Scientists discuss characteristics of the rocks and internal structure of the landslide deposit at Mount St. Helens. More than 100 scientists from around the world examined the landslide and other volcanic deposits during a field trip to Mount St. Helens, sponsored by the International Association of Volcanology and Chemistry of the Earth's Interior in 1984.
The dark red and black rock layers behind the scientists consist of shattered or brecciated lava flows that were erupted and then cooled on the upper flanks of Mount St. Helens; the lava flows were shattered as the landslide moved away from the volcano. The flows are less than 2,500 years old. The tan-colored rocks in the upper right are from dacite lava domes erupted between 3,000 and 2,500 years ago; the hard lava was subsequently altered and weakened by hot acidic groundwater. Both rock units and the contacts between them are preserved in the landslide deposit even after moving >5 km from the volcano.
More about the landslide deposit at Mount St. Helens
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