutle River.

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