Mount St. Helens is relatively young compared to other volcanoes in the Cascade Mountains, it is still active and is considered the most active in the Cascade Mountains in the last 10,000 years.
Eruptions: Most famous for its May 18, 1980 eruption, which is the most disastrous volcano eruption in United States history. It happened after two months of earthquakes and steam-venting episodes. The eruption of 540 million tons of ash shot up 80,000 feet before depositing ash in eleven states and five Canadian provinces. It killed 57 people and caused 1.1 billion dollars of damage.
Photo of the eruption from a weather satellite