Mount St. Helens

The eruption of Mount St. Helens in 1980 and the series of geologic and volcanic events that followed provided a unique opportunity for geologists to study a classic volcanic disturbance in detail using modern technological tools.

Warning Signs

In the weeks leading up to May 18, 1980, Mount St. Helens exhibited warnings of an impending eruption, including internal rumbles and small bursts. A large bulge had developed over 98 meters on the mountain's north face, indicating a highly pressurized buildup of old, trapped magma just below the mountain's surface. As days passed, the bulge grew more prominent while the tiny eruptions at the summit continued.

What followed, however, could not have been predicted. First, deep internal disturbances forced magma (molten rock) up to the earth's surface through the volcano's central, or tunnel, vent. As the magma spread upward, it exerted a large amount of pressure on the sides of the central vent and, as the pressure increased, caused the bulge of old magma to push even farther outward. Then, when the pressure became stronger than the mountain's ability to contain it, the magma blasted through two enormous holes—one at the summit and one on the north flank. The release of energy resulted in a massive explosion.

A Mountain Beheads Itself

At 8:32 on the morning of May 18, 1980, an earthquake in the Mount St. Helens region of the Washington State Cascade Range started a scenario of startling events, beginning with the sudden and spectacular eruption of Mount St. Helens and culminating in the destruction of hundreds of square kilometers of valuable timberland, vacation resorts, and foraging area for wildlife. Considered by many scientists to be a classic series of volcanic activities, the eruption blasted away the top 396 meters (one-third) of the mountain in a cataclysmic explosion that exuded 360 billion kilograms of rock and ash. Forests more than 27 kilometers away were completely leveled; pyroclastic debris (steam, gas-filled rock, and wet ash), moving at the amazing speed of 321 kilometers per hour, flew horizontally across the expanse of the Cascade Range together with suffocating volcanic gases, eventually taking the lives of fifty-seven people and countless birds, fish, and animals; twenty-six lakes and 241 kilometers of freshwater streams were destroyed. In the end, the scene in the Cascades was one of utter desolation for some 300 square kilometers.

The fury of the initial blast, comparable to a 10-megaton bomb, immediately tore away one-third of the entire mountain and was followed by a huge landslide. Hot gases, steam, and molten rock shot laterally from the mountain's flank at nearly the velocity of a rifle bullet. Explosion after explosion shook the volcano as debris poured down the mountainside, carrying hot lava, timber, and wet ash. The two gigantic fissures spewed volcanic ash in heavy, dense clouds. Driven by mighty pressures from the Earth's depths, the ash clouds funneled outward, reaching beyond 33 kilometers in less than two minutes and beginning to spread over the northwestern region of the United States. Mount St. Helens' snow-capped summit quickly turned to water and steam under the heat and pressure of the eruption, releasing 174 billion liters of water down the mountainside. The flood swept mud, solidified lava, tree splinters, pulverized rock, and other material in its path, creating an avalanche that clogged streams and bulldozed forestation as it continued its outward spread. Attacked by volcanic gases, fumes, ash, and intense heat from the bowels of Mount St. Helens, the atmosphere immediately surrounding the erupting volcano became a cauldron of earsplitting thunder and crackling lightning.

Another eruption occurred on May 25, 1980, spreading an additional ash deposit, up to 70 millimeters thick, for hundreds of square kilometers, mostly westward. Then, in October of the same year, Mount St. Helens erupted again, sending yet more ash clouds southward. The caldera's floor grew to more than 260 meters in diameter, with a dome 49 meters high.

Ashes, Ashes

The picturesque valleys around the mountain were smothered with debris. Gray ash fell like a huge rainstorm in areas farther from the mountain, covering the ground with a blanket 1.5 to 2 meters thick. Even four days later, the ash was extremely hot, registering temperatures as high as 323 degrees Celsius. Rescuers had to move swiftly through the deep ash to keep their legs from burning. People in Yakima reported removing 540 million kilograms of ash from streets, rooftops, and other structures. Elsewhere, the weight of the volcanic ash caused the flattening of hayfields and similar agricultural problems, resulting in an estimated $300 million worth of damage to crops. Once fallen, the ash dried to a powder that was blown in all directions by new winds. Where the ash was thickest, people had to wear protective face masks to keep from choking. Not all the ice from the peak melted during the first several blasts; huge chunks—some larger than trucks—lay scattered along the valley floors, where they then melted.

The ecological damage from the eruption was widespread. Streams were choked with debris, killing fish and destroying watering holes; dense clouds of ash, composed of tiny glass particles driven like shrapnel by the foaming winds, sanded down the wings of birds and insects and clogged their breathing; the rolling landslides inundated timberland and covered wildlife with a shroud of ooze, ash, gases, and debris. Decades were required for the forests and lakes to rebuild themselves.

Origins of the Blast

The Mount St. Helens eruption is among history's most closely studied volcanic events. Scientists could record in detail the physical and chemical activities that occur during a cataclysmic volcanic eruption. Eyewitness accounts were available from people who had been caught amid the eruption and narrowly escaped death. These accounts furnish important data about many different aspects of volcanism.

The Cascade Range in western Washington State was created by a subduction process beginning about 80 million years ago. Subduction is one of the main methods of producing earthquakes; it occurs when the edges of two tectonic plates meet. The stronger, or perhaps more rigid, plate “dives,” or subducts, under the other plate. In subducting, the plates scrape along each other and cause great jolts, which are experienced as earthquakes. In the Mount St. Helens scenario, the tiny Juan de Fuca plate subducted under the larger North American plate (the North American continent), a process still occurring. The Pacific plate has been moving closer to the North American continent as the Juan de Fuca plate disappears, and it is now abutted next to the San Andreas fault, which runs across western California. The Juan de Fuca plate is almost entirely subducted.

