Mount St. Helens: The Morning the Mountain Blew Sideways

Mount St. Helens had been quiet for 123 years when the earthquakes started in March 1980. Within weeks, the north face of the mountain was bulging outward at a rate of five feet per day. Scientists knew something was coming. They did not know it would come from the side.

On the morning of May 18, 1980, at 8:32 AM Pacific Daylight Time, a magnitude 5.1 earthquake triggered the largest landslide in recorded history. The entire north face of the mountain collapsed. What followed was not a conventional eruption. It was a lateral blast, a superheated wall of gas, rock, and ash that traveled at speeds exceeding 300 miles per hour. The blast flattened 230 square miles of forest in minutes. Fifty-seven people died.

The eruption of Mount St. Helens changed volcanology. It demonstrated that the primary threat from certain types of volcanic eruptions is not lava flowing downhill. It is the explosive lateral force that no one standing within range can outrun.

Before the Eruption

Mount St. Helens sits in the Cascade Range of southwestern Washington State, roughly 50 miles northeast of Portland, Oregon. The mountain was part of a chain of volcanoes stretching from British Columbia to Northern California, all formed by the subduction of the Juan de Fuca Plate beneath the North American Plate.

Before 1980, Mount St. Helens was considered the most likely of the Cascade volcanoes to erupt. The U.S. Geological Survey had published a hazard assessment in 1978, two years before the eruption, identifying the mountain as a significant threat. The report predicted an eruption within the near future, likely before the end of the century. The prediction was correct. The timeline was generous.

On March 20, 1980, a magnitude 4.2 earthquake struck beneath the mountain. Over the following days, earthquake activity intensified. By March 27, the mountain produced its first phreatic eruption, a steam-driven explosion caused by groundwater heated by rising magma. A crater opened at the summit. Ash fell on nearby communities.

The earthquakes continued through April and into May. More than 10,000 earthquakes were recorded in the two months before the eruption. The mountain was breaking apart from the inside.

The North Flank Bulge

The most visible warning sign was the bulge. Beginning in late March, the north face of Mount St. Helens began to deform. Instruments placed on the mountain by USGS geologists measured the bulge expanding outward at approximately five feet per day.

The bulge was caused by a cryptodome, a mass of magma that had intruded into the mountain without breaking through to the surface. Instead of erupting vertically through the summit, the magma pushed horizontally into the north flank, inflating it like a balloon.

By mid-May, the bulge extended more than 450 feet outward from the original slope of the mountain. The north face was unstable. The question was not whether it would fail. It was when.

David Johnston Inside the Crater

David Johnston was a 30-year-old USGS volcanologist assigned to monitor Mount St. Helens. He was one of the scientists who understood most clearly the danger the mountain posed. Johnston had studied volcanic gases at Augustine Volcano in Alaska and recognized the chemical signatures of a magma system building toward eruption.

Johnston had entered the crater of Mount St. Helens during the weeks before the eruption to collect gas samples and take measurements. He understood the risk. He told colleagues that the mountain was a “dynamite keg” and that the north flank bulge made it especially dangerous.

On the night of May 17, Johnston was stationed at the Coldwater II observation post, roughly six miles north of the summit. His job was to monitor instruments and report any changes. He was directly in the path of what would come the next morning.

8:32 AM: The Earthquake

At 8:32 AM on May 18, 1980, a magnitude 5.1 earthquake struck directly beneath Mount St. Helens. The earthquake destabilized the already overloaded north face. What happened next occurred in a sequence so rapid that each phase was measured in seconds.

The north face of the mountain began to slide. The landslide occurred in three massive blocks, each containing hundreds of millions of cubic yards of rock and ice. The slide moved northward, down the slope of the mountain, and into the North Fork Toutle River valley.

The volume of the landslide was approximately 0.67 cubic miles of material. It was the largest landslide ever recorded. The debris traveled at speeds up to 150 miles per hour, filling the valley to an average depth of 150 feet over an area of 24 square miles. Spirit Lake, at the base of the mountain, was buried under 600 feet of debris.

The Mountain Collapses

The landslide did not cause the eruption. It uncorked it.

When the north face slid away, it removed the weight holding back the pressurized magma and superheated water inside the mountain. The sudden depressurization triggered an explosive release of energy from the exposed cryptodome.

The blast was not directed upward. It was directed laterally, northward, following the path of the landslide. The lateral blast is what made Mount St. Helens different from most volcanic eruptions in modern memory. The energy did not go up. It went sideways.

The mountain lost 1,300 feet of elevation in the collapse. Where a symmetrical peak had stood at 9,677 feet, there was now a horseshoe-shaped crater open to the north, with a rim elevation of roughly 8,365 feet.

The Lateral Blast

The lateral blast was a pyroclastic density current: a ground-hugging flow of superheated gas, pulverized rock, and volcanic debris. It emerged from the exposed interior of the mountain at temperatures estimated between 660 and 930 degrees Fahrenheit. It traveled at speeds exceeding 300 miles per hour. In some areas near the summit, speeds may have reached 670 miles per hour.

