OREGON STATE UNIVERSITY

Mount St. Helens Eruptive Activity, 1980-1984

The Two-Month Precursory Period

 

The Mount St. Helens volcano reawakened in March 1980 after more than a century of quiet. A magnitude 4.0 earthquake on March 20 was followed by two months of intense earthquake activity, and phreatic "steam-blast" eruptions which began on March 27. Ejecta from these phreatic eruptions were composed of fragments of pre-existing rocks; no magma was tapped during these eruptions. These events were caused by the intrusion of viscous magma into the volcano, shoving the north flank outward more than 300 feet and creating the famous `bulge.' Repeated surveys during April and May showed that the bulge was growing northward at an average rate of about five feet per day.

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The Eruption

A magnitude 5.1 earthquake on May 18 (8:32 a.m. PDT) shook loose the steepened bulge on the volcano's north flank, resulting in the largest known landslide in historic time, 2.3 cubic km (0.56 cubic miles). The entire north flank was described by an aerial observal as "rippling" and "churning" moments before "the north side of the summit began sliding north along a deep-seated slide plane."

As the avalanche reached the north base of the cone, the topography it encountered caused it to be divided into three sections:

1.    

Part of the avalanche slid into Spirit Lake, raising the lake bed roughly 180 feet, and damming its natural outlet. Water displaced by the avalanche surged up the surrounding hillslopes, washing the blown-down timber from the lateral blast into the lake.

2.    

Part of the avalanche "ramped" up and over a 1,200 foot high ridge five miles north of the volcano (Johnston Ridge) depositing debris on top of the ridge and in the South Colwater Creek drainage.

3.    

The bulk of the avalanche was deflected westward down the North Fork of the Toutle River valley. The front of the avalanche traveled a distance of 15 miles in about 10 minutes. The resulting deposit covers the valley floor to an average depth of 150 feet, but it is more than 500 feet deep in a few places (such as 1.5 miles west of Harry Truman's Lodge).

The hummocky avalanche deposit covers a total area of about 24 square miles. It consists of intermixed volcanic debris of various sizes, including blocks, pebbles, sand and silt, and blocks of glacial ice.

Lateral Blast

The sudden removal of the volcano's north flank released pressure on the hydrothermal and magmatic system within the volcano, triggering a devastating lateral blast to the north. The abrupt pressure release, or "uncorking," of the volcano by the avalanche can be compared in some ways to the removal of the cap from a vigorously shaken bottle of soda pop, or to punching a hole in a boiler tank under high pressure.

The northward-directed lateral blast of rock, ash, and hot gas devastated an area of about 150 square miles. The blast stripped trees from most hill slopes within six miles north of the volcano and leveled nearly all vegetation for as far as 13 miles in a 180-degree arc north of the mountain. The blast deposited blocks and smaller rock fragments and organic debris over the devastated area in layers to more than three feet in thickness. Surrounding this zone of toppled vegetation is a narrow band of scorched but standing timber in which sandy deposits are as thick as four inches; this zone has an area of about 25 square miles.

Plinian Column (Vertical Eruption)

A vertically-directed ash column erupted from the newly formed horseshoe-shaped crater within minutes of the lateral blast. Within ten minutes, the ash column reached an altitude of more than 12 miles. Ash from this eruption cloud was rapidly blown east-northeastward by the prevailing winds, producing lightning and starting hundreds of small forest fires, and causing darkness eastward for more than 125 miles. Ash fell visibly over the Great Plains, and fine ash was detected by systems used to monitor air pollution in several cities of the northeastern United States. Some ash drifted around the globe within about two weeks. The eruption subsided by late afternoon on May 18; by early May 19 the eruption had stopped.

The air-fall ash deposited during the nine hours of vigorous eruptive activity amounted to about 540 million tons distributed over an area of more than 22,000 square miles. The volume of uncompacted ash is equal to about 0.05 cubic mile of solid rock, or only about ten percent of the amount of material that slid off the volcano during the avalanche.

Lahars (Mudflows)

Lahar is an Indonesian term used to describe dense, viscous flows of volcanic debris and water resembling wet concrete that form during a volcanic eruption or originate on the slopes of a volcano. These mixtures typically contain 60 percent sediment and 40 percent water by volume. Lahars occurred on nearly all streams draining the volcano during the eruption and were formed in three major ways:

1.    

Within minutes of the eruption's onset, hot pyroclastic surges mixed with snow and ice on the upper flanks of the cone, forming major lahars in the South Fork Toutle River, Pine Creek, and Muddy River drainages. Subsequent calculations indicate the surges, which are air-mobilized, low density, turbulent clouds of volcanic debris, were moving initially up to 120 ft./sec. (80 mph), but slowed considerably as they transformed into denser, water-mobilized lahars on the lower slopes of the volcano. The Pine Creek lahar reached Swift reservoir by about 9 a.m.; the Muddy River lahar arrived at about 9:40 a.m.

2.    

