Mount St. Helens Eruption: Facts & Information

The May 18 eruption sent volcanic ash, steam, water, and debris to a height of 60,000 feet (18,288 meters), while the mountain lost 1,300 feet (396 meters) of altitude. Fifty-seven people were killed or are still missing. USGS Photograph taken on May 18,
The May 18 eruption sent volcanic ash, steam, water, and debris to a height of 60,000 feet (18,288 meters), while the mountain lost 1,300 feet (396 meters) of altitude. Fifty-seven people were killed or are still missing. USGS Photograph taken on May 18, 1980, by Robert Krimmel.
Credit: Robert Krimmel, USGS

Mount St. Helens erupted on May 18, 1980. The volcano, located in southwestern Washington, used to be a beautiful symmetrical cone about 9,600 feet (3,000 meters) above sea level. The eruption, which removed the upper 1,300 feet (396 meters) of the summit, left a horseshoe-shaped crater and a barren wasteland. Today the land is healing, having recovered its natural beauty, but the landscape has been permanently altered.

Native Americans in the Pacific Northwest called the mountain "Louwala-Clough" or "Smoking Mountain." The modern name came from Capt. George Vancouver of the British Royal navy, who named it in honor of Alleyne Fitzherbert, the British Ambassador to Spain, who also held the title Baron St. Helens.

The volcano was relatively active in the early 19th century, and may have had a major explosive eruption in 1800, according to the Mount St. Helens Forest Learning Center. There were minor eruptions in 1898, 1903 and 1932, but for most of the 20th century, the mountain was seen as a peaceful, beautiful mountain and recreation area. That serenity was shattered in 1980.

On shaky ground

On March 1, 1980, a new system of seismographs at the University of Washington went into operation to monitor earthquake activity in the Cascades. On March 20, it recorded a magnitude-4.2 earthquake deep beneath Mount St. Helens, inaugurating a round-the-clock watch that was to save many lives. From March 25 to March 27, quakes of magnitude 4.0 rocked the mountain as many as three times a day, and smaller quakes occurred several times every hour.

At 8 a.m. PST on March 27, the U.S. Geological Survey issued an official Hazard Watch for Mount St. Helens; around noon, the first eruption of steam from the summit sent a column of ash and steam 6,000 feet (1,829 meters) into the air. Twin fissures opened on the mountain’s north face.

A "bulge" developed on the north side of Mount St. Helens as magma pushed up within the peak.
A "bulge" developed on the north side of Mount St. Helens as magma pushed up within the peak. Measurements suggested the bulge was growing at a rate of up to 5feet (1.5 meters) per day. By May 17, part of the volcano's north side had been pushed upward and outward more than 450 feet (135 meters). This image, a view is from the northeast, was taken on April 13, 1980 by Austin Post, USGS.
Credit: Austin Post, USGS

Volcanologists set up reflective targets between the fissures and used lasers placed on a mountain ridge six miles away to record changes. On March 31, instruments began to record the rhythmic pulses known as “volcanic tremor.” Such pulses indicated that magma was on the move.  In April, the laser equipment began to detect changes to the mountain’s profile; an ominous swelling bulge was forming between the fissures. By late April, the laser measurements showed that the bulge was growing at a rate of about five feet per day.

'This is it!'

On the morning of May 18, USGS volcanologist David A. Johnston, camped on the ridge with his lasers, radioed in his regular 7 a.m. report. The changes to the bulging mountain were consistent with what had been reported several times daily since the watch began. At 8:32, a magnitude-5.1 earthquake registered on the seismographic equipment. His excited radio message, “This is it!” was followed by a stream of data. It was his last transmission; the ridge he camped on was within the direct blast zone. [Gallery: The Incredible Eruption of Mount St. Helens]

Overhead, Keith and Dorothy Stoffel were making an aerial survey of the volcano when they noticed a landslide on the lip of the summit crater. Within 15 seconds, the whole north face of the mountain was on the move. Just as they passed around to the east side of the mountain, the north face collapsed, releasing superheated gases and trapped magma in a massive lateral explosion. Keith put the plane into a steep dive to gain the speed to outrun the cloud of incandescent gas; Dorothy continued to photograph the eruption through the rear windows of the plane as they made their escape.  

The abrupt release of pressure over the magma chamber created a “nuees ardentes,” a glowing cloud of superheated gas and rock debris blown out of the mountain face moving at nearly supersonic speeds. Everything within eight miles of the blast was wiped out almost instantly. The shockwave rolled over the forest for another 19 miles, leveling century-old trees; all the trunks neatly aligned to the north. Beyond this “tree down zone” the forest remained standing but was seared lifeless. The area devastated by the direct blast force covered an area of nearly 230 square miles (596 square kilometers).

Shortly after the lateral blast, a second, vertical explosion occurred at the summit of the volcano, sending a mushroom cloud of ash and gases more than 12 miles into the air. The cloud of ash darkened the skies, causing streetlights to come on as far away as Spokane, Wash., more than 300 miles (480 km) away. Ash continued to erupt for more than nine hours. Ultimately, an estimated 540 million tons (490,000 kilotons) of ash drifted up to 2,200 square miles (5,700 square km) settling over seven states.

As the north face was blown apart, the heat instantly vaporized glacial ice and snow around the remaining parts of the mountain. By 8:50 a.m., massive mudflows were moving through the river systems to the west and southeast of Mount St. Helens. The hot mud moved in excess of 90 mph (145 kph), sweeping away everything in its path.

Most destructive U.S. volcano

In terms of economic impact, the Mount St. Helens eruption was the most destructive in U.S. history. Fifty-seven people are known to have died. More than 200 homes were destroyed. More than 185 miles of roads and 15 miles of railways were damaged. Ash clogged sewage systems, damaged cars and buildings, and temporarily shut down air traffic over the Northwest. The International Trade Commission estimated damages to timber, civil works and agriculture to be $1.1 billion. In addition, Congress approved $950 million in emergency funds to the Army Corps of Engineers, FEMA, and the Small Business Administration to help recovery efforts. [Related: Why Was Mount St. Helens So Destructive?]

Today Mount St. Helens and other volcanoes in the Pacific Northwest are closely monitored by geologists at the David A. Johnston Cascades Volcano Observatory in Vancouver. In partnership with two other USGS installations located in Alaska and Hawaii, they are providing valuable information on volcanic activity to the United States and countries along the Pacific Ring of Fire.

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