Mount St. Helens Facts

One of North America's Most Active Volcanoes

Mount St. Helens Sunset With Wildflowers

TerenceLeezy / Getty Images

Mount St. Helens is an active volcano located in the United States' Pacific Northwest region. It is positioned about 96 miles (154 km) south of Seattle, Washington and 50 miles (80 km) northeast of Portland, Oregon. Mount St. Helens is found within the Cascade Mountain Range, which runs from northern California through Washington and Oregon into British Columbia, Canada.

This range, as part of the curved stretch of extreme seismic activity known as the Pacific ​Ring of Fire, features many active volcanoes. In fact, the Cascadia Subduction Zone itself was formed by plate convergence along the North American coast. Today, the land surrounding Mount St. Helens is rebounding and most of it has been preserved as a part of the Mount St. Helens National Volcanic Monument.

Geography of Mount St. Helens

Compared to other volcanoes in the Cascades, Mount St. Helens is fairly young geologically speaking because it was formed only 40,000 years ago. Its top cone, which was destroyed in the 1980 eruption, began developing only 2,200 years ago. Due to its rapid growth, many scientists consider Mount St. Helens the most active volcano in the Cascades within the last 10,000 years.

There are three main river systems in the vicinity of Mount St. Helens. These include the Toutle, Kalama, and Lewis Rivers. These were all significantly impacted by the eruption of 1980.

The nearest town to Mount St. Helens is Cougar, Washington, which is around 11 miles (18 km) away. Gifford Pinchot National Forest comprises the rest of the immediate area. Other nearby but much farther cities such as Castle Rock, Longview, and Kelso, Washington were affected by the 1980 eruption because they are low-lying and near the region's rivers.

1980 Eruption

On May 18 of 1980, the eruption of Mount St. Helens removed 1,300 feet of mountaintop and ravaged surrounding forests and cabins in a destructive avalanche. In addition to avalanches, the area endured the aftermath of earthquakes, pyroclastic flow, and ash for several years.

Activity on the mountain began on March 20, 1980, when a magnitude 4.2 earthquake struck. Steam soon began to vent from the mountain and by April, a bulge appeared on the north side of Mount St. Helens. This bulge would cause a historically catastrophic avalanche. When another strong earthquake struck on May 18, the entire north face of the volcano tumbled into a debris avalanche that is believed to have been the largest in history.


This massive landslide caused Mount St. Helens to erupt in a violent explosion on the same day. The volcano's pyroclastic flow—a swift river of hot ash, lava, rock, and gas—leveled the surrounding area almost instantly. The "blast zone" of this deadly eruption spanned 230 square miles (500 sq km): rocks were hurled, waterways flooded, the air poisoned, and more. 57 people were killed.

Ash alone had disastrous effects. During its first eruption, the plume of ash from Mount St. Helens rose as high as 16 miles (27 km) and moved east until it spread upwards of 35 miles. Volcanic ash is highly toxic and thousands of humans were exposed. Mount St. Helens continued erupting ash from 1989 to 1991.

In addition to the spread of ash, heat from eruptions and force from numerous avalanches caused the mountain's ice and snow to melt, which led to the formation of fatal volcanic mudflows called lahars. These lahars poured into neighboring rivers—the Toutle and Cowlitz, in particular—and caused widespread flooding. This devastation blanketed miles and miles of land. Material from Mount St. Helens was found 17 miles (27 km) south in the Columbia River along the Oregon-Washington border.

Five smaller explosions, accompanied by countless eruptive episodes, would follow this reawakening in the next six years. Activity on the mountain continued until 1986 and a giant lava dome formed in the newly-developed crater at the volcano's summit.


The land around this volcano has almost fully rebounded since 1980. The area that was once completely scorched and barren is now a thriving forest. Just five years after the initial eruption, surviving plants sprouted through the thick layer of ash and debris and flourished. Since 1995, biodiversity within the previously damaged area has even increased—there are many trees and shrubs growing successfully and animals that inhabited the land pre-eruption have returned and resettled.

Most Recent Activity

Mount St. Helens' devastating 1980 modern eruption was not its most recent activity. The volcano has continued to make its presence known. Since its historic explosion, Mount St. Helens experienced a period of much smaller eruptions lasting from 2004 to 2008.

During this four-year period, the mountain was again very active and eruptive. Fortunately, none of the explosions were particularly severe and the land has not suffered too greatly because of them. Most of these smaller eruptions only added on to the growing lava dome at Mount St. Helens' summit crater.

In 2005, however, Mount St. Helens erupted a 36,000 foot (11,000 m) plume of ash and steam. A minor earthquake accompanied this event. Ash and steam have been visible on the mountain several times in more recent years.


mla apa chicago
Your Citation
Briney, Amanda. "Mount St. Helens Facts." ThoughtCo, Sep. 1, 2021, Briney, Amanda. (2021, September 1). Mount St. Helens Facts. Retrieved from Briney, Amanda. "Mount St. Helens Facts." ThoughtCo. (accessed May 30, 2023).