The largest lahar originated in the slumping and flowing of water saturated parts of the debris avalanche deposit during the afternoon. This lahar peaked near the mouth of the Toutle River at midnight, flowing at velocities between 25 and 40 feet per second, and left deposits three feet thick on parts of the flood plain, and 15 feet thick in the channel. The mudflow in the Toutle River drainage area deposited more than 65 million cubic yards of sediment along the lower Cowlitz and Columbia rivers. The water-carrying capacity of the Cowlitz River was reduced by 85 percent, and the depth of the Columbia River navigational channel was decreased from 39 feet to less than 13 feet, causing disruption of river traffic and temporarily choking off ocean shipping.

3.    

The smallest lahars formed from the erosion and turbulent mixing of snow and ice by small, hot pyroclastic flows on the afternoon of May 18, and from small landslides of water-saturated tephra that liquefied.

 

Pyroclastic Flows

The term "pyroclastic" -- derived from the Greek words "pyro" (fire) and "klastos" (broken) -- describes materials formed by the fragmentation of magma and rock by explosive volcanic activity. Pyroclastic flows are composed of hot gas, entrapped air, and different-size particles of fragmented magma and old volcanic rock (ash, blocks, bombs). Pyroclastic flows travel at great speeds in response to gravity (up to 60 to 100 miles per hour).

Pyroclastic flows were first directly observed at 12:17 p.m. and continued intermittently during the next five hours of strong eruptive activity. Smaller pyroclastic flows were erupted during the first few minutes of the avalanche lateral blast sequence. The successive outpourings of pyroclastic material consisted mainly of pumice and ash derived from new magma. Fragments of preexisting rocks were minor components.

The pyroclastic flow deposits formed a fan-like pattern of overlapping sheets, tongues, and lobes that extend five miles north of the crater. Temperature measurements made in these pyroclastic flows were still 780 degrees Fahrenheit two weeks after the eruption. Many "rootless" steam-blast explosions formed small craters on the northern margin of the deposits near Spirit Lake, as encroaching ground water was flashed into steam by the hot material. These steam-blast explosions continued intermittently for several weeks or months after the emplacement of the pyroclastic flows.

 

Explosive Eruptions

Following May 18, Mount St. Helens erupted explosively five times during 1980. None of these eruptions was as large as the events on May 18, but each eruption produced ash columns 25,000-50,000 feet above sea level and hot, dry pyroclastic flows of pumice and ash that swept down the north flank as fast as 60 miles per hour. These pyroclastic flows deposited ash and pumice fragments in fan-like patterns of sheets, tongues, and lobes in an area extending up to five miles north of the vent. Individual pyroclastic-flow units were generally less than 15 feet thick, and maximum temperatures recorded several hours after their deposition ranged from about 570 to 1,350 degrees Fahrenheit. The thickness of air-fall deposits ranged from one-third to one-fortieth that of the May 18th air-fall deposit at a given distance from the volcano.

Lava extruded from the vent and formed lava domes within a few days after the June 12, August 7, and mid-October explosive eruptions. The June and August domes were blown away by subsequent explosive eruptions, but the October dome survived to form the core of the present dome.

Domes are formed by thick, pasty masses of lava too sticky to flow very far from the vent. Lava of the Mount St. Helens dome is dacite. It contains a higher percentage of silica than the Hawaiian basalts and is about one million times more viscous.

Dome-building Eruptions

Eleven eruptions after October 1980 were dominantly nonexplosive events that built a composite lava dome about 800 feet high and 2500 feet in diameter in the crater. Each eruption extruded near the top of the dome and crept three to 15 feet per hour down one side over a period of several days; between 100 and 150 million cubic feet of new lava was added to the dome during each of these episodes.

Dome-building eruptions in 1981-82 were episodic, occurring every one to five months. Between February 1983 and February 1984, the dome grew continuously both by the intrusion of magma into it and by the extrusion of lava onto its surface. As of May 1984, it appeared that Mount St. Helens had returned to the episodic style of dome growth. At the then current rate of dome growth, which averaged about 35 million cubic feet per month, it was estimated that some 150 to 200 years would be needed to build Mount St. Helens to its former height. However it was considered unlikely that such a simple scenario would prevail.

Small Explosions

Small explosions sometimes precede or accompany the dome-building eruptions at Mount St. Helens. If they occur when snow mantles the crater floor, they can produce mudflows and snow avalanches. The explosive onset of the March 19, 1982, eruption hurled hot pumice and dome rocks against the 2,000-feet-high south crater wall, dislodging snow and rock that avalanched through the crater and down the north flank of the volcano. Deep snow in the crater melted quickly from the volcanic heat, forming a temporary small lake from which a destructive flood swept down the north flank and into the North Fork Toutle River. About a day later a new lava lobe began to flow down the southeast flank of the dome.

Tephra Emissions

In addition to the dome-building eruptions, vigorous emissions of gas and tephra have occurred from fractures and small craters on top of the dome since late 1980. These periodic outbursts usually last several minutes, occasionally sending ash plumes as high as 15,000 to 20,000 feet above the volcano. Most of the tephra consists of fragmented pieces of dome rock, not new liquid magma, in contrast to the more hazardous magmatic explosions of 1980. These events are intermittent, sometimes occurring several times per day, and at other times not occurring at all for several weeks.