The map shows the known distribution of recognizable ash layers from three fairly typical explosive eruptions of Mount St. Helens in the past few thousand years. The eruption that produced the Yn ash was about four times as large as that of 1980 and was probably the largest explosive eruption in the volcano's history.
https://pubs.usgs.gov/fs/2005/3045/
Today, scientists are using the 10-year anniversary of the eruption at Mount St. Helens to bring awareness to the new and crucial eruption prediction technology that they’ve installed all over the Northwest since 2004, the Associated Press reports.
https://weather.com/science/news/mount-st-helens-new-eruption-prediction-technology-20140929
This simplified chronology shows the volcanic history of Mount St. Helens from its earliest beginnings about 300,000 years ago (300 ka) to its devastating and deadly eruption on May 18, 1980. The major volcanic deposits of each stage and period are listed to the right of the time columns.
https://pubs.usgs.gov/fs/2005/3045/
Mount St. Helens, famous for its explosive 1980 eruption, has long been the most active volcano in the Pacific Northwest. U.S. Geological Survey (USGS) scientists have documented the volcano's 300,000- year geologic history, including powerful explosions of ash, outpourings of lava, and huge landslides and volcanic mudflows.
https://pubs.usgs.gov/fs/2005/3045/
On May 18, 1980, Mount St. Helens, Washington, exploded in a spectacular and devastating eruption that brought the volcano to the attention of the world. Few people realized that Mount St. Helens had long been the most active volcano in the Cascade Range of the Pacific Northwest.
https://pubs.usgs.gov/fs/2005/3045/
It looks like Mount St. Helens is getting ready to erupt again and it can happen in the order of years to decades, USGS seismologist Seth Moran told the Associated Press. The USGS has been monitoring Mount St. Helens since a far more catastrophic eruption in 1980 claimed 57 lives. The eruptions that began in 2004 and continued until 2008 were of the less violent dome building kind, where lava seeping into the volcano’s crater gradually builds up over the years and eventually fills the crater back in.
https://weather.com/science/news/mount-st-helens-new-eruption-prediction-technology-20140929
The primarily ash-producing eruptions of Mount St. Helens during Smith Creek time did not significantly change the volcano's shape. Pine Creek Eruptive Period (2.9 to 2.55 ka)--During the Pine Creek Eruptive Period, Mount St. Helens erupted ash and produced pyroclastic flows and dacite domes, and two small debris avalanches occurred on its north flank.
https://pubs.usgs.gov/fs/2005/3045/
The last significant eruption of Mount St. Helens before 1980 is generally considered to have occurred in 1857. Minor explosions reported in 1898, 1903, and 1921 were probably steam-driven and not magmatic (molten rock) eruptions.
https://pubs.usgs.gov/fs/2005/3045/
The Cougar Stage culminated with the eruption of the largest lava flow in the history of Mount St. Helens (Swift Creek flow). The vent for this andesite (53 to 63% SiO2) lava flow, at an elevation of 6,000 feet on the south flank of Mount St. Helens, marks the summit of the volcano at that time.
https://pubs.usgs.gov/fs/2005/3045/
Ten years ago, on Oct. 1, 2004, Mount St. Helens awoke dramatically from an 18-year slumber, explosively shooting steam, gas and volcanic ash thousands of feet in the air. Fast-forward a decade and we’ve recently witnessed a similar, and far deadlier, scenario play out as Japan’s Mount Ontake erupted without warning on Sept. 27, killing at least 36 people.
https://weather.com/science/news/mount-st-helens-new-eruption-prediction-technology-20140929
when was mt. st. helens last eruption
The last eruption of  Mount St. Helens  was on 1980. The last significant eruption of Mount St. Helens have occurred in 1857.