Study of Mount St. Helens

More scientific experience and technology were applied to the Mount St. Helens eruption investigation than to any other volcanic event in North America. One of the most valuable of all the data sources, however, was eyewitness accounts provided by both trained and untrained observers. Dozens of geologists and volcanologists worked near St. Helens, some paying with their lives. In addition, hundreds of local citizens who witnessed the week-long scenario furnished information. The narratives, offered by citizens who had lived for many years in the region, enabled researchers to construct an almost minute-by-minute catalog of events, complete with time estimates, sizes, volumes, and intensities—much of which replaced the data that went unrecorded when scientific instruments were destroyed in the eruption.

The seismometer is the most frequently used instrument for studying volcanic activity, which measures the frequency, location, and intensity of tremors. Seismometers were placed in as many as twelve locations near Mount St. Helens, furnishing a constant monitoring system. In this instance, the instruments recorded dozens of fair-sized tremors up to the moment of the 5.0-magnitude earthquake that triggered the eruption.

Another valuable instrument is the geodimeter, which uses a laser beam to measure minute changes in ground swelling, such as the huge subsurface bulge on the mountain's north flank. By monitoring changes in the height of such a dome, geologists can determine its rate of growth or subsidence. A fast-growing bulge would signal an impending crisis, perhaps a vent-creating explosion.

Tiltmeters measure a mountain's slope angle. They tell geologists about the rate of change of the mountain's growth and the altering angle of its sides. These measurements can help predict an eruption because internal volcanic pressures and upwelling magma tend to push outward against the volcano's crust, increasing the slope angle.

Regular cameras are used to record an eruption's progress. Cameras on tripods are wired to receive radio signals that automatically advance the film for the next picture, making it unnecessary for the cameras to be operated by humans. Cameras can, therefore, be placed in areas that would be life-threatening for researchers, perhaps because of intense heat or because they are in the path of a shock wave. Cameras in various positions can be loaded with color or black-and-white film. Color is essential for assessing various activities; variations in grayness in an ash cloud tell geologists about the density of different portions of the cloud. Cameras are also vital in recording the aftermath of an eruption. They help to determine the seriousness of such problems as stream choking, ash fallout, and forest destruction.

Geologists use a stream gauge to record water temperature, amounts of material suspended in the water, and the water's chemical content. Using such measurements, scientists can determine the seriousness of the stream choking, its cause, and the rate at which the stream is being filled with alien material. It is important to know the temperature of the stream, too, because of the possible negative impact that sudden temperature changes might have on fish, fish eggs, and other aquatic life.

Finally, a vital instrument for geologists is the gas sensor, used at ground level and in airplanes. It is essential to assess the presence or absence of various gases in the region of an eruption or impending eruption, especially hydrogen, carbon dioxide, and sulfur dioxide. Although these gases are potentially life-threatening, they can reveal the subsurface movement of magma.

Twenty-First Century Activity

Beginning in 2004, Mount St. Helens displayed further eruptive activity, though to less of an extent than in 1980. In October of that year, ash and steam had started to fill the air when the volcano exploded after an earthquake swarm, also emitting lava that continued to flow until it created a substantial new lava dome in its crater, which had been created by the eruption of 1980. Occasional bursts of volcanic activity that typically involved the release of plumes of steam occurred periodically in 2005 and 2006 as the dome continued building and changing shape. After another eruption in January 2008, scientists reported later that year that the growth of the lava dome had officially ceased and that with several months of no new activity, the alert level could be reduced back to normal.

While no eruptions have occurred since then, scientists' observation of earthquake swarms occurring between March and May 2016 and April and May 2017 caused some concern as to whether another eruption might be imminent. However, some experts stressed that the earthquakes remained small and that there were no other signs, such as more significant volcanic gas emissions, accompanying them that would more definitively indicate a forthcoming eruption. Volcanologists predicted that another eruption of Mount St. Helens would most likely occur within the lifetimes of those living in the early 2020s. In a 2018 United States Geological Survey study, scientists ranked Mount St. Helens at a very high risk of eruption.

An increase in the occurrence of smaller earthquakes over a short period of time occurred again in both 2023 and 2024, with those in 2023 representing the most of this type of activity since the 2008 eruption. However, scientists insisted that this did not necessarily mean an eruption was imminent, as the increased seismic activity was due to magma flows. At the same time, it was still generally believed that an eruption of Mount St. Helens in the twenty-first century was likely.

Principal Terms

caldera: the sunken area at the summit of a volcano caused by the internal collapse of magma

fissure: an extensive crack or fracture; a linear volcanic vent

magma: molten rock material formed deep in the earth's interior; when thrown out of a volcano, it is known as lava

pyroclastic material: rocks formed from the debris of explosive volcanic eruptions and fragments from the walls of the vents

subduction: a process that occurs when one tectonic plate dives beneath another tectonic plate; it may occur over thousands or millions of years

tectonics: the study of the processes that formed the structural features of the earth's crust, especially the creation and movement of immense crustal plates

tunnel vent: the central tube in a volcanic structure through which material from the earth's interior travels

vent: a break or tear on the side of a mountain through which magma and pressure can escape

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