The Cascade Mountains extend from British Columbia, Canada, through Washington and Oregon and into Northern California. Among the range’s largest mountains are Mount Baker, Mount Rainier, and Mount Hood, all of which are active volcanoes. While Mount St. Helens is not as tall as its neighbors, it has been by far the most dangerous in recent geological history. Approximately 275,000 years ago, Mount St. Helens initially formed over the course of several eruptive stages; today, it is the most active volcano in the Cascades. According to the United States Geological Survey (USGS), the mountain is an explosive dacite volcano, which means that its eruptions consist of dacite, a specific type of igneous rock. Dacite volcanoes like Mount St. Helens generally have powerful, sudden eruptions and eject large amounts of ash and thick lava flows.

This precise type of eruption occurred on May 18, 1980: a lethal event that shocked many residents of Washington State, although the mountain had long been known for its volcanic activity. Its volatile nature is reflected in the local Indigenous people’s names for it; the Klickitat refer to it as “Louwala-Clough,” which means “smoking mountain,” while the Cowlitz people use the term “Lawetlat’la,” meaning “mountain of fire.”

Various European Americans who moved to the area in the 1800s noticed the mountain’s volcanic activity when settlers first saw small eruptions on the Goat Flanks area of the mountain. However, this factor did not deter them from settling near the mountain and making use of its abundant natural resources, and they proceeded to log its timber and fish in its lakes and rivers.

After going through several small eruptions in the early 19th century, Mount St. Helens sat dormant for decades. These years of calm prompted the locals to consider the mountain nothing more than a peaceful place to enjoy the natural world. The USGS explains:

Although minor steam explosions may have occurred in 1898, 1903, and 1921, the mountain gave little or no evidence of being a volcanic hazard for more than a century after 1857. Consequently, the majority of 20th-century residents and visitors thought of Mount St. Helens not as a menace, but as a serene, beautiful mountain playground teeming with wildlife and available for leisure activities throughout the year (“Description: Mount St. Helens Volcano, Washington.” USGS).

As a result, the volcano’s powerful and deadly eruption in 1980 came as a surprise to many. An earthquake triggered the nine-hour eruption, causing the mountain’s north face to crumble into a massive avalanche of rock. As this debris rolled down the mountain, the volcano also ejected gases and hot winds that destroyed over 150 square miles of the forest and the local wildlife. Meanwhile, clouds of volcanic ash drifted into the air and eventually rained down over Washington and British Columbia. Approximately 57 people died in the eruption, from local cabin owners and hikers to scientists studying the mountain. Today, visitors can learn more about this historic event in person at the Mount St. Helens National Volcanic Monument, which is located on the mountain.