Mount St. Helens

1980 Cataclysmic Eruption

Magma began intruding into the Mount St. Helens edifice in the late winter and early spring of 1980. By May 18, the cryptodome (bulge) on the north flank had likely reached the point of instability, and was creeping more rapidly toward failure.

Seismogram from station CPW, 112 km (70 mi) northwest of Mount St. ...

Annotated seismogram indicates the signals for a Low-Frequency (LF) volcanic earthquake, relative quiescence, and then harmonic tremor as the eruption of May 18, 1980 accelerated. Each horizontal line represents 15 minutes of time. (Public domain.)

Summary of Events

On May 18, 1980, a magnitude-5+ earthquake was accompanied by a debris avalanche, which in turn unloaded the confining pressure at the top of the volcano by removing the cryptodome. This abrupt pressure release allowed hot water in the system to flash to steam, which expanded explosively, initiating a hydrothermal blast directed laterally through the landslide scar. Because the upper portion of the volcano was removed, the pressure decreased on the system of magma beneath the volcano. A wave of decreasing pressure down the volcanic conduit to the subsurface magma reservoir, which then began to rise, form bubbles (degas), and erupt explosively, driving a 9-hour long Plinian eruption.

Steam-blast (phreatic) eruption from the summit crater of Mount St....

Steam-blast eruption from summit crater of Mount St. Helens. Aerial view, April 6, looking southwest, showing a roiling, gray-brown, ash-laden cloud that envelops and almost completely hides an initial fingerlike ash column, and an upper white cloud formed by atmospheric condensation of water vapor in the convectively rising top of the eruptive column. Image and caption taken from Professional Paper 1250 and not scanned from original slide. (Credit: Moore, James G.. Public domain.)

Precursory Activity

On March 16, 1980, the first sign of activity at Mount St. Helens occurred as a series of small earthquakes. On March 27, after hundreds of additional earthquakes, the volcano produced its first eruption in over 100 years. Steam explosions blasted a 60- to 75-m (200- to 250-ft) wide crater through the volcano's summit ice cap and covered the snow-clad southeast sector with dark ash.

Within a week the crater had grown to about 400 m (1,300 ft) in diameter and two giant crack systems crossed the entire summit area. Eruptions occurred on average from about 1 per hour in March to about 1 per day by April 22 when the first period of activity ceased. Small eruptions resumed on May 7 and continued to May 17. By that time, more than 10,000 earthquakeshad shaken the volcano and the north flank had grown outward about 140 m (450 ft) to form a prominent bulge. From the start of the eruption, the bulge grew outward—nearly horizontally—at consistent rates of about 2 m (6.5 ft) per day. Such dramatic deformationof the volcano was strong evidence that molten rock (magma) had risen high into the volcano. In fact, beneath the surficial bulge was a cryptodome that had intruded into the volcano's edifice, but had yet to erupt on the surface.

Debris Avalanche

With no immediate precursors, a magnitude 5.1 earthquake occurred at 8:32 a.m. on May 18, 1980 and was accompanied by a rapid series of events. At the same time as the earthquake, the volcano's northern bulge and summit slid away as a huge landslide—the largest debris avalanche on Earth in recorded history. A small, dark, ash-rich eruption plume rose directly from the base of the debris avalanche scarp, and another from the summit crater rose to about 200 m (650 ft) high. The debris avalanche swept around and up ridges to the north, but most of it turned westward as far as 23 km (14 mi) down the valley of the North Fork Toutle River and formed a hummocky deposit. The total avalanche volume is about 2.5 km3 (3.3 billion cubic yards), equivalent to 1 million Olympic swimming pools.

Bulge on the north side of Mount St. Helens developed as magma push...

A "bulge" developed on the north side of Mount St. Helens as magma pushed up within the peak. Angle and slope-distance measurements to the bulge indicated it was growing at a rate of up to five feet (1.5 meters) per day. By May 17, part of the volcano's north side had been pushed upwards and outwards over 450 feet (135 meters). (Lipman, Peter. Public domain.)

Bulge (right) and small crater, Mount St. Helens summit. Crater are...

Bulge (right) and small crater, Mount St. Helens summit. Crater area dropped in relation to the summit, and bulge shows pronounced fracturing because of its increased expansion. View looking south. (Credit: Krimmel, Robert M.. Public domain.)

Lateral Blast

Blowdown of trees from the shock-wave of the directed (lateral) bla...

