Mount St. Helens, 1980 to now— What’s going on?

U.S. GEOLOGICAL SURVEY and the U.S. FOREST SERVICE—OUR VOLCANIC PUBLIC LANDS

M ount St. Helens seized the world’s attention in 1980 when the largest historical landslide on Earth and a powerful explosive eruption reshaped the volcano, created its distinctive crater, and dramatically modified the surrounding landscape. An enormous lava dome grew episodically in the crater until 1986, when the volcano became relatively quiet. A new glacier grew in the crater, wrapping around and partly burying the lava dome. From 1987 to 2003, sporadic earthquake swarms and small steam explosions indicated that magma (molten rock) was being replenished deep underground. In 2004, steam-and-ash explosions heralded the start of another eruption. A quieter phase of continuous lava extrusion followed and lasted until 2008, building a new dome and doubling the volume of lava on the crater floor. Scientists with the U.S. Geological Survey and University of Washington’s Pacific Northwest Seismograph Network maintain constant watch for signs of renewed activity at Mount St. Helens and other Cascade volcanoes. Now is an ideal time for both actual and virtual visitors to Mount St. Helens to learn more about dramatic changes taking place on and beneath this active volcano.

Civilization exists by geological consent, subject to change without notice. –Will Durant

The spectacular 1980 eruption at Mount St. Helens opened a new episode in the volcano’s history that began more than 250,000 years ago. Fifty-seven people lost their lives as a result of the eruption, and dramatic changes to the landscape occurred and will continue long into the future. Lava domes grew in the 1980 crater during 1980–86 and 2004–8. A new glacier— Crater Glacier —formed, and streams began eroding and transporting millions of tons of sediment downstream. Years later, hot dome rocks are still steaming, Crater Glacier is still advancing, rock falls send plumes of dust skyward, and streams continue to erode and transport sediment away from the volcano. Meanwhile, magma (molten rock) is accumulating again beneath the volcano. Scientists are keeping a close watch on Mount St. Helens and the other Cascade volcanoes to assess hazards and provide timely warnings of future activity. Arming yourself with knowledge and following a few simple recommendations will help you prepare for future eruptions.

1980—Earth’s Inner Fury Uncorked

On May 18, 1980, Mount St. Helens in southwest Washington State erupted violently, killing 57 people. The picturesque conical volcano had been rumbling for months, but on that morning its north side collapsed in one of the largest debris avalanches (landslides) in history, triggering a lateral blast that devastated 150 square miles and sent up an eruption column that reached the stratosphere and blanketed areas hundreds of miles downwind with ash. The eruption also initiated lahars (volcanic mudflows) that choked nearby rivers. The landslide deposit—its surface strewn with huge blocks of shattered rock called hummocks—buried the North Fork of the Toutle River valley as much as 600 feet (180 meters) deep for a distance of 13

miles (21 kilometers). Part of the landslide overtopped a ridge 1,150 feet (350 meters) high 6 miles (10 kilometers) north of the volcano, leaving a deposit called The Run-Up or The Spillover. The eruption destroyed nearly all of the volcano’s glaciers, and lahars choked the Toutle, Cowlitz, and Columbia River channels with sediment. Pyroclastic flows—ground-hugging avalanches of hot volcanic ash, pumice, rock fragments, and gases that destroy everything in their path—rushed out of the crater left by the landslide and formed the Pumice Plain in the valley below. Five more explosive eruptions during summer 1980 sent columns of ash jetting into the stratosphere, disrupting life in the Pacific Northwest and threatening air travel.

From December 1980 to October 1986, a lava dome grew episodically on the crater floor, eventually reaching a height of nearly 1,000 feet (305 meters) and a volume of 120 million cubic yards (92 million cubic meters). Snowfall and avalanches from the steep crater walls collected on the crater floor, where they were shaded from sunlight during most of the year. As a result, Crater Glacier formed and began to flow—the newest and only expanding glacier in the Cascade Range. Newly formed streams began the monumental task of eroding and transporting millions of tons of sediment downstream. More than three decades later, only about 8 percent of the sediment dumped into the river system in 1980 has been eroded away. The downstream flood threat from rivers clogged with mobilized sediment will require attention for decades to come.

2004—The Volcano Erupts Again

In September 2004, almost 18 years after the 1980–86 lava dome stopped growing, a swarm of shallow earthquakes and a rapidly growing welt on the crater floor signaled that Mount St. Helens was about to erupt again. Scientists warned that a new eruption could start explosively, and on October 1 the first of several explosions shot a plume of volcanic ash and gases into the sky. Lava emerged from the top of the welt about 10 days later. For the next 3+ years, scientists and spectators alike marveled as a series of hot, solid, smooth-sided lava “spines” rose from the vent, bulldozed their way across the crater floor, and piled up to form a new dome 1,500 feet (455 meters) high.

Thick, pasty lava flow emerges onto the surface of the 1980-86 dome on June 26, 1981. The 1980-86 dome grew episodically by accumulation of more than a dozen flows like this one, plus by internal growth from magma that pushed upward but did not reach the surface. USGS photo by Dan Dzurisin.

Crater Glacier was split in two by the growing dome and shoved against the surrounding crater walls. The glacier responded by flowing northward around the east and west sides of the 1980–86 dome at rates

PRODUCTS AND LEGACY OF MOUNT ST. HELENS’ ERUPTIONS

of several feet per day. In early 2008, the two arms of the glacier met on the north side of the dome. By the time the eruption ended in early 2008, the new dome had grown to a volume of 124 million cubic yards (95 million cubic meters)—very nearly the same as the 1980–86 dome. Today (2013), no new lava is being added to the crater and the rate of advance

2004–PRESENT

of Crater Glacier has slowed to the pre-2004 value of about 1 foot per day (0.3 meters per day).

