The pristine, snow-covered summit of Mount St. Helens in Washington State had been living up to its American Indian name Louwala-Clough, or “smoking mountain,” for two months when dawn broke on May 18, 1980. The volcano that some called “America’s Mount Fuji” had been rumbling and emitting steam for so long that many nearby residents had nearly stopped hearing it. But at 8:32 that morning, a magnitude 5.1 earthquake centered a mile beneath the mountain caused its entire north flank to collapse.
A huge avalanche of melting ice, earth, and debris raced downslope at speeds of up to 180 miles per hour. And without the burden of its outer shell, gas and steam that had built up to tremendous pressure within the mountain caused it to literally explode in a lateral blast that was heard hundreds of miles away.
Moving horizontally at nearly 670 miles per hour, just below the speed of sound, a hellish wall of magma, ash, and volcanic debris destroyed everything in its path to a distance of 19 miles from the mountain.
Following the lateral explosion, a plume of ash and steam launched skyward, reaching an altitude of 12 miles in ten minutes, finally punching through the stratosphere where it began to spread out. Around the remains of the mountain, lightning forked through the thick, swirling ash, starting forest fires. By noon, the cities of Yakima and Spokane were covered in a thick blanket of gray, choking ash. Before it was over, fifty-seven people were dead.
Scenes of the eruption of Mount St. Helens burned themselves into the national consciousness, and the explosion was the largest in the recorded history of the United States. But there have been far larger explosions, and their effect on the atmosphere has been much greater.
Television crews usually focus on spectacular fountains and streams of lava when covering erupting volcanoes, and no one will argue that they don’t make compelling subjects. Volcanoes can eject lava bombs—blobs of magma the size of coconuts or larger—which are just as hazardous as they sound, and produce pyroclastic flows, which are avalanches of superheated gases, hot ash, pumice, and melted rock that can race down a mountainside at 100 miles per hour. But those effects are confined to a small area around the eruption. Far greater and more widespread are the effects of ash and other particles flung into the stratosphere by these exploding mountains.
Once dust and ash are injected into the upper atmosphere by a volcanic eruption, the debris can remain suspended there for years. Fortunately, the eruption of Mount St. Helens had no significant effect on global climate, due largely to the fact that most of its force was directed sideways instead of upward.
During the twentieth century two volcanoes had a huge impact on Earth’s climate: El Chichon in Mexico and Mount Pinatubo in the Philippines. The Mexican volcano spewed 120 tons of material in 1982 into the atmosphere, and Pinatubo disgorged a stunning 310 tons in 1991, laden with caustic chemicals such as sulfur dioxide and hydrogen chloride, which can damage Earth’s ozone layer.
As sulfur particles from volcanic eruptions drift in the upper atmosphere, they combine with water vapor, and as the particles are bombarded with sunlight, they mutate into sulfuric acid, which forms a hazy layer that reflects solar radiation back into space. With less solar energy reaching the surface, temperatures begin to drop. After the eruption of Mount Pinatubo, average global temperatures fell 1.5°F.
The Cascade Range of the Pacific Northwest is home to more than a dozen potentially active volcanoes, including Mount St. Helens, and most of them tend to erupt explosively. The US Geological Survey has set up a Volcano Hazards Program to watch these potential trouble spots, since some, like Mount Hood and Mount Rainier, are close to major cities.
A link between volcanic eruptions and global climate change was first established after the eruption of Krakatau, in the Sunda Strait between Java and Sumatra. The volcano, also known as Krakatoa, exploded in 1883 with a blast said to have been the loudest sound ever heard on Earth. Ten times more powerful than Mount St. Helens, the eruption instantly vaporized two-thirds of the volcano and caused tsunamis—giant waves more than 100 hundred feet high—that killed more than 36,000 people. Its dust cloud covered two million square miles, lowering global temperatures for five years. The blast set off seismometers all over the world, and for the first time, scientists were able to gather observations from widely spaced locations in order to study the explosion.
Accounts of Krakatau’s aftermath include tales of brilliant sunsets that could be seen all over the world for weeks after the eruption, caused by the enormous quantity of ash pumped into the stratosphere by the blast. The sky became so bright in New York and Connecticut that some residents thought their cities were ablaze and called local fire departments.