Blowdown of trees from the shock-wave of the directed (lateral) blast from the May 18, 1980 eruption of Mount St. Helens. Elk Rock is the peak with a singed area on the left.

(Credit: Topinka, Lyn. Public domain.)

The landslide removed Mount St. Helens' northern flank, including part of the cryptodome that had grown inside the volcano. The cryptodome was a very hot and highly pressurized body of magma. Its removal resulted in immediate depressurization of the volcano's magmatic system and triggered powerful eruptions that blasted laterally through the sliding debris and removed the upper 300 m (nearly 1,000 ft) of the cone. As this lateral blast of hot material overtook the debris avalanche; it accelerated to at least 480 km per hr (300 mi per hr). Within a few minutes after onset, an eruption cloud of blast tephra began to rise from the former summit crater. Within less than 15 minutes it had reached a height of more than 24 km (15 mi or 80,000 ft).

The lateral blast devastated an area nearly 30 km (19 mi) from west to east and more than 20 km (12.5 mi) northward from the former summit. In an inner zone extending nearly 10 km (6 mi) from the summit, virtually no trees remained of what was once dense forest. Just beyond this area, all standing trees were blown to the ground, and at the blast's outer limit, the remaining trees were thoroughly seared. The 600 km2(230 mi2) devastated area was blanketed by a deposit of hot debris carried by the blast.

Plinian eruption column from May 18, 1980 Mount St. Helens. Aerial ...

Plinian eruption column from May 18, 1980 Mount St. Helens. Aerial view from the Southwest. (Credit: Krimmel, Robert. Public domain.)

Plinian Eruption

Removal of the cryptodome and flank exposed the conduit of Mount St. Helens, resulting in a release of pressure on the top of the volcano's plumbing system. This caused a depressurization wave to propagate down the conduit to the volcano's magma storage region, allowing the pent-up magma to expand upward toward the vent opening. Less than an hour after the start of the eruption, this loss of conduit pressure initiated a Plinian eruption that sent a massive tephra plumehigh into the atmosphere. Beginning just after noon, swift pyroclastic flows poured out of the crater at 80 - 130 km/hr (50 to 80 mi/hr) and spread as far as 8 km (5 mi) to the north creating the Pumice Plain.

The Plinian phase continued for 9 hours producing a high eruption column, numerous pyroclastic flows, and ash fall downwind of the eruption. Scientists estimate that the eruption reached its peak between 3:00 and 5:00 p.m. When the Plinian phase was over, a new northward opening summit amphitheater 1.9 x 2.9 km (1.2 x 1.8 mi) across was revealed.

Advancing mammatus ash cloud from the Mount St.Helen's eruption on ...

Ash cloud from Mount St. Helens over Ephrata, Washington (230 km (145mi) downwind), after May 18, 1980 eruption. (copyright by Douglas Miller)

Over the course of the day, prevailing winds blew 520 million tons of ash eastward across the United States and caused complete darkness in Spokane, Washington, 400 km (250 mi) from the volcano. Major ash falls occurred as far away as central Montana, and ash fell visibly as far eastward as the Great Plains of the Central United States, more than 1,500 km (930 mi) away. The ash cloud spread across the U.S. in three days and circled the Earth in 15 days.

Lahars

During the first few minutes of this eruption, parts of the blast cloud surged over the newly formed crater rim and down the west, south, and east sides of the volcano. The turbulently flowing hot rocks and gas quickly eroded and melted some of the snow and ice capping the volcano, creating surges of water that eroded and mixed with loose rock debris to form lahars. Several lahars poured down the volcano into river valleys, ripping trees from their roots and destroying roads and bridges.

The largest and most destructive lahar occurred in the North Fork Toutle and was formed by water (originally groundwater and melting blocks of glacier ice) escaping from inside the huge landslide deposit through most of the day. This powerful slurry eroded material from both the landslide deposit and channel of the North Fork Toutle River. Increased in size as it traveled downstream, the lahar destroyed bridges and homes, eventually flowing into the Cowlitz River. It reached maximum size at about midnight in the Cowlitz River, about 80 km (50 mi) downstream from the volcano.

Lahars resulting from the May 18, 1980 eruption of Mount St. Helens...

Nearly 135 miles (220 kilometers) of river channels surrounding the volcano were affected by the lahars of May 18, 1980. A mudline left behind on trees shows depths reached by the mud. (Credit: Topinka, Lyn. Public domain.)