Dramatic Changes Occur Before Your Eyes and Beneath Your Feet

Years after the construction of the lava domes of 1980–86 and 2004–8, hot dome rocks are still steaming. Crater Glacier is still advancing and surrounds both domes. Rock falls send plumes of dust skyward. Sediment is the lasting legacy of past eruptions—erosion of 1980 deposits is creating steep-sided channels in the North Fork of the Toutle River valley, and millions of tons of sediment are being carried downstream, increasing flood hazards. This material originated from the upper part of Mount St. Helens, which slid away on May 18, 1980, and it is now on its way to the Columbia River and beyond.

Beneath the volcano, magma is accumulating again and building for another eruption—quite possibly within our lifetimes. Scientists are carefully tracking these and other changes in this most dynamic landscape in the Pacific Northwest.

Helicopter flying past lava spine emerging from vent on crater floor. A series of hot, solid lava spines like this one piled up to form the 2004-08 dome. During an eruption, when semisolid magma scrapes against vent walls, it forms a smooth, soft “fault gouge” surface—a texture also seen on rocks lining California’s San Andreas fault. USGS photo by Dan Dzurisin, April 28, 2006.

Silent Forces Beneath Us Create Volcanoes of the Cascade Range

Slow collisions of Earth’s tectonic plates sustain Cascade volcanoes. Mount St. Helens is the most active volcano in the Cascade volcanic arc, a chain of volcanoes stretching 700 miles (1,100 kilometers) from Lassen Peak in northern California to Mount Garibaldi and beyond in southern British Columbia, Canada—part of the famous Pacific “Ring of Fire.” The Cascadia section of the Ring of Fire exists because, off the Pacific Northwest coast, the Juan de Fuca Plate is colliding with and sinking beneath the North American Plate at a rate of 1–2 inches (3–5 centimeters) per year. Scientists call this type of boundary between colliding plates a subduction zone, with one plate subducting (sliding beneath) the other. As the plates move, strain builds up in the rocks along subduction zones. This strain is sometimes released in great earthquakes, such as the 1700 Cascadia earthquake off the Pacific Northwest coast and the 2011 Tōhoku earthquake in Japan.

Subduction plays a key role in generating the magmas that form and feed Cascade volcanoes. The surprise ingredient in the magma-generating process is—water. Deep within the Earth, extreme heat breaks down water-bearing minerals in the sinking Juan de Fuca Plate, releasing hot water into the overlying rocks of the Earth’s mantle. The added water lowers the melting temperature of mantle rock, causing some of it to melt. The resulting magma is less dense than surrounding rock, so it rises into the Earth’s crust, where its heat triggers more melting and the formation of magma reservoirs. As magma approaches Earth’s surface, water vapor and other gases in the magma expand and provide additional energy for a volcanic eruption.

Monitoring Can Provide Advance Warning of Future Eruptions

Volcanoes generally don’t produce large earthquakes like those that occur along plate boundaries, but sometimes swarms of hundreds to thousands of small quakes occur beneath a volcano and provide clues to processes occurring deep underground. Ground deformation (swelling, stretching) occurs when the pressure beneath a volcano changes, which can mean that magma is accumulating or moving toward the surface. A change in the amount or composition of gases released from magma as it rises toward the surface also can foretell an impending eruption. By monitoring and analyzing earthquakes, ground deformation, and volcanic gas emissions, scientists are better able to understand a volcano’s behavior, to assess hazards and potential impacts, and to provide timely warnings of future events.

Mount St. Helens is a world-class natural laboratory where USGS and other scientists are learning about processes of rapid landscape change, the magma system that feeds the volcano, and the volcanic and tectonic forces that shape the Pacific Northwest, as well as the warning signs and hazards associated with eruptions. Mount St. Helens is the volcano in the Cascades most likely to erupt again in our lifetimes. The exact timing and magnitude of the next eruption cannot be forecast long in advance, but our growing knowledge base and continued monitoring will enable the USGS to provide short-term forecasts and warnings—as was done in 1980 and 2004.

Those who cannot remember the past are condemned to repeat it. –George Santayana

Be Prepared for Future Eruptions

Experience has shown that Mount St. Helens tends to be relatively quiet

VOLCANO HAZARDS IN THE MOUNT ST. HELENS REGION

between eruptions, with occasional earthquake swarms and minor amounts of ground movement (deformation) and volcanic gas emission. Scientists continue to monitor the volcano closely for any sign of a change from the normal background level of activity, but until that happens the timing of the next eruption cannot be known. On the other hand, we know from the geologic record at Mount St. Helens that additional explosive eruptions are possible and that dome building can go on episodically for decades to centuries. So the dome-growth episodes in 1980–86 and 2004–08 are probably not the last in this series. If past trends are repeated, renewed dome growth will be preceded by a few days to weeks of heightened earthquake activity, ground deformation, and gas emission.

People who are knowledgeable and prepared can better survive a volcanic eruption. Following the recommendations

ANSWERS (to questions posed in caption of lidar image)

below will help you prepare for, and recover from, the next eruption.

The hazard zonation map for Mount St. Helens shown above reflects current understanding and will be updated as conditions warrant. Becoming familiar with the hazard map is a good start to ensuring your safety in the event of an eruption. Stay informed about the status of Mount St. Helens and other Cascade volcanoes through the news media and via the USGS Volcano Hazards Program website: http://volcanoes.usgs.gov/.

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