| This day in 1980, an earthquake with a 5.1 Richter magnitude triggered a gigantic landslide on the north face of Mount St. Helens in southwestern Washington. The north slope fell away in an avalanche that was followed by a lateral air blast, which carried a high-velocity cloud of superheated ash and stone outward more than 20 km (12 miles) from Mount St. Helens's summit. The avalanche and lateral blast were followed by mudflows, pyroclastic flows, and floods that buried the river valleys to the ... |