Mount St. Helens

Mount St. Helens is the most historically active volcano in the Cascade range, having produced four major explosive eruptions since 1479, and dozens more smaller eruptions, including pyroclastic flows, lava flows and domes, and lahars.  It is located approximately 80 km NE of Portland, OR.

Mount St. Helens is best known for its large explosive eruption, summit collapse and directed blast of May 18, 1980, which was the most expensive and deadly volcanic event in United States history. The volcano continued erupting during 1980-1986, producing a lava dome within the 1980 crater. Mount St. Helens again erupted in 2004-2008, when it produced only minor explosive activity but a series of spectacular lava spines with a cumulative volume of almost 100 million cubic meters that doubled the size of the lava dome. Tremors and millions of small earthquakes accompanied both of the recent eruptions. Since early 2008 no eruptions have taken place and the lava dome has shrunk in height as its steep sides crumble into a broader, more symetrical shape.

Here are links to some of the descriptive web pages provided by the Cascade Volcano Obervatory.

More information about Mount St. HelensMore information about the 1980 eruptionMore information about the 1980-1986 eruptionMore information about the 2004-2008 eruption

Background Seismicity

The Cascades Volcano Observatory and the PNSN cooperatively operate 21 seismometers on or near Mount St. Helens. On average, we locate 4 to 14 earthquakes within 10 km of the volcano each week. Mount St. Helens and the nearby St. Helens Seismic Zone occasionally produce swarms of many small earthquakes, but these are not usually directly associated with magmatic activity. An additional challenge on Mount St. Helens is that small rockfalls commonly occur on the steep crater rim.

For lots of detail on the past seismicity at Mount St. Helens see the seismicity page.