Even Krakatau wasn’t the largest volcanic eruption in recorded history: sixty-eight years earlier, a volcano named Tambora exploded in Indonesia, spewing 150 times more ash into the upper atmosphere than Mount St. Helens did in 1980. The column of dust, ash, and debris rose to a height of 28 miles before collapsing on itself, causing devastating pyroclastic flows to sweep down to the ocean. Around 10,000 people died in the explosion and its immediate aftermath, but Tambora wasn’t through causing misery.
In the months to come, thousands more would die on nearby islands as epidemics and famine swept through their midst. As the unimaginably huge cloud of ash spread farther across the globe, it began to trigger climate changes far and wide. In China the skies above Hainan Island went black, obliterating the Sun and destroying crops and trees.
On the shores of Lake Geneva, Switzerland, in 1816, the gloomy weather prompted Lord Byron to suggest that his houseguests join him in a ghost story–writing contest. Byron and Percy Shelley abandoned their attempts before long, but Shelley’s wife, Mary, conjured a story of a scientist named Frankenstein that made literary history.
By the next year, known as “the year without a summer,” economic losses were piling up as far away as North America. In June 1816, severe frosts killed Northeast farmers’ crops as they repeatedly tried to replant. In Vermont foot-long icicles hung from the trees while ice an inch thick covered ponds and lakes. Newly shorn sheep died by the thousands, and so did migratory birds caught in the icy weather’s grip. At Williamstown, Vermont, the temperature at 5 p.m. one June day was recorded at 30.5°F.
Things weren’t much better in Europe, which was just recuperating from the Napoleonic Wars. In Britain and France riots broke out as food shortages loomed, and in Switzerland the violence caused the government to declare a state of national emergency.
After studying eruptions for decades, volcanologists now understand that a volcano’s history is one major key in forecasting its future. Volcanoes such as Kilauea in Hawaii erupt on a regular basis and in a predictable manner, and because they don’t generally erupt explosively, they can be monitored with sensitive instruments that provide clues to the conditions that exist just before an eruption.
Volcanoes give many clues before erupting, including underground earthquakes that become more frequent and intense before a blast—activity that scientists call seismicity. Volcanologists can now measure and track the shock waves generated by these temblors with seismometers, which record quakes’ magnitude and epicenters as magma flows deep below the surface.
Melted rock can also push upward toward the surface until the volcano’s “skin” is distorted into a pressure-filled dome. Before Mount St. Helens erupted in 1980, the mountain began to develop a huge bulge on its north face that grew at the rate of more than 5 feet per day. Geologists were able to track the growth of the bulge using a device called a geodimeter, which can measure very small changes in distance with reflected light. By the day of the eruption, the instruments showed that some parts of the north face had bulged outward more than 450 feet from their original positions.
Scientists are getting some assistance from space in their continuing quest to accurately predict volcanic outbursts. Earth-observing satellites can spot rising underground pools of magma from orbit. The upwellings, which often precede an eruption, show up as “hot spots” on digital images.
Scientists also employed tiltmeters at St. Helens. As their name implies, these devices can measure minute changes in the slope angle or tilt of the ground. Tiltmeters work much like a carpenter’s level: the movement of a bubble floating in conducting fluid is monitored electronically, and any change in the bubble’s position is translated into a degree of tilt and relayed to a base station. Tiltmeters are so sensitive they can measure a change in angle as small as 0.00006 of a degree!
Another tool used in forecasting eruptions is a correlation spectrometer, which measures how much sulfur dioxide is being emitted by a volcano. These emissions tend to increase markedly before an eruption.
In 1991 scientists were able to use all these devices to successfully predict the eruption of Mount Pinatubo. It first gave warning by generating a series of earthquakes, followed by an increase in sulfur dioxide emissions. Then, the sides of the volcano began to swell outward. Alarmed authorities evacuated nearby Clark Air Base and 58,000 residents who lived within 20 miles of the volcano just days before the mountain exploded in one of the most powerful eruptions of the twentieth century.