The blast overtook the landslide within seconds. It spread outward in a fan-shaped pattern covering 230 square miles. Every tree within the blast zone was flattened. The trees closest to the mountain were stripped of bark and branches and laid down in radial patterns pointing away from the source.

The blast killed everything in its path. Animals, vegetation, structures, and people. There was no time to react. At 300 miles per hour, the blast covered a mile in roughly 12 seconds. David Johnston, at Coldwater II, had time to radio one transmission: “Vancouver! Vancouver! This is it!” His observation post was overrun moments later. His body was never recovered.

Within the blast zone, the destruction was total. Beyond the zone of complete devastation, there was a zone of singed but standing trees, and beyond that, a zone where trees were toppled but not carbonized. The boundaries between these zones were sharp, a reminder that the physics of the blast were governed by topography as much as distance.

Lahars

The heat of the eruption melted glaciers and snow on the mountain, mixing with volcanic debris to create lahars: fast-moving flows of volcanic mud. The lahars traveled down river valleys at speeds of up to 90 miles per hour.

The largest lahar flowed down the North Fork Toutle River, picking up trees, boulders, bridges, and buildings. The flow reached the Cowlitz River, 50 miles from the mountain, where it deposited so much sediment that the river’s navigable channel was reduced from 40 feet deep to less than 15 feet. The Columbia River, farther downstream, was partially blocked by sediment, and shipping was disrupted for weeks.

The lahars destroyed 27 bridges and nearly 200 homes. The flows continued for hours after the initial eruption, fed by ongoing melting and rainfall on hot deposits.

Scientists in the Aftermath

The eruption provided an unprecedented natural laboratory. Scientists arrived within days to study the blast zone, the deposits, and the processes of destruction and recovery.

The eruption deposited more than 540 million tons of ash over an area of 22,000 square miles across 11 states. Ash fell as far east as the Great Plains. In Yakima, Washington, 80 miles to the east, visibility dropped to near zero, and the city was covered in several inches of ash.

The eruption column reached 80,000 feet in altitude. The ash cloud circled the Earth in 15 days. Global temperatures dropped by a fraction of a degree for the following year due to sulfur dioxide injected into the stratosphere.

Researchers found that the lateral blast had produced pressures comparable to nuclear weapons at close range. The energy released was estimated at 24 megatons of TNT, roughly 1,600 times the yield of the bomb dropped on Hiroshima. But unlike a nuclear explosion, the energy was released directionally, concentrated in a 180-degree arc to the north.

Remembering David Johnston

David Johnston became the most recognized casualty of the eruption. The Johnston Ridge Observatory, built near the site where he died, now serves as the primary visitor center for the Mount St. Helens National Volcanic Monument.

Johnston was not reckless. He was doing the work that needed to be done, monitoring a volcano that scientists knew was dangerous. He had advocated for keeping the public out of the danger zone. His warnings contributed to the evacuation orders that kept the death toll far lower than it might have been.

Fifty-seven people died in the eruption. Many of them were outside the restricted zone, in areas that scientists believed were safe. Some were loggers who had been allowed back to work. Some were residents who had refused to leave. Harry Truman, an 83-year-old lodge owner at Spirit Lake, had famously refused evacuation orders. He was buried under hundreds of feet of debris.

Without the restricted zone and the evacuation orders that scientists like Johnston pushed for, the death toll would have been significantly higher. The communities downriver from the mountain were given enough warning to evacuate as lahars approached. The science worked. It just could not save everyone.

Why the Mountain Blew Sideways

The lateral blast was not an anomaly. It was a consequence of the specific geometry of the eruption.

The cryptodome had intruded into the north flank rather than rising vertically through the conduit. When the landslide removed the north face, it exposed the pressurized interior from the side, not the top. The path of least resistance for the expanding gases was horizontal, out through the opening created by the landslide.

This type of eruption had occurred before at other volcanoes, but never in modern times with the kind of monitoring, documentation, and scientific presence that Mount St. Helens had. The eruption provided the data that volcanologists needed to understand lateral blasts as a fundamental volcanic hazard, not an aberration.

Since 1980, hazard assessments for volcanoes worldwide have incorporated the possibility of lateral blasts. Monitoring programs have expanded. Evacuation zones have been redesigned. The eruption of Mount St. Helens did not just destroy a mountain. It changed the science of how we prepare for volcanoes.

The mountain is still active. Minor eruptions occurred between 2004 and 2008, rebuilding a lava dome inside the crater. Monitoring continues. The Cascades Volcano Observatory, established after the 1980 eruption, watches Mount St. Helens and every other major volcano in the range.

The mountain blew sideways because the pressure was sideways. The lesson is that volcanoes do not always behave the way people expect. The science at Mount St. Helens proved that, and 57 people paid the price for the knowledge that followed.