Mount St. Helens National Volcanic Monument
Ref: US Forest Service (2025). Mount St. Helens National Volcanic Monument. WA.
___________________________________________________________________________
Cascades
The Cascadia subduction zone is a plate boundary that parallels the Pacific NW coast, 48-80 km offshore. Here, the Juan de Fuca Plate sinks under the North American Plate.
___________________________________________________________________________
Chronology
18 May, 1980: Volcanic eruption of Mount St. Helens in the US state of Washington. A 5.2M earthquake at 0832 triggers a massive landslide that collapses the northern flank of the mountain and relieves pressure off the underlying magma chamber- the cryptodome. This results in a powerful lateral blast from the side of the mountain, relieving additional pressure, and resulting in cloud of volcanic ash and rock that rises 15 miles tall within 15 minutes (Mount St. Helens, US Forest Service).
___________________________________________________________________________
