Washington
The 1980 eruption of Mount Saint Helens blew off the north wall of the peak, creating an asymmetrical summit. In this photo, the south wall of the volcano’s top casts a shadow over the crater and the breach in the north side.
On May 18, 1980, Mount Saint Helens in Washington State blew its top in the largest volcanic eruption to strike the lower forty-eight states since the 1915 eruption of Mount Lassen in California. The eruption was categorized a 5 on the Volcanic Explosivity Index—on par with infamously destructive eruptions such as Mount Vesuvius in Rome in AD 79 and Mount Fuji in Japan in 1707.
To those who flew over the mountain before its eruption, and have seen it from the air since, the sight is a sobering lesson that not all geologic activity occurs over millennia. Geologic time truly includes right now.
In the months leading up to the blast, Mount Saint Helens showed signs of reawakening from a 123-year nap, including a series of escalating small earthquakes and episodes of steam venting from a newly formed summit crater. On the morning of the eruption, at 8:32:17 a.m., a magnitude 4.2 earthquake set off a rock slide that snowballed into a catastrophic, world-record-setting landslide down the north face of the volcano. When the weight of the rock lifted off the underlying magma chamber, the change in internal pressure triggered an eruption. The resulting blast was so powerful that older rock ejected from the depths of the volcano overtook younger rocks still avalanching off the flanks, depositing a huge amount of mixed material from the north face of Mount Saint Helens into Spirit Lake, a large lake north of the mountain.
In the explosive eruption of Mount Saint Helens on May 18, 1980, an ash plume rises fifteen miles high.
Mount Saint Helens, one day before its eruption May 18, 1980 (left), and from the same vantage point two years later (right). In the later image, steam and gases escape from a post-eruption lava dome that has formed inside the crater.
Before-and-after satellite photographs of Mount Saint Helens (left and right) show a devastated and vastly altered landscape caused by the 1980 eruption.
Once started, the event quickly evolved into a Plinian eruption, one similar to the eruption that devastated Mount Vesuvius. This type of event is characterized by explosive, powerful plumes of ash and volcanic material. In under ten minutes, the column of ash and super-heated gas from Mount Saint Helens rose to a height of more than fifteen miles above the volcano’s summit. The column eventually transported ash across eleven states, as far away as Oklahoma. Heat generated by the eruption melted snow, ice, and glaciers on the summit and flanks of the volcano, and the resulting meltwater mixed with ash and debris to form multiple, highly mobile mudflows or lahars. These lahars moved at speeds up to ninety miles per hour and one traveled as far as the Columbia River, fifty miles to the southwest. Searing hot gases spewed out laterally from the blast zone, incinerating everything in a nineteen-mile radius from the volcano and leaving a ring of devastation that is still recovering.
The volcano continued to erupt over the next few days, followed by smaller blasts over the next year, before going quiet again. The disaster left fifty-seven people dead and millions of acres of forests reduced to scorched wasteland. In the years following the eruption, Mount Saint Helens National Volcanic Monument was created for scientists to study the process of ecological recovery and for the public to visit the site of the historic eruption.
Today, nearly four decades later, intermittent swarms of earthquakes and a dynamic landscape inside the crater serve as constant reminders that this is still very much an active volcano. Lava domes have formed and settled over the years, and recently two giant magma chambers were discovered below the surface, one three to seven miles down and the other seven to twenty-four miles deep. Geologists around the world keep a close and wary eye on the not-so-saintly behavior of North America’s most restless stratovolcano.
Lava domes in the crater of Mount Saint Helens still exude steam and gases, nearly forty years after the last major eruption. Mount Rainier, another of the Cascade Volcanoes, can be seen in the distance to the north, across Spirit Lake.
You might catch a glimpse of Mount Saint Helens en route to Portland, Oregon, or Seattle, Washington. From the air, the pattern of the eruption on the north side of the volcano is striking, with the entire north wall of the summit blown out, creating a mammoth crater where the side of the mountain used to be.
