Résumés
Summary
The massive rockslide-debris avalanche ofthe May 18, 1980, eruption of Mount St.Helens began with a retrogressive failure triggered by the 08:32 PDT earthquake. It depressurized the volcano's magmatic and hydrothermal system and produced a hummocky deposit with a volume of 2.5 km3. Detailed work provides a comprehensive understanding of a previously poorly understood type of event.
The deposit consists of relatively intactpieces (block faciès) of the pre-1980 mountain and mixed material (mixed facies) that is primarily rocks from the pre-1980 Mount St.Helens and the 1980 cryptodome. Travelpaths of rockslide blocks are interpreted from a geologic map of the deposit. The material was fractured and dilated during the rockslide, after which grain-to-grain dispersive stress facilitated flow. During transport, the dilated material mixed but significant finematerial was not